displacement yacht speed

Displacement Speed vs. Hull Speed: What’s the Difference?

Ship displacement speed and hull speed are important concepts in the marine world since they indicate the maximum speed at which a ship can travel.

Their main difference is that displacement speed is solely determined by the size and shape of the vessel, while hull speed is also determined by the amount of power.

Table of Contents

Displacement speed

Displacement speed is the speed of a boat when it is at its most efficient and is considered to be the best performance that the vessel can provide.

It is the slowest speed at which the boat will still move forward through the water and is calculated based on the hull’s shape, size, and weight.

This type of speed does not take into account any external forces such as wind or current and is solely dependent on the boat’s ability to move through the water with minimal resistance.

At this speed, the boat will not feel like it is being pushed along, but instead, it will feel like it is gliding through the water with relative ease.

The faster the boat is traveling, the more drag it will create, resulting in a decrease in speed and an increase in fuel consumption.

However, if the boat is traveling at displacement speed, it will use the least amount of fuel while still maintaining a steady, fast rate of speed.

How do you calculate displacement speed?

Displacement speed, also known as wave-making resistance, is the maximum speed of a boat in deep water. It is the speed at which the wave created by the hull’s motion has the same shape as it had when the boat was stationary.

Calculating displacement speed can be done by finding the square root of the waterline length multiplied by the acceleration due to gravity, divided by the displacement of the vessel.

In other words, calculating displacement speed requires knowing the length of the waterline, the gravity acceleration, and the displacement.

The shorter the waterline, the faster the boat can travel with less effort. A longer waterline means more resistance, which reduces the speed of a boat.

The displacement is the weight of the vessel and all its cargo or passengers, and it will affect how fast the boat can move through the water.

If you have a smaller vessel that is carrying a lot of cargo, you’ll need to adjust for this in your calculations to ensure you get an accurate result.

The gravity acceleration is also an important factor, as a higher number will result in a faster speed.

It’s important to note that although displacement speed is often considered to be the maximum speed of a boat, there are several factors that may slow down its performance in actuality.

Factors such as wind, waves, and hull drag will all have an effect on the overall speed of a boat and should be taken into account when attempting to achieve a fast displacement speed.

How fast can a displacement hull go?

Displacement hulls are designed to be slower than other types of vessels, and their speed is limited by the laws of physics.

Generally, displacement hulls can only travel at speeds that are equal to or below their displacement speed, also known as their “hull speed”.

The displacement speed of a vessel is determined by the length of the vessel and the weight of the water it displaces. This speed is usually determined by using a mathematical formula, known as the “Hull Speed Formula”.

The Hull Speed Formula states that the top speed of a displacement vessel is equal to 1.34 times the square root of its waterline length in feet.

This means that the longer the vessel is, the faster it can travel . For example, a 25-foot-long displacement hull would have a top speed of approximately 7 knots (nautical miles per hour).

It is important to note, however, that this formula is only an approximation and not an exact value. There are many variables at play, such as the type and size of the vessel, as well as environmental conditions like wind and waves.

As such, it is difficult to predict the exact speed a displacement hull can reach. Generally speaking, however, most displacement hulls should not be able to travel faster than their hull speed.

Hull speed, also known as theoretical velocity, is the maximum speed of a boat with a given length and waterline beam. It is a function of the square root of the waterline length of the vessel.

At this speed, the hull is theoretically making the best use of its length and beam by creating the least amount of drag. A boat moving at hull speed will not necessarily be going as fast as possible; this is only the theoretical maximum speed.

In general, for displacement hulls, hull speed is about 1.34 times the square root of the waterline length in feet.

So, for example, a 30-foot boat will have a hull speed of approximately 7.7 knots. However, the actual hull speed may vary slightly depending on the type of hull, construction materials, and other factors.

When a boat is moving faster than its hull speed, it is said to be “planing”. This means that it is riding on top of the water instead of pushing through it.

When planing, a boat is able to travel faster due to less drag on the hull. However, it takes more power to achieve planing speeds than displacement speeds and can increase fuel consumption. As such, it is usually best to stay within the range of hull speed when cruising.

How fast is the hull speed?

The hull speed, also known as the theoretical maximum speed of a displacement boat, is calculated using a formula.

The formula uses the length of the boat’s waterline and an “f” value, which is a coefficient determined by the shape of the boat’s hull. Generally speaking, a boat’s hull speed is about 1.34 times the square root of its waterline length in feet.

As an example, a boat with a 30-foot waterline length would have an approximate hull speed of 7.7 knots (about 8.9 mph).

The hull speed is the theoretical maximum speed of a displacement boat and is typically only achievable under certain conditions, such as light wind, flat water, and in certain types of boats.

It is important to note that most vessels will not be able to reach their hull speed because of drag and other external factors, so it should not be used as a reliable indicator of top speed.

What is critical hull speed?

The critical hull speed is a term used to describe the maximum speed a displacement hull can go without experiencing instability or cavitation.

A displacement hull is any boat that sits in the water and displaces its own weight, as opposed to a planing hull which uses its own weight and the shape of its hull to glide across the water.

At critical hull speed, there is an increase in drag on the hull due to air bubbles forming along the bottom of the boat. This drag reduces the efficiency of the propulsion system, making it difficult for the boat to continue at this speed.

The critical hull speed is determined by calculating the square root of the waterline length of the boat multiplied by 1.34. This number represents the theoretical top speed a displacement hull can reach.

As such, it is important for sailors to be aware of their boat’s waterline length and understand that critical hull speed will be the upper limit for their vessel.

Can you exceed hull speed?

The short answer is yes, you can exceed hull speed. The hull speed is a theoretical limit that is based on the length of a boat’s waterline and its displacement.

This means that if a boat is long enough, and if it is powered by an engine that has enough thrust, then it is possible to exceed the theoretical hull speed limit.

However, it is important to remember that it is still not recommended to push your boat to the point of exceeding hull speed.

This is because if you go too fast, you could risk damaging the boat’s hull or motor. Additionally, the higher speeds can cause increased drag, resulting in lower fuel efficiency.

Therefore, it is best to stay within the hull speed limit unless you are a skilled boater who knows what they are doing.

The difference between displacement speed and hull speed

Displacement speed and hull speed are interchangeable at most times.

Displacement speed is the maximum speed a ship can travel without creating a bow wave or cavitation.

It is calculated by dividing the length of the hull at the waterline by 1.34. This calculation is based on Froude’s Law, which states that a ship’s displacement speed is proportional to the square root of its waterline length.

The hull speed is the speed at which a boat’s hull generates more resistance than it can overcome, resulting in decreased efficiency.

This occurs when the wave-making drag generated by the boat’s hull is greater than the forward thrust generated by the engine.

The hull speed is also known as “critical hull speed” or “maximum hull speed.” The formula for calculating hull speed is 1.34 times the square root of the boat’s waterline length (LWL).

The main difference between displacement speed and hull speed is that displacement speed is only affected by the size and shape of the hull, while hull speed also takes into account the amount of power being applied to the vessel.

Displacement speed is typically lower than hull speed, but some vessels are designed to operate beyond their theoretical hull speed. In this case, it is necessary to use more powerful engines to push the boat beyond its theoretical limits.

Hull Speed Calculator

Table of contents

Welcome to the hull speed calculator . If you've ever seen a boat go so fast that its nose started rising, then you've seen the concept of hull speed in action. In this article, we'll explain what hull speed is and what it means for a ship's design. Later, we'll show you how to calculate hull speed with the hull speed formula, so that you can work out how to calculate hull speed for your own boat.

What is hull speed?

Hull speed is the speed at which a vessel with a displacement hull must travel for its waterline to be equal to its bow wave's wavelength. A displacement hull travels through water, instead of on top of it as a planing hull (like a kiteboard ) would, thereby displacing water with its buoyancy as it sails. The pressure that this displacement exerts on the water creates a wave; this wave is known as the vessel's bow wave . A slow-moving boat's bow wave might make small waves, but, as the boat sails faster, the bow wave's wavelength λ \lambda λ grows. When the wavelength meets the waterline length (that's also when the bow wave's first and second crests are at opposite tips of the waterline), the boat is said to be traveling at hull speed. Take a look at the picture below to see what we mean:

Why does hull speed matter?

Although it's not perfect, hull speed remains a useful concept that can help us answer questions about how fast a sailboat can go, and the optimal amount of thrust you need to keep a boat moving forward.

A boat's hull speed limits how fast it can travel efficiently. When traveling at hull speed, the boat's bow wave and stern wave have synchronized and constructive interference occurs, which allows the boat to move very efficiently. However, at speeds greater than hull speed, a vessel's nose automatically starts rising as the vessel tries to climb its bow wave. This process is called planing , and it wastes lots of energy. Trying to move faster than the hull speed will therefore require more and more thrust (whether that comes from sails, rowing, or engines) in exchange for smaller and smaller gains in speed as more energy is wasted angling the boat upwards. Hull speed can therefore be said to impose a flat limit on how fast a sailboat can go.

Shortcomings of hull speed

Although the physics behind hull speed is sound, it is heavily dependent on the hull's shape. Long and thin hulls with piercing designs can easily break their hull speed without planing. Such hulls are found on:

• Catamarans; and
• Competitive kayaks.

A hull's design can enable it to circumvent the workings of hull speed. It is for this reason that hull speed is not used in present-day ship design; naval institutions nowadays favor more modern measurements of speed-to-length ratio, such as the Froude number .

How to calculate hull speed

The formula for hull speed only needs the length of the vessel's waterline in feet, denoted as L waterline L_\text{waterline} L waterline ​ . With this length, the vessel's hull speed in knots can be calculated with

If you want to instead work out exactly how long your new boat's waterline must be for it to have a certain hull speed, you can invert the formula to obtain

How to use the hull speed calculator

The hull speed calculator is just as easy to use as the formula.

Enter your vessel's waterline length into the first field. This is the length of your boat's hull at the height of the waterline. Your vessel's hull speed will then be calculated and presented in the second field.

You can also use the hull speed calculator backward to work out how long a vessel's waterline must be if you know its hull speed.

You can freely change the units of your measurements without interfering with the hull speed formula.

How can I increase my boat's hull speed without changing its hull?

Load your boat heavier! If you think about a normal displacement hull, it's usually narrower near the bottom than at the deck. So pushing it down with some weight will lengthen the boat's waterline, and so its hull speed is increased. Of course, heavier boats are harder to move, so while your loaded boat now has a higher hull speed, you would need more power to move it.

Waterline length

The length of the ship at its waterline.

The speed at which the ship's waterline length equals its bow wave's wavelength.

Yachting Monthly

• Digital edition

Busting the hull speed myth

• julianwolfram
• December 13, 2021

Waterline length is not the defining factor in maximum boat speed that we all think it is. Julian Wolfram busts the hull speed myth

Modern hull forms, like this Jeanneau SO440, use chines to create volume forward while keeping a narrow entrance at the waterline

Every sailor is delighted when the breeze picks up and the boat really starts to get going with a bone in her teeth.

Julian Wolfram is a physicist, naval architect, former professor of ocean engineering at Heriot-Watt in Edinburgh and a Yachtmaster Offshore who has cruised and raced for 45 years

The crew will want to know how fast she will go and perhaps surreptitiously race her against any similar sized boat in the vicinity.

Speculation may start about what allows one boat to go faster than another – is it the hull shape or the sails?

It is easy to spot good, well-trimmed sails but what about the hull ?

The important part is not visible below the water surface. However there is one key indicator that is often very apparent – the waves generated by the sailing yacht.

When a yacht picks up speed the wave pattern around it grows and the greater the speed the bigger the waves .

The energy in these waves is proportional to the square of their height – double the height and the energy goes up by a factor of four.

This energy comes from the wind , via the sails and rig , making the hull push water out of the way.

If less of this wind energy was wasted in producing waves the yacht would go faster.

When a typical displacement monohull reaches a speed-to-length ratio of around 1.1 to 1.2 (speed in knots divided by the square root of the waterline in feet) up to half the wind energy driving it is usually wasted in generating waves.

The hull speed myth: Half angle of entrance

So how can we tell if a yacht will sail efficiently, or have high wave resistance and waste a lot of energy generating waves?

The answer starts back in the 19th century with the Australian J H Michell.

In 1898 he wrote one of the most important papers in the history of naval architecture in which he developed a formula for calculating wave resistance of ships.

Light displacement cruising boat: The bow of this Feeling 44 is finer than older cruising boats

This showed that wave resistance depended critically on the angle of the waterlines to the centreline of the ship – the half angle of entrance.

The smaller the angle the smaller the height of the waves generated and the lower the wave-making drag.

A knife blade can slice through water with minimal disturbance – drag the knife’s handle through and you generate waves.

The big hull speed myth

For a displacement hull the so-called ‘hull speed’ occurs when the waves it generates are the same length as the hull.

This occurs when the speed-length ratio is 1.34.

It is claimed that hulls cannot go significantly faster than this without planing. It is called ‘the displacement trap’ but is a myth.

Heavy displacement cruising boat: An older design has a bow that is several degrees wider

As an example, consider a 25ft (7.6m) boat that goes at 10 knots in flat water.

This is a speed-length ratio of two. That is the average speed over 2,000m for a single sculls rower in a world record time.

The reason for this high speed is a half angle of entrance of less than 5º. Hobie Cats, Darts and many other catamarans have similarly low angles of entrance and reach even higher speed-length ratios with their V-shaped displacement hulls.

These hulls also have almost equally fine sterns, which is also critically important to their low wave resistance.

The monohull problem

Now a monohull sailing yacht needs reasonable beam to achieve stability and, unless waterline length is particularly long, the half angle of entrance will inevitably be much larger than those on rowing skulls and multihulls .

In his 1966 Sailing Yacht Design Douglas Phillips-Birt suggests values of 15º to 30º for cruising yachts.

Many older cruising yachts with long overhangs and short waterline lengths, for their overall length, have values around the top of this range.

Busting the hull speed myth: A Thames barge is a similar length and beam to a J-Class, but its bluff bow, built for volume, makes it much slower. Credit: Alamy Stock Photo

Newer sailing yachts, with plumb bows, have somewhat smaller half angles and a modern 12m-long fast cruiser may have a value around 20º and a racing yacht 17º or 18º.

Size matters here as, to achieve stability, a little yacht is likely to have a bigger half angle than a large one, such as the German Frers-designed 42m (138ft), Rebecca which has a half angle of entrance of under 13º.

Rebecca also has a fine, elegant stern which helps minimise the stern wave – I’ll come back to sterns and stern waves.

Interestingly the half angle of entrance is not mentioned in the otherwise excellent 2014 Principles of Yacht Design by Larsson et al, although it is currently used as one of the parameters in the preliminary estimation of wave resistance for ships.

While it is still particularly applicable to very slender hulls, naval architects are not generally familiar with Michell’s work.

His formula for wave resistance involves quadruple integrals of complex functions.

German-Frers’ designed Rebecca has a half angle of entrance of just 18°. Credit: Cory Silken

These are not ‘meat and drink’ for your average naval architect, and only a few mathematically inclined academics have much interest in theoretical wave resistance.

Michell’s work is rarely, if ever, covered in naval architecture courses now.

Nowadays the emphasis is much more on numerical methods, high-speed computers and computational fluid mechanics (CFD) using the so called Navier-Stokes equations.

Examining these equations, which apply to any fluid situation, does not give any insights into wave resistance, albeit they can model wave resistance very well when used in the piecewise manner of CFD.

It is very easy to measure the half angle of entrance at the design waterline when a yacht is out of the water.

Take a photograph directly upwards from the ground under the centreline at the bow.

Busting the hull speed myth: Multihulls achieve high speeds due to fine hulls, light displacement and ample stability. Credit: Joe McCarthy/Yachting Monthly

Now blow this up on a computer screen, or print it off at a large scale, and measure the angle with a protractor.

Alternatively, if you have a properly scaled accommodation plan drawn for a level close to the design waterline this will yield a reasonable approximation of the half angle of entrance.

Unfortunately there is not a simple relationship between the fineness of the bow and the wave drag.

But, all other things being equal, the smaller the half angle the better.

It is easy to measure and is a useful parameter to know when comparing yachts.

Stern shape and hull speed

The half angle of entrance cannot be taken alone as a measure of wave drag, and the fairness of the hull and in particular the run aft is also critical.

Just as the half angle of entrance dictates the height of the bow wave, so the fineness of the stern is a key influence on the height of the stern wave.

Consider the water flowing around both sides of the hull and meeting at the stern.

Modern race boats, like Pip Hare ‘s IMOCA 60, combine a fine angle of entrance with wide, flat hulls for maximum form stability and planing ability. Credit: Richard Langdon

If these streams meet at a large angle the water will pile up into a high stern wave.

On the other hand if they meet at a shallow angle there will be less piling up. A fine stern can maintain a streamline flow of water.

However if the sides of the hull meet at the stern at a large angle then the streamline flow will tend to separate from the hull, leaving a wide wake full of drag-inducing eddies.

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In many modern designs the hull sides are not far off parallel at the stern and it is then the upward slope of the buttock lines that are critical and, again, the shallower the slope the better from a hull drag perspective.

The slope of the buttocks can easily be measured if the lines plan is available and a good indication can be obtained from a profile drawing or a photo taken beam on with the boat out of the water.

Drawing a chalk line parallel to the centreline and half a metre out from it will provide a buttock line that can be checked visually for fairness when the boat is viewed from abeam.

A rowing scull easily exceeds its theoretical max hull speed. Credit: Alamy Stock Photo

Again, the smaller the angle the better – provided the transom is clear of the water.

An angle of more than 17º will lead to separated flow and eddy making. This also happens if the transom is immersed.

The greater the immersion the greater the drag, so weight in stern lockers on modern boats can be critical.

Modern hull design

The modern wedge shape attempts to resolve the conflicting demands of a small angle of entrance, good stability and a fine stern.

The plumb bow extends the waterline forward and, with the maximum beam taken well aft, the hull forward can be relatively narrow, providing a low half angle of entrance.

The stern is wide, which helps achieve good stability, but at the same time the buttocks rise slowly at a shallow angle to the water surface.

This gives a smooth and gradual change in the hull’s cross section area ensuring the water flow remains attached to the hull and that the stern wave is kept low.

A modern cruising boat gains stability from a wide stern, but needs twin rudders

This wide, flat stern also helps surfing down waves and possibly planing.

Some designs have chines just above the design waterline which increases usable internal volume and gives a little more form stability when heeled.

However, as soon as the chine is immersed there will be separation along the chine edge as water will not flow smoothly around a sharp edge.

It is just not possible to get the chine perfectly aligned with the streamlines of the water flow in all sailing conditions and there will be some extra drag at times.

There are two downsides to the wedge- shaped hull.

Overloading aft will create a large increase in drag

First the boat has to be sailed at a small angle of heel to keep the rudder properly immersed and to avoid broaching. This can be offset to some extent by using twin rudders .

The second is that the weight must be kept relatively low.

This is because a relatively small increase in weight causes a big increase in wetted surface area at the stern and hence in the frictional drag which makes the boat slower, particularly in light airs.

This is the downside of slowing rising buttocks and the reason why dinghy sailors get their weight forward in a light breeze .

Displacement Length Ratios

Traditionally for sailing yachts the displacement-length ratio has been used as a measure of speed potential, partly because it is easy to calculate from the yacht particulars.

It is waterline length (in metres) divided by the cube root of displacement (in cubic metres or tonnes).

A heavy boat, such as the Heard 35, will have a value of about 4 to 4.8.

A more moderate displacement boat, such as the Hallberg Rassy 342 or Dufour 32 Classic, will have a value in the range 5 to about 5.5; whilst a racing boat may a value of up to, and even over, 7.

A heavy displacement cruising boat with a fair run aft is less affected by additional weight

However the displacement length ratio can be misleading as making a hull 20% deeper and 20% narrower will keep the displacement the same but will significantly reduce the half angle of entrance and the wave drag.

It is interesting to note a Thames barge in racing trim has the same length-displacement ratio as a J class yacht, but their speed potential is vastly different.

Finally I should mention the older ‘length-displacement’ ratio, which is quoted in imperial units.

This is calculated by dividing a boat’s displacement in tons (2,240 pounds) by one one-hundredth of the waterline length (in feet) cubed.

Credit: Maxine Heath

It is still used in the USA and should be treated with caution.

The myth that your boat’s speed is only restricted by it waterline length does a disservice to its designers, and does little to help you understand how to get the best from her when the wind picks up.

Have a look at how the boat is loaded, how you sail on the wind, your boat handling and how much canvas you ask her to carry and you may discover more speed than you expect.

The remarkable John Henry Mitchell

Pioneer of wave theory

It’s worth saying a little more about the remarkable John Henry Michell.

He produced a series of ground-breaking papers including one that proved a wave would break when its height reached a seventh of its length.

He was the son of Devon miner who had emigrated to the gold mining area near Melbourne.

He showed such promise that he got a scholarship to Cambridge.

He was later elected a fellow of the Royal Society at the age of 35 – not bad for the son of a Devonshire miner.

His brother George was no slouch either – he invented and patented the thrust bearing that is named after him.

The half angle of entrance became the traditional factor for assessing the fineness of hulls.

It is defined as the angle the designed waterline makes with the centreline at the bow.It varies from less than 5º for very fine hull forms up to 60º or more for a full-form barge.

At higher speeds, modest increases in the half angle can give rise to substantial increases in wave resistance.

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Hullspeed and the Speed/Length Ratio

So what gives one boat better hullspeed than another? This question was pondered long and hard by William Froude (1810 to 1869), a British engineer who had a special fascination with the sea and ships.

Funded by the Admiralty, who were clearly very keen to get some answers to this question, he built a tank testing facility at Torquay, where he experimented with various model hull forms.

As an early expert in model analysis he was well acquainted with the 'law of mechanical similitude' , which demonstrates among other things that there are few linear relationships in hull design.

So just what is the answer?

Let's take a look...

Hullspeed and the Matchbox Analogy

Consider your hull as a matchbox - not wonderfully efficient hydrodynamically, but stick with it for a moment.

Dissatisfied with the constraints of matchbox living, you decide to double its size. You add another matchbox ahead to double its length, two alongside to double its beam and four on top to double its draft.

Now wetted area has increased by four, volume and displacement by eight and stability - as the product of its mass and acceleration - has increased sixteenfold.

So by doubling a hull's dimensions, wetted area is squared, displacement is cubed and stability increases by the power of four.

With this knowledge and that gained by carefully measuring applied force and resultant movement, Froude was able to both calculate and demonstrate that a relationship existed between hull speed and waterline length - that relationship being known and described in the metric world as 'Froude Numbers'.

The Speed/Length Ratio

However, most of us more accustomed to units of feet and knots are probably more familiar with the Froude Number's close relation - the Speed/Length Ratio.

S/L Ratio = hullspeed (in knots) divided by the square root of the waterline length (in feet)

This discovery enabled Froude to compare the performance of boats of different length. For example a 25ft sailboat moving at 5 knots would have the same S/L Ratio at a 100ft patrol boat steaming along at 10knots, and consequently both would develop the same resistance per ton of displacement at those speeds.

For Froude's models, having no rig above the waterline to create windage, this resistance was caused by two principal factors; hull drag and wave making resistance.

Maximum Hull Speed

Maximum hull speed (in knots) = 1.34 x the square root of the waterline length (in feet)

These figures relate to a boat in displacement mode. If sufficient power can be applied to overcome hull drag and enable the boat to plane, then other criteria will affect ultimate hullspeed.

Any Questions?

What is the theoretical hull speed of a non-planing boat?

The theoretical hull speed is the maximum speed that a non-planing boat can achieve in displacement mode, when the wavelength of its bow wave is equal to its waterline length. Beyond this speed, the boat will encounter increasing wave resistance and will need more power to overcome it.

What factors affect the theoretical hull speed of a boat?

The main factor that affects the theoretical hull speed of a boat is its waterline length, which determines the wavelength of its bow wave. The longer the waterline length, the higher the theoretical hull speed. Other factors that may influence the actual speed of a boat include its hull shape, displacement, draft, trim, sail area, wind and sea conditions, and propeller efficiency.

What is the difference between planing and non-planing boats?

Planing boats are boats that can lift themselves partially or fully out of the water and ride on top of their own bow wave, reducing their wetted surface area and drag. Planing boats can exceed their theoretical hull speed and reach higher speeds with less power. Non-planing boats are boats that remain fully submerged in the water and cannot climb over their own bow wave. Non-planing boats are limited by their theoretical hull speed and require more power to increase their speed.

What is the 'half angle of entrance' and how does it affect wave resistance?

The half angle of entrance is the angle between the waterline and the centerline of a boat at its bow. The smaller the half angle of entrance, the finer the bow shape and the lower the wave resistance. A fine bow can slice through water with minimal disturbance, while a blunt bow can generate large waves and drag. The half angle of entrance is one of the key factors that determines the wave-making resistance of a boat.

How can I increase the speed of my non-planing boat?

There are several ways to increase the speed of your non-planing boat, such as:

• Increasing your sail area or using more efficient sails;
• Reducing your displacement or weight;
• Optimizing your trim or balance;
• Improving your propeller efficiency or reducing your propeller drag;
• Choosing a finer or longer hull shape;
• Sailing in favorable wind and sea conditions.

What are some common misconceptions about hull speed?

Some common misconceptions about hull speed are: - Hull speed is a fixed limit that cannot be exceeded by non-planing boats. In reality, hull speed is a theoretical estimate that can be surpassed by some boats with sufficient power or sail area, but at the cost of increased wave resistance and drag.

• Hull speed is the same for all boats with the same waterline length. In reality, hull speed can vary depending on the hull shape, displacement, draft, and trim of the boat, as well as the wind and sea conditions;
• Hull speed is the optimal speed for non-planing boats. In reality, hull speed is often too high for non-planing boats to maintain efficiently or comfortably, especially in adverse conditions. A lower speed that minimizes wave-making resistance and maximizes fuel or power efficiency may be more desirable.

The above answers were drafted by sailboat-cruising.com using GPT-4 (OpenAI’s large-scale language-generation model) as a research assistant to develop source material; to the best of our knowledge,  we believe them to be accurate.

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The owner's guide to superyacht naval architecture part 1 - learning your lines

'Two years ago,' says Peter Bouma, a naval architect at Vripack in Holland, 'we had a Brazilian owner who wanted a displacement trawler. During our initial talks about the design, he put to us a requested design speed that was 2.5 knots above the design's hull speed. For us naval architects, this is a terrible speed to design to. Not only is it complex to get a good estimation for, it's also not an efficient speed from a fuel economy point of view. His reasoning was logical he wanted to sail in quite heavy seas, but his typical weekend trip was going to an island and back in a day. He wanted that particular speed to be able to have dinner at a certain time and still get back before nightfall. The answer was to install a big, expensive engine and to burn a lot more fuel on that specific trip a compromise that, once explained, he decided he was prepared to make.'

I am sitting in Vripack's offices in the Dutch town of Sneek to talk about the basic elements of naval architecture that every owner should know. With me are Bart Bouwhuis, director of design; Ierring Faber, manager of the naval architecture side; and Aleksandr Markov and Peter Bouma, both Vripack naval architects. Bouma's example of the Brazilian owner's request highlights the importance of understanding what you require from your new yacht, and therefore what compromises you might have to make. It's a key aspect of the concept design phase, and a key element to ensure your dreams can, in fact, become reality. And for all the styling up top, naval architecture is still king. 'For sure, you can create a body under any object,' says Bouwhuis, 'and it will float and have the right stability, but how will it behave in a seaway? Well, that's a different matter, so it all really depends on the operational profile.'

Design revolves around a golden triangle that must always be in balance speed, weight and power. Whenever you alter one of these factors, the others must also change to keep the triangle balanced. So, for example, if you add weight, you also have to add power, or lose speed; if you want more speed, you have to add power or reduce weight; and if you reduce power, you either have to reduce weight, or lose speed.

'There's no magical solution to break this triangle,' explains Bouma, 'and when a client comes with an idea that apparently does break the triangle, there's always an explanation perhaps a different material has been specified so the overall weight is less, for example.'

If you look at the three general hull forms available for yacht design, they all demonstrate the different solutions of this balancing act. A displacement hull has a limited top speed (more on that shortly), and within the speed range if you want a high volume, heavy displacement boat, you need to add more power. Similarly, if you want smaller engines for the same size yacht, you either accept that you will go slower, or will have to compromise on volume and weight lessening the displacement, in effect. Displacement, of course, is simply the weight of the water that is displaced by the yacht the hole in the water, if you will. We have Archimedes to thank for that discovery.

A semi-planing or planing hull requires a different balance of the triangle. In these cases, your power requirement goes up with the speed for a given weight, or you have to reduce weight in order to get the higher speeds. Naturally, it is never quite as simple as this, and there are some key factors that determine what can and can't be done with a hull design. It is these factors that show why you can't have a large, high-volume and heavy displacement steel yacht that planes at 60 knots, for example. If you've ever talked to naval architects about the design of your new project, it is likely you will have heard them bandy around arcane terms such as prismatic coefficients and Froude numbers. But what do these mean? Having a basic grasp of these concepts means that you will not only have some idea of what the naval architect is talking about, but will also help at the concept stage in realising why your 60-knot planing steel giant is just not possible.

Hull speed The defining characteristic between different hull forms, particularly between displacement hulls and semi-planing or planing hulls, all revolves around theoretical hull speed. Many of us will know the standard, simple formula for working out the theoretical hull speed in knots of any given yacht it's nothing more than the square root of the waterline length in feet multiplied by 1.34. So a yacht with a waterline length of 150 feet (45.14 metres) will have a theoretical hull speed of root 150 x 1.34, or 16.4 knots. But what does this mean, and why is there a boundary here?

'It's just physics,' says Faber. 'It's just how water behaves. It's the speed a wave propagates through the water, and that depends on its wavelength. At theoretical hull speed, the length of the wave created by the hull equals the waterline length (essentially, the hull is the same length as one peak and one trough of a wave). Above this theoretical hull speed you have to climb the hill of the wave created. The behaviour of the boat at this point is that it trims aft and gets sucked down effectively creating even more displacement and more drag. So it's a very steep part of resistance you encounter exactly at this point.'

If you want to see this effect in action, hold the end of a spoon between your thumb and forefinger and let it hang in a stream of water from a tap, so the water passes over the convex curve of the spoon. You will notice that the water curves around the spoon, but also that the spoon is drawn into the flow of the water. The same thing happens with the underwater body of a yacht it doesn't matter whether it's the water moving or the object.

This is why a standard displacement hull finds it impossible to climb over the wave, no matter how much power you try to pack in engine room. The only way to break through the wave the so-called 'hump' is not only to increase the power, but also to modify the hull shape, in effect designing a semi-displacement or a planing hull form, or one of the specialist hybrid hulls like the LDL or Fast Displacement Hull Form (also known as its acronym FDHF).

Froude for thought While theoretical hull speed is fairly common knowledge, the Froude number is not, yet is directly related and far more relevant in determining what sort of hull is required.

The Froude number (Fn) was invented by a 19th Century British naval architect called William Froude as a way of measuring and analysing ship resistance in towing tanks. Rather than rely on specific dimensions (such as a given length) to calculate, for example, hull speed, the Froude number is a coefficient a dimensionless number that can be applied to any size of vessel, so what applies to a small-scale towing tank model will also scale up to apply to a full size version. So essentially, while hull speed is just one number, the Froude number is a speed:length ratio. The formula for Fn is V divided by the square root of (g*L), where V is speed in m/s, g is gravity and L is waterline length in metres.

But why is it useful to know this? 'When you think about hull speed, the Froude number around 0.4 is a good value to have in your head,' says Markov. 'This is the point it becomes very power inefficient if you want to go faster. And at Froude numbers of 0.3 and 0.5 you have your humps. These are major numbers. For yacht design, Fn 0.3 is largely irrelevant it's the point at which the hull length equals two wavelengths but at Fn 0.5 hull length equals one wavelength, the big hump. For Fn between 0.5 and 1, you are looking at semi-displacement hulls, and at Fn 1 dynamic forces start lifting the hull. At Fn 3 you are fully planing. When an owner comes with a request for a boat and having a certain speed in mind, in practice the first thing a naval architect does is basically put that speed in relation to the length and that's what the Froude number is, a relationship between speed and length. That's what gives your basic hull form displacement, semi-planing or planing.'

Vive la resistance As mentioned above, one of the key factors determining speed is the increasing resistance the hull experiences around the hump that point at which the yacht is trying to climb up its own bow wave, around that Froude number of 0.5. But what is resistance?

'Resistance,' explains Bouwhuis, 'is the force required to pull the boat through the water nothing more, nothing less!' If you were to tow a yacht through the water and measured the weight on the towing line, this effectively gives you a force the amount of resistance the hull is encountering. This resistance is primarily broken down into two key types: wave resistance (largely a function of the weight of the vessel) and frictional resistance (related to the wetted surface of the vessel essentially, all the things you've stuck anti-fouling paint on). 'Wavemaking resistance is the resistance to generating waves,' Markov continues, 'so the more waves you generate the more your resistance is. You use your power to generate waves. It's essentially the time that the water needs to adjust to the speed of the object that is passing through it.' For semi-planing and planing hulls, wave resistance is a major characteristic to optimise. Frictional resistance, on the other hand, is somewhat more linear it just increases as you go faster. How do you reduce friction and increase speed? By reducing wetted surface, and one way to do that is to put less weight in the boat and we are back to that speed:weight:power triangle again.

There are other tricks too with planing hulls, all those steps, and hydrofoil shapes and catamaran and trimaran designs are all ways of reducing wetted surface, and therefore reducing frictional resistance. Of course, weight is disproportionate to length, as length is one-dimensional whereas weight is three-dimensional. This is why the larger your yacht, the harder it is to get it planing.

Reducing one type of resistance using a bulb on the bow, say, to reduce wavemaking resistance often impacts inversely on the other type of resistance. In the case of a bulb, you may achieve a 30 per cent reduction in wave resistance (which accounts for perhaps 60 per cent of overall resistance) but you add wetted surface area, which increases frictional resistance so your overall reduction may actually only be 10 per cent

Prism break Hull shapes are often talked about in terms of the prismatic coefficient (Cp), but what is this? In its most basic terms, it is a measure of the fullness of a hull. Think of a supertanker, with a box-like cross section and beam that is the same at the bow, midships and stern. This is a very full hull, compared to a J Class yacht, which has max beam amidships but almost no volume at all in the bow or stern. The prismatic coefficient is calculated by effectively cutting through the underwater part of the hull to find the cross section that has the largest surface area. This figure is then multiplied by the waterline length, and Cp is the ratio between the actual displacement volume and this figure. The coefficient is therefore always less than 1, and typically ranges from about 0.5 (the relatively slender J Class hull) to about 0.9 (for the extreme, very full tanker hull). In other words, it's the volume distribution through the length of the hull. Planing yachts (both sail and power) will tend to have more volume in the aft end due to higher beam, and therefore will have a higher Cp.

Buoyant feelings There are a couple of other factors that are important longitudinal centre of buoyancy (LCB) and longitudinal centre of gravity (LCG). LCB is the centre of gravity of the displaced volume of water the point where the upwards force of the water will push against the hull; LCG is the centre of gravity of the vessel on a fore/aft measure and represents the downward force of the boat. LCG should be directly above the LCB if you want to have no fore-aft trim. To imagine the impact that design has on LCB and LCG, consider an explorer-style vessel with the bulk of the superstructure located quite far forward, compared to a sleek speedboat with a long, slender nose and main superstructure aft. It is more critical on planing boats, as LCG is not just a measure of volume distribution but also has an impact on dynamic stability.

Beam on What about beam? Why isn't beam included in these basic calculations? 'We tend to look at length over beam,' says Faber, 'which has a lot of influence over resistance components discussed above. The influence of beam on resistance is more complicated and not that straightforward as it is with length. Ideally you want to have a slender boat for Froude numbers in that critical area between 0.3 and 0.5, but once you get above those numbers closer to the planing mode you would like to have some beam and waterplane area to generate lifting force in the aft ship.' Beam is usually regarded as a high resistance component. For wavemaking resistance, beam is a negative factor and the wavemaking resistance is predominantly an issue in displacement and semi-displacement hull forms hence why a slender hull can be more efficient in that speed range.

But beam has quite a different influence in planing mode, where it is very much more related to the mechanics of how a planing hull operates. It helps provide dynamic lift, and you need a certain beam to carry a certain weight, plus it's good for resistance up to a point beyond that, it becomes negative for resistance, so there is an optimum beam. If you extend the beam further, you create a lot more wetted surface and therefore more resistance. Too little beam, on the other hand, means the bottom load is too high because there is no support anymore, and with it a lot more trim, which means wetted surface and resistance

Conclusion While the three basic hull types form the platform from which all yachts are built, there are of course tricks to optimise for certain speed requirements or operating conditions, and specialised hull forms that offer particular advantages for particular purposes, usually as highly optimised versions of one of the three basic hull types. We will look at these aspects in more detail when we consider the science of hull optimisation and what compromises this brings.

Stay tuned for owner's guide to superyacht naval architecture part 2, where we will look at one of the critical elements of that design triangle weight and the importance not only of controlling the overall weight of a project, but also in managing its distribution throughout the hull.

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Hull speed is a phenomenon of displacement boats, and not of planing boats. Most sailing boats and all ships displace water—move it aside—as they plow through it. Planing craft, such as most motor boats, glide over the top like a surfboard. It takes more energy to push water aside than it does to slide over the top of it, and so displacement boats move at a more sedate pace than their lighter planing cousins. Some small sailing boats can be made to plane, but the general rule is that sailing boats are of the displacement type. Hull speed is usually an upper limit to the speed of displacement boats.* It is unsurprising that such a limit exists: we have seen how drag increases with speed, and so sooner or later drag will balance out the drive force and a sailboat will not be able to go faster. Yet there is a surprise in store for those of you who are not familiar with sailing: the hull speed of a given boat depends on its hull length at the waterline. It is not obvious from a simple consideration of drag why this should be so, but it is a well-attested fact, often quoted in the sailing literature, that the maximum natural speed of a displacement boat (in knots) is 4/3 the square root of waterline length in feet.

A key feature of the phenomenon, again well known to any sailor, is that hull speed has been reached when the bow wave of the boat lengthens to the waterline length. At lower speeds, there may be three or four complete waves seen to lap along the boat hull, but this number decreases as the boat picks up speed and reaches, pretty closely, one complete wave by the time the boat reaches her hull speed. It may be possible for her to go faster than hull speed, but this requires a disproportionate amount of effort. In other words, the hydrodynamic drag

* There is one trick by which a small displacement boat can exceed hull speed without expending enormous effort, and that is by surfing. Riding along the front of a wave is not the sole preserve of surfboards.

Figure 6.1. (a) Your hull-speed raft, viewed from above. Note the direction of motion. (b) When the bow wavelength is less than the distance between the long beams, drag is reduced compared to the case of (c). In (c) bow wavelength equals the distance between beams because the aft beam is more submerged. So hull speed is reached when hull length equals bow wavelength. Consequently, hull speed is limited by hull length.

force that is acting to hold back the boat increases rapidly once hull speed is reached. My goal in this section is to explain to you, in simple physics terms, why these phenomena occur.

Which is why I have press-ganged you into service onboard the undignified vessel illustrated in figure 6.1. She is a wooden raft with two long logs fore and aft that stretch way beyond her beam. These logs are not there to provide flotation, please note—we will suppose that the raft has enough buoyancy without them—but rather to illustrate hull speed. You set the primitive sail and drift off to the right. The forward log generates a bow wave which spreads out in the wake, as waves do. You notice something that you have seen many times before in other craft: the bow wave size (amplitude) increases as the vessel speed increases. This makes sense because the hull is pushing water aside, the displaced water has to go somewhere, and the faster you go, the more water is moved. So the wave size increases. Now you pick up speed, and so the wavelength of the wake, as observed alongside your hull, stretches out until exactly one wave lies between the two extended logs at bow and stern. The raft speed that gives rise to this condition is her top speed, you

Figure 6.2. Your hull-speed barge. Bow waves forward of the center of gravity, CG (open circle) exert a buoyancy force (vertical arrows) proportional to wave height that acts to rotate the barge hull counterclockwise. Similarly, waves aft of the CG act to rotate the hull clockwise. If we can assume that drag forces are proportional to counterclockwise torque (a dominant CCW torque means that the barge is climbing a hill created by its bow wave), we can show that hull speed occurs when bow wavelength equals hull length.

find. It is clear why: the aft log is now submerged, and so experiences more drag than it did earlier, when there was no wave crest at the hull stern (see fig. 6.1). So, drag force peaks when bow wavelength equals hull length, in this simple example.

Now we are able to see where the old formula for hull speed comes from. The speed of a bow wave, or of any other surface water wave,1 is c where c2 = gk/2p. Here l is the water wavelength, and g is the constant acceleration due to gravity. Now the raft speed, v, equals the water wave speed, c, so that v = VgL/2p (since hull length, L, equals water wavelength at hull speed, as we just saw). Substitute numbers and we arrive at the old formula.

The ungainly raft has served her purpose, and you can now abandon her. The lesson learned is intuitive, and yet it gives us a basis for understanding quantitatively what hull speed is about. Now I can do another calculation, this time a little more realistic. The math is more involved (you need not wade though it), but the basic idea is again quite intuitive. Figure 6.2 shows the profile of a steep-sided hull plowing through water and generating a bow wave, which oscillates along the line of the hull. This vessel is kept afloat by the buoyancy force, and we can see that the buoyancy force is going to be different at different points along the line of the hull because the wave height varies along the hull. Buoyancy that acts forward of the hull CG (shown in fig. 6.2) will create a counterclockwise torque that tends to twist the hull about the CG—trying to make it do a backflip. The buoyancy force aft of the CG produces a torque that acts in the clockwise sense. These two more or less cancel* but not quite. If the counterclockwise buoyancy torque is just a little bigger than the clockwise torque, the boat will tilt backwards, until her stern goes deep enough to generate a compensating torque. We would then be left with a boat that is going uphill, trying to reach the crest of her own bow wave.

Where am I going with all this? Roughly speaking, counterclockwise torque equates to uphill motion, and uphill motion leads to increased drag, for reasons that will soon be made clear. So, I am saying that increasing the unbalanced counterclockwise torque generated by a bow wave will increase drag. If this increase should suddenly take off at a certain speed, then we have found our hull speed. In fact, I can calculate the torque generated by the bow wave. You can see that as the bow wavelength changes, the torque will also change because the manner in which buoyancy force is distributed along the hull length changes with wavelength (fig. 6.2). The results of this calculation are plotted in figure 6.3. (For those interested, the math is provided in this endnote 2 in sufficient detail for you to reproduce the calculation.2) In figure 6.3 we see once again that drag force takes off for water wavelengths exceeding hull length, more or less.3

For simplicity, the hull of figure 6.2 was given vertical sides, but most boats don't have vertical sides, for a host of reasons. Recall that, in the Age of Sail, ships of the line were given a tumblehome cross section to deter boarders. Nowadays we are less likely to have to repel nefarious enemies swarming over our gunwales with cutlass in hand, casting a single bloodshot eye (the other being patched) in search of our gold doubloons. Hull sides are angled but the other way, with cross sections resembling a martini glass rather than a brandy glass. In plain language: more V-shaped. Here are some physics reasons for different hull cross sections.

——'Rounded hull bottoms are stronger than V-shaped hulls, but the latter will be deeper for the same displacement and so will better resist leeway.

*Just as well, because backflipping boats would be pretty uncomfortable.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Water wavelength / L

Figure 6.3. Hull speed is limited by drag. In the simple model described in the text, the drag increases with water wavelength, l, as shown (L is hull waterline length). Here, drag force is set arbitrarily to 1 at zero speed. If the bow wave is assumed to have constant amplitude, independent of speed, then drag changes with speed as shown. For a more realistic model, with bow wave amplitude increasing with speed, the curve looks similar. In this simple model, hull speed occurs at l « 1.2L because for longer waves (higher boat speed) the drag force becomes too strong.

•—A large deck area is desirable, but large hydrodynamic drag is not. For a hull of a given displacement, the choice of hull shape is constrained by the trade-off between these two characteristics. •—'An angled hull—say one that is V-shaped—will have greater reserve buoyancy. That is, the righting moment will increase as the hull heels further and further. •—'During heeling, the waterline along an angled hull will not be symmetric about the longitudinal axis; the port side waterline length and shape will be different from that on the starboard side. This asymmetry can assist the boat to head up while heeling. Thus, even without aerodynamic assistance from her sails, a boat may automatically

point to windward when heeling solely because of hydrodynamic forces acting on the hull. •—'Different angled hull shapes beneath the waterline assist with planing. For certain boats, such as racers, this is important because planing requires less displacement, less wetted area, and so less drag—and hence increased speed.

The physics of angled hull shapes casts an interesting light on the capabilities of some ancient ships . Certain ancient ships were built with a lot of overhang at the bow and stern, but this practice is usually thought to have been of little value for the old square-riggers because these ships were supposed to be nippy only when running or on a broad reach. Today, such hull shapes are utilized to increase hull speed while heeling because the waterline length is increased when the hull is heeled over. This lengthened waterline increases boat speed on a beam reach, for example. It seems plausible to suppose that ancient vessels with overlapping bows and sterns may have been capable of traveling across the wind at speed. Indeed, such a hull design offers no other advantage for these square-rigged vessels. (An overhanging bow and stern increases deck area, but for merchantmen—and in ancient times most of the sailing ships were merchant vessels because warships were oar-powered—deck area was not such a big deal. Volume of the hold was what mattered.) For a downwind point of sail, extended hull length above the waterline will increase pitching motion when traveling downwind; this is bad, and yet the overhanging bow and stern must have conferred some advantage or these ancient ships would not have been built this way.

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Readers' Questions

Is it possible for a displacement boat to exceed hull speed?

No, it is not possible for a displacement boat to exceed its hull speed. Hull speed is the theoretical maximum speed that a displacement boat can reach, and it is determined by the length of the waterline. When a boat exceeds its hull speed, it starts to climb up on its own bow wave and create excessive drag, making it difficult to go any faster.

What can you say about the speed of a boat that makes a bow wave?

The speed of a boat that makes a bow wave is usually quite fast, as the bow wave is usually associated with a boat moving at high speeds.

How to calculate hull speed?

Hull speed, also known as displacement speed, is the speed at which a boat hull moves through the water. It is calculated by taking the square root of the waterline length of the boat in feet and dividing it by 1.34. The formula is: Hull Speed = √LWL / 1.34 where LWL = waterline length in feet.

What is maximum hull speed for a boat?

The maximum hull speed for a boat is typically 1.34 times the square root of the waterline length of the boat in feet. For example, the maximum hull speed for a boat with a waterline length of 20 feet would be about 24 knots (1.34 x √20).

Why catamarans sail faster than hull speed?

Catamarans sail faster than hull speed because of their unique hull design. Their twin hulls provide greater stability and lift than a single hull, which results in less drag on the boat. This reduced resistance allows the boat to move more quickly through the water, resulting in higher speeds than what is normally achieved with a traditional hull design. Additionally, the width of the catamarans hulls also distributes the weight of the boat more evenly, which further reduces drag and increases speed.

Ben Shank December 12, 2007

(submitted as coursework for physics 210, stanford university, fall 2007).

If you've spent any time near a commercial port, you have no doubt watched barges carrying cargo back and forth and seen the great mound of water that builds up at their head when they are under way. Although their massive wake helps keep the local population of JetSkiers in check, it comes at considerable expense. All of the energy transferred to those waves must be replaced by the engines to maintain a constant speed. In essence, the barge must push the water in its path aside to make headway. The inefficiency with which flat-nosed barges do this makes their wake particularly noticeable, but all ships that float by displacing water must displace different water in order to move forward. From the bow and the stern of every moving displacement hull comes a series of waves called a wake that carries away the energy of displacement. These waves travel at a speed v = (Lg/2π) 1/2 , where L is the wavelength and g is the local acceleration due to gravity. At sea level, where most ships travel, this works out to v = 1.34 L 1/2 when L is measured in feet and v is given in knots. [2] (A knot is one nautical mile, about 6080 feet, per hour.) Because a wake arises at the bow and the stern of a ship, its wavelength can be approximated as the length of the ship at the waterline. When the ship is travelling more slowly than its wake, water is simply displaced and the associated energy travels away from the sides without further interaction. However as the ship approaches this critical speed, it will build up a barrier of water in front of it and a trough behind it because the water simply cannot get away fast enough. To go any faster the ship will have to push uphill, requiring considerably more energy. This will make the barrier of water yet higher, making the next increment in speed even more costly. Because the critical speed v depends only on the length of the hull it is referred to as the ship's "hull speed." The value 1.34 knots/ft 1/2 in the equation given above is often called the speed to length ratio for a hull despite the fact that it is not strictly a speed divided by a length.

Obviously this analysis is oversimplified. Naval architects and professional shipwrights perform much more sophisticated analyses of their craft to account for interaction with ocean waves and wind, the precise shape of the hull, the modeled shape of the wake, and other factors. However the concept of a speed to length ratio is so useful and fast that many professionals work out a value for a specific hull shape and then refer to it as they scale the model up or down in size. Values range from 1.18 (in nautical units) for barges to 1.42 for very long, sleek vessels. Most amateurs use 1.34 as a good approximation for most common hull shapes.

Hull speed is sometimes treated as the highest speed a ship can attain. This is not strictly the case. It simply measures a critical speed at which the ship catches up to its own wake. Typically the energy required to speed up a displacement hull then becomes exponential in speed rather than quadratic. If the engine was already working hard to reach this speed, chances are it will not get much faster. However several options exist to beat the rising mound of water and press on to greater speed. The most common, particularly for smaller craft, is to climb up over the barrier. If the hull can be shaped in such a way as to generate lift, the boat is no longer displacing water equal to its weight and therefore experiences less displacement-related drag. This process is referred to as hydroplaning, or simply planing. Motorboats on plane have their noses raised high out of the water as they climb their own bow wake. Specialized racing boats almost seem to leap from their own wakes as they skitter across the surface of the water. Planing is useful mostly for smaller boats which almost always want to travel faster than their sluggish hull speeds and often have plenty of power to spare when they get there. Somewhat larger vessels can gain some of the benefits of planing without the inherent loss of stability by travelling in a semi-displacement mode. By receiving some lift, but not enough to balance their weight, these craft significantly reduce the hull speed barrier without removing it entirely.

Another tactic for breaking the hull speed barrier is to simply cut through one's own bow wake. [2] This method is popular with modern navies. Fast-attack warships cannot afford the instability of planing or even semi-displacement travel, but they would hardly live up to their name if they could not overtake larger vessels with greater hull speeds. These ships are given specially designed wave-piercing prows and gigantic engines. They do as much as they can to reduce wake-drag and spend most of their time patrolling well below their hull speed. However when the need arises, these ships solve the hull speed problem by throwing more power at it. As might be expected, this strategy does not result in substantial speed increases, often only allowing a two to three hundred foot destroyer to actually reach its hull speed. [3] In this limited sense, hull speed does seem to place an effective maximum on the speed of a single displacement hull.

Although almost all fighting ships today are equipped with wave-piercing bows, true fast attack is increasingly accomplished by submarines. Subs ignore the wake problem by going under it. This highlights an important aspect of wave-induced drag. It is not just the displacement of water that limits the speed of a ship so sharply. By getting away from the complex propagation of wave energy at the air-water interface, submarines have no hull speed despite the fact that they displace more water submerged than when surfaced. Because the enhanced speed of a submarine is one of its key strategic advantages, it is difficult to clearly demonstrate. All of the submarine classes identified by the US Navy [4] are listed as having a top speed of "25+ knots submerged." The smallest of these, the Cold War era Sturgeon class, has a length of 292 feet, which would give it a hull speed of 22.9 knots. The modern 350-foot Seawolf has a hull speed of 25.1 knots, but we can be certain that it, as well as the Sturgeon, are capable of far greater speeds than 25 knots when submerged.

Displacement is not the only drag force on a body moving through water. Surface tension, the tendency of water to stick to any object immersed in it, creates an effect called 'skin drag.' Long hulls which present a sufficiently small cross-section to oncoming water have their speeds dominated by skin drag. This category includes rowing shells and racing kayaks [5] as well as multi-hulled vessels such as catamarans. The wavelengths of the wakes of these vessels are not simply the length of the hull and they are often free to travel much faster as a result. Super-narrow mono-hulls show up almost exclusively in human-powered races, but multi-hulls can take more varied forms. The US Navy is experimenting with catamaran hulls as a more flexible alternative to submarines for fast response in shallow waters. The latest venture, named Swift, is 321 feet long, giving it a hull speed of 24 knots, but has a top speed twice that fast. Over the years multi-hulled designs have been used for high speed ferries, exceptionally stable fishing boats and, of course, racing.

The concept of hull speed was developed as a practical rule-of-thumb by mariners in the days of wind and steam to describe a phenomenon that arises from a basic application of the physics of waves and, for them, placed a strict limit on the speeds they could attain. But a ship's hull speed is not some unattainable velocity like the speed of light, which cannot be accessed by any imaginable application of power. Instead it is a practical limit for engineers, a place in the physics of surface vessels where the nature of the forces involved changes dramatically. Certainly fantastic power can be expended to little avail near the hull speed of a traditional hull, but creative engineers have found ways around, over or under the walls of water that bar their way.

© 2007 Benjamin Shank. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

[1] H. Y. H. Yeh, "Series 64 Resistance Experiment on High-Speed Displacement Forms," Marine Technology 2 , 248 1965.

[2] Inc. U.S. Coast Guard Auxiliary Association, Sailing Skills & Seamanship (U.S. Coast Guard Auxilary Assn., 1978).

[3] http://www.milnet.com/pentagon/usnship.htm

[4] http://www.milnet.com/pentagon/subclass.htm

[5] J. Winters, "Speaking Good Boat, Part II (Kayak Hull Speed and Beyond)," http://www.qcckayaks.com/resources/speakboat2.asp .

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The displacement hull, comfort and autonomy

The displacement hull allows sailing at reduced speeds by reducing fuel consumption. But how does it work? What are its characteristics?

Boats limited by their hull speed

The displacement hull is designed for moderate speeds, i.e. it does not lift off and simply "pushes the water forward". This hull shape does not promote speed , but ensures good flexibility in waves and great stability. The displacement hull allows sailing in all seas, even in heavy weather , because it does not hit the waves . The stability of course is generally ensured by a slightly V-shaped hull bottom equipped with a long and deep keel .

Since it does not lift off, the speed of a displacement boat is limited to its hull speed . This means that the boat cannot exceed a theoretical speed - the critical or limit speed - even with a larger engine. Its calculation depends only on the waterline length. The larger the boat , the higher the speed it can reach.

Hulls used in pleasure craft and workboats

Today, these hulls are mainly found on cargo ships or on professional boats, working or fishing. They are also used on inland waterways, where speed is not an issue. By sailing at reasonable speeds, consumption is also reduced.

In yachting , these hulls can be found on many old sailing boats and today on boats that seek maximum autonomy and want to limit consumption. This is the case of trawlers or electric propulsion .

Naval architect Pierre Delion summarizes: "A ith energy savings, the displacement hull is not about to disappear! Displacement hulls are used for boats that do not exceed their hull speed. If you're looking for a boat with low speed and low fuel consumption, they're ideal. They're mostly used for low-speed boats that aren't going to exceed 10 knots."

Advantages of the displacement hull

• passage to the sea
• requires low engine power

Disadvantages of the displacement hull

• speed limited by the length of the hull
• more roll in general, due to a more limited width

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Hull Speed Calculator and Waterline Length

Waterline length is the length of the boat from bow to stern where it sits in the water. In other words, as it floats in the water, if you were to mark the point on the bow where the water touched and marked the point on the stern where the water touched and then measured that distance this would be the waterline length.

By the way, if you were to connect the marks you made at the bow and stern and paint a line on the boat’s hull you would have created what is called a “boot stripe”. The boot stripe is the line painted on many boats which separates the bottom which is underwater from the sides that are above the water.

Yes, the length of the water line does affect the speed of some boats. In particular displacement type boats. These are boats that have a large underwater profile such as sailboats and trawlers. Since these types of boats are not able to get up on top of the water and plane they are pushing a tremendous amount of water.

How Does the Bow Wave Affect Hull Speed?

If you were to watch a displacement vessel move through the water you would notice that they create both a bow wave and a stern wave as they push through the water . The vessel’s bow wave wavelength increases as the boat’s speed increases. The faster the boat goes, the larger these two waves become until at some point they become a single wave. It is at this point that the boat has reached its “hull speed”. That means that this is as fast as it can go. It can’t go faster because it is caught in this wave. The longer the boat, the faster it can “theoretically” go because it takes longer for the bow and stern wave to become one wave. You can calculate the hull speeds or displacement speed of a displacement vessel with the following formula.

The Hull Speed Formula

Hull speed = 1.34 X (Square root of waterline length)

For example, if your displacement sailboat was 36 feet long the hull speed would be calculated as follows:

square root of 36 = 6

6 X 1.34 = 8.04 kts. hull speed

What Difference Does Hull Speed Make?

You can actually learn a lot about how a vessel travels when you understand hull speed. At hull speed, those bow and stern waves are synchronized and that’s important. Something called constructive interference happens and that means the boat is travelling as efficiently as it possibly can.

There’s nothing stopping you from exceeding hull speed. It’s not like the speed of your car which is determined by the engine and other performance factors. A sail boat or a boat under power could very reasonably exceed hull speed. However, it’s not going to perform well when it does so. Hull speed is basically your boat’s sweet spot, the speed at which it does the best it can. The speedometer won’t show you this.

If you exceed maximum hull speed, you’re going to see problems arise. You’ve likely seen it before if not in your boat than in someone else’s. The boat begins to plane and the bow will rise out of the water. That indicates the boat speed is simply too fast.

Once your boat starts planing , it’s going to have to struggle to keep that pace. Whether it’s powered by a motor or by sails, much of the energy going into thrust will now be wasted. So you’ll be pushing harder and achieving less. This can stress the engine and waste fuel. It also makes it more difficult to manage the sails on a boat under wind power.

Because of this, hull speed is general the optimal speed for any boat and you don’t want to exceed it if it can be avoided. You’ll save time, money and energy if you know your hull speed and are able to stick to it as much as possible.

On a related topic, you may wonder just how fast a boat can go. Are there speed limits? As mentioned above, displacement type boats are limited to how fast they can go.

Is There Another Hull Besides Displacement Hull Vessels?

Another type of boat is the planing vessel. Planing vessels are designed to actually rise up and ride on top of the water when power is applied. These vessels have flatter bottoms than displacement hulls.

They require considerably more horsepower to get the boat up on top of the water, but they can attain much higher speeds because of the reduced friction of moving on top of the water rather than through the water.

Some planing vessels are used for professional boat racing and can reach speeds over 100 miles per hour. Recreational planing vessels may reach speeds of 25-40 miles per hour.

On the water you will also find speed limits just as you do on land. You should be on the lookout for speed limits and abide by them. Also, just like in a car, you should never operate at a speed that is unsafe for the current conditions. You should constantly be on the lookout for other traffic, visibility, wave action around you, and other elements which may require a reduced speed.

No Wake Zones

Not every body of water is going to have a speed limit posted on a buoy for you to see. Some places are listed simply as no wake zones, for instance. In order to ensure no wake you’d need to be at a fairly slow pace, around 5 miles per hour, probably. This is a speed used around swimming areas, bridges, piers, docks and so on. These are places where excess speed would be a serious safety concern. This doesn’t give you a hard and fast number, but it does indicate you need to watch your speed. If your vessel is going fast enough to produce a disruptive wake, then you are violating the rules.

The Bottom Line

When you can calculate hull speed, it can be a lot of help. You’ll be better able to understand how your boat should be performing on the water and can optimize your speed as a result. Anything that can make your boat run more efficiently while taking stress of the engine and lowering fuel costs can’t be a bad idea, right?

About Chris

Outdoors, I’m in my element, especially in the water. I know the importance of being geared up for anything. I do the deep digital dive, researching gear, boats and knowhow and love keeping my readership at the helm of their passions.

Categories : Tools and Calculators

Keith Rogers on June 14, 2022

If a semi-displacement yacht has a waterline length of 183 feet, what would be LOA of this yacht?

AzlDraken on December 13, 2022

approx 18knts

Knapweed on July 7, 2023

LOA is Length Overall not hull speed. The answer to your question is it will probably be longer than 183 feet but by how much, nobody could know from the information you provided.

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Planing, displacement and semi-displacement yachts. Explanation without formulas

•   Nautical Expert
• Yacht Buyer's Guide

Updated on September 9th, 2023

The pebble that you threw into the lake or into the sea as a child, making it jump on the water surface (stone skipping), is nothing more than a planing mode. Of course, you know that in order for the stone to do this, it must be fairly flat, have a high flight speed and an optimal angle of attack, about 20 degrees.

The flat surface of the stone, which is in contact with the water surface, is similar to the bottom of a motor boat with low deadrise, which is a prerequisite for planing. It is obvious that an absolutely flat boat bottom, with zero deadrise, has less drag, more lift and, therefore, the highest coefficient for the planing mode.

Planing yachts

However, moving on a completely flat-bottomed boat will shake your brain quite well even with weak waves, and besides, it is fraught with the destruction of the hull due to strong shock loads. The seaworthiness and handling of such a boat, of course, will be disgusting. But the engine power to switch to planing mode will require a minimum.

On large water bodies, where at least small waves are present most of the time, you will have to choose a compromise: a boat with variable deadrise. This is a bottom with a V-shaped bow, smoothly turning into a flatter surface at the transom. Such a hull can significantly increase seaworthiness and reduce shock loads when passing through a wave, while maintaining the possibility of planing. At the same time, the aft part of the bottom cannot be made completely flat, as this will greatly increase the yaw.

Planing is a mode when the hull of the boat stops floating due to only the Archimedes force of buoyancy. When the required speed is reached, the hull begins to support the oncoming flow of water and, to some extent, air. That is, it is supported by hydrodynamic forces, and the value of the Archimedes force in this case is significantly reduced.

Remember stone. When he jumps over the surface of the water, the buoyancy force of Archimedes is extremely small. In a static state, the pebble cannot float on the surface of the water. The planing effect helps pilots make an emergency landing on the water.

Planing yacht hulls often have transverse redans – ledges on the bottom to reduce the wetted area and cut off excess water, as well as stern plates – to stabilize the boat and reduce excessive trim.

If you are looking to buy a new outboard motor for your tender or RIB but are unsure of the minimum power required for planing, there is a simple calculation. For every 25 kg of boat weight, 1 hp is required. Please note that this refers to the total weight of the boat, including the motor, skipper and beer. At the same time, if the tender has a pronounced deadrise, then it is necessary to reduce the estimated weight to 20-22 kg.

In other words, to achieve the planing effect, it is necessary that the boat has a sufficiently high power-to-weight ratio – at least 30 hp per ton of gross weight of the boat and a special hull shape.

Of course, do not forget that a lot depends on the parameters of the propellers, which is a separate large and interesting topic.

Displacement yachts

Deadrise has its limit, after which the hull of a motor yacht ceases to be planing. A yacht with displacement lines has a much greater deadrise throughout the entire bottom, and the chines in the bow are smoother. The absence of flat surfaces does not allow such a boat to enter the planing mode. Therefore, such a hull has to push the water mass in front of it, and not fly over it. And, although a displacement yacht has a low speed limited by the ratio of its length at the waterline to the width, seaworthiness, efficiency, safety and cruising range increase incredibly.

The speed limit is related to the wave formation process. Surprisingly, a megayacht and a small displacement fishing boat, having similar proportions and hull contours, when moving at the same speed, form the same wavelength. As the speed increases, the wavelength also increases. Given the length of the hull of a megayacht, one can imagine how many such waves can be along it. That practically does not affect the speed and power consumption. But the size of a fishing boat at some point may turn out to be less than the wavelength that it itself forms, and it turns out that the boat is between two waves, at their soles.

Increasing the speed will not help in this case. This will only lead to an increase in the height of the bow wave and a sharp increase in fuel consumption. A boat of the same length, but less width, could go faster, since its hull would form transverse waves of a lower height.

In addition to increasing the length-to-width ratio, one way to reduce this wave drag on a displacement hull is with a bow bulb located below the waterline. When the yacht moves, the bulb creates its own additional wave, which partially dampens the wave created by the hull.

At the same time, if the boat had planing bottom contours and sufficient engine power, it could pass over the crest of the bow wave. So the boat would switch from displacement mode to planing mode.

To overcome the bow wave somewhat more power is required than to maintain the planing mode. Therefore, the transient mode consumes much more fuel. But after overcoming it, excess gas should be dumped and switched to cruising mode.

A catamaran, from the point of view of hydrodynamics, should be considered not as a single vessel, but as two separate ones. Each hull is displacement. But you will notice that these hulls have a very high length to width ratio, which allows them to move with minimal wave formation and drag. This is what determines the high speed and efficiency of multihulls.

Semi-displacement (semi-planing) yachts

Today this is a very popular solution for motor yachts, but it is not suitable for all hull sizes. Trying to get the best of both worlds, as always, is full of compromises, but meets the basic requirements of buyers. These boats feel good in rough seas, and in good weather they are able to reach high speed.

At its core, to get this kind of hull, you simply take the bow of a displacement boat and connect it to the stern of a planing boat. Smooth chines and deep deadrise, turning into an almost horizontal plane of the stern, provide a wide range of speeds. That being said, you are guaranteed a good autonomy if you stay in displacement mode.

Important parameters

Speed is fun, but let’s look at what interests us much more, namely range and economy.

The cruising range, of course, strongly depends not only on the technical features, but also on the size of the yacht. However, planing boats with a range of only 250-500 nautical miles clearly do not claim to be the most autonomous. Semi-displacement yachts can achieve much better cruising ranges of up to 1000 NM or more if you don’t push the throttle too hard. Displacement yachts will leave everyone far behind if there are no filling stations along the way, because they are able to remain autonomous at incredible distances from 2500 to 8000 NM and more.

Average power consumption for planing yachts is 60 hp/t or more. Semiplaning boats have an average of 10-40 hp/t. And here, displacement yachts win everyone not only with incredible seaworthiness, but also with an average power consumption of 5 hp/t. It becomes clear how they manage to achieve a range of several thousand nautical miles, and why we love trawlers so much.

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Vessel Parameters

In the context of boats, displacement refers to the weight of the volume of water that the hull displaces when it is placed in the water. To calculate displacement, you should consider the boat’s fully loaded weight, which includes:

• Full crew and passengers
• All normal ship’s stores and gear
• Two-thirds of the fuel and water tanks’ capacity
• Two-thirds of the cargo being carried

A vessel’s length at the waterline is the length of a boat at the level where it sits in the water (the waterline). The waterline length will be shorter than the length of the boat overall.

Hull speed refers to the maximum speed at which a displacement hull can travel efficiently through the water. As a boat approaches its hull speed, the resistance from the water increases exponentially, requiring a disproportionate amount of power to achieve higher speeds. Therefore, it is generally advised to operate a displacement boat within or below its hull speed for optimal efficiency and performance.

Additional Notes

• The provided calculator is designed specifically for displacement boats and may not be suitable or applicable to other types of boats or hull designs
• While the calculator has proven to be valuable to many users, it is important to recognise that it proves only an approximation
• Users often find that operating their boats at maximum motor output is rarely necessary. This understanding effects the calculations and estimation related to the duration of their boat’s performance
• The equations used in this calculator were developed based on the research and expertise of Dave Gerr, as presented in his book “Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers.” We have further refined the equations based on real-life examples.

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Calculating power requirements for full displacement hull

Discussion in ' Boat Design ' started by Annode , Sep 2, 2019 .

Annode Previous Member

It seems to take very little power to move even large and heavy full displacement hulls up until about 6-8kts. Then the power curve vs speed seems to go up exponentially after that, and nearly vertical at about 13kts. Assuming that 8kts is sufficient, both in terms of efficiency and speed, what is the nominal (flat water) calculation for power vs weight. After that is known, what is the calculation for additional power for manoevering a heavy steel hull with a lot of inertia, and for wind, sea. Since its not trawling, dont need monster trawler engine, but something extra I imagine. How is this calculation typically made?  

DCockey Senior Member

Annode said: ↑ It seems to take very little power to move even large and heavy full displacement hulls up until about 6-8kts. Then the power curve vs speed seems to go up exponentially after that, and nearly vertical at about 13kts. Assuming that 8kts is sufficient, both in terms of efficiency and speed, what is the nominal (flat water) calculation for power vs weight. Click to expand...

to calculate engine power... which for some strange reason is measured in horsepower in boat engines - that only rev to 1500 or 2000 rpm thus disguising the the torque that the engine generates... a more relevant number since a turbo diesel 500hp can generate 2000 ft/lbs of torque  

Annode said: ↑ to calculate engine power... which for some strange reason is measure in horsepower in boat engines that only rev to 15 or 200rpm thus disguising the the torque that the engine generates... a more relevant number since turbo diesel 500hp can mean 2000 ft/lbs of torque Click to expand...

JSL Senior Member

how about we get all the facts* about this installation so contributors to the solution find it a bit easier. (* wl length, displacement, power, etc etc. Power, speed, & * etc. also governs propeller size.  

Mr Efficiency Senior Member

For practical purposes, rather than strictly theoretical ones, it is the wave making property of the typical displacement hull that causes the rapid rise in resistance, and is related to boat length, the wave system set up that is parallel to the direction of travel, propagates predictably according to speed, in that a given speed gives a given wave length, it is when the crest of the second wave falls behind the stern, that the resistance goes off the chart. Waves on the ocean behave according to similar rules, long wave length equates to speed, short wave lengths slow. Simply, physics.  

Ad Hoc Naval Architect

As already noted by JSL, there are far too many variables to consider before attempting to provide any kind of reply. Annode said: ↑ to calculate engine power... which for some strange reason is measured in horsepower in boat engines - that only rev to 1500 or 2000 rpm thus disguising the the torque that the engine generates... a more relevant number since a turbo diesel 500hp can generate 2000 ft/lbs of torque Click to expand...

> boats usually have "reduction gears" between the engine and propeller which reduce rotational speed while increase the torque. yeeeees ... just talking about engine right now. 800 - 2000 rpm for larger boats is normal I am told (with a 3:1 reduction in the gearbox) Specifics... no hab This is a THEORETICAL rough approximation discussion. I am going backwards from the power requirements for reasons that are not relevant right now. answers that are basically "why is that the question?" are not helpful. It is the question. so.. back to the topic...  

Chuck Losness Senior Member

Dave Gerr's "The Nature of Boats" discusses power requirements. It might be too basic and not give you what you are looking for. It would be a place to get started. I am sure that this topic has been discussed in the past. Try doing a search for calculating the power for displacement boats.  

fredrosse USACE Steam

A rough estimate of horsepower for displacement type hulls is fairly simple: 1. Maximum "hull speed", in knots, is equal to about 1.3 X the square root of the waterline length of the hull in feet. For example, say you have a displacement hull with a waterline length of 25 feet, then the hull speed is 5 x 1.3 = 7 knots. Trying to propel the boat faster than this will result in enormous increasing power requirements. 2. For displacement hulls of reasonable shape (not a square box, which would require more power, and not a rowing shell used for competition, using less power), about 1 horsepower per long ton of displacement will get the boat to hull speed in calm water. This value is based on a propeller of good efficiency, generally a large prop at relatively low RPM. Some additional margin is usually prudent to cope with wind and waves, or non-optimum propeller conditions, but anything over about 2 horsepower per ton of displacement is not required. 3. Much more detail is provided in the FAQ section of thesteamboatingforum.net, as virtually all of the steamboats use displacement type hulls, a general exception to the pleasure boat industry which has much faster boats.  

Fred. Thank you. thank you. That was a great answer. These numbers are for flat water and a reasonable shape hull obviously. OK. So the next part of the question is what facto do you use to give you some get out of trouble power. I have been reading on this forum for a while and one thread about a backup 9.9hp outboard and stories of string winds and currents got me wondering how you would calcualte a reasonable margin of "get out of trouble" power on top of the requirement to move at hull speed in clam seas. Obviously you can never have too much power, but these engines go up significantly in price with each step up in power. (for the sake of discussion lets assume a good size boat 25-30m with a steel hull and a weight of 100 - 150 tons. this is out of the category of small fibreglass hulls so things like wind, waves and inertia become significant.  

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KeithO Senior Member

Take a look at this paper. I think it is on topic for your thread. http://oa.upm.es/14340/2/Documentacion/3_Formas/Savitskyreport_conSemidesplazamiento.pdf  

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Hull Smoothness – What Matters for Speed?

How much effort should you spend on hull smoothness? We decided to investigate this after seeing a variety of approaches. Many (maybe most) fast sailors put time into polishing the hull, but others don’t bother.

Our primary source for this article is A Smooth Bottom is a Fast Bottom from the GP14 class website. This article is an easy read and the best summary we found. Author Paul Grimes was a Collegiate All-American sailor at Brown University and has experience in hydrodynamics and marine yacht services. We also referred to Sailing Theory and Practice , by C.A. Marchaj

Hull Smoothness and Speed – Data

Hull drag results from several factors. These factors have different names, depending on which book you read.

• Skin friction drag – the friction from the hull sliding through the water. A smooth hull reduces skin friction.
• Form drag – related to the streamlining of the hull and foils.
• Wave-making resistance – related to the slowing effect of the waves produced at the bow and stern by the boat’s movement.
• Induced resistance – due to leeway as the boat slides to leeward while sailing upwind.

Skin friction causes a substantial portion of total drag. Marchaj’s data from towing tests shows that skin friction for an International Canoe is 80% of total drag at 3 knots. Skin friction increases with boat speed, but other the forms of hull drag increase more, so skin friction is only 40% of total drag at 6 knots.

What is the speed advantage of a smooth hull? We could not find definitive speed data. Marchaj’s data only compares a boat with a clean bottom to a boat with a foul bottom. With the same driving force, the clean-bottom boat travels 0.27 knots faster than the foul-bottom boat when moving at 4 knots. The difference shrinks to 0.14 knots when the boats are moving at 6 knots. These are significant differences.

Since we couldn’t definitive data beyond foul and clean hulls, we’ll have to review the concepts of laminar and turbulent flow to get more answers about hull smoothness.

Laminar and Turbulent Flow

The no-slip condition and the boundary layer.

It may be counterintuitive, but the water molecules immediately next to the moving hull are pressed against the hull and adhere to it – they don’t slip. This is true regardless of the hull’s smoothness. These molecules slow down the water molecules “above” them, and so on until the water further from the hull is no longer affected. The affected layer is called the boundary layer. Skin friction drag is determined by the type of flow within the boundary layer.

The type of flow in the boundary layer determines the amount of skin drag.

• In laminar flow, the water molecules in the boundary layer all flow in the same direction – parallel to the hull surface.
• In turbulent flow, the water molecules move more chaotically.

Benefits and Limitations of Laminar Flow

Laminar flow reduces skin friction by as much as 80%, compared to turbulent flow. However laminar flow is fragile. It turns into turbulent flow under several conditions.

• Surface is not fair (bumps or dents).
• Surface is not extremely smooth (highly polished), especially in the forward part of the hull.
• Water is flowing fast. At speeds greater than 4 knots or so, boats can’t sustain laminar flow over the hull length, regardless of smoothness.
• Distance traveled along the surface is long. As the distance traveled becomes long, it becomes impossible to sustain laminar flow, no matter how smooth the hull.

Turbulent Flow

Although turbulent flow causes more drag, there’s still a very thin laminar layer in turbulent flow. The skin drag is minimized if the surface roughness is less than this thin laminar layer.

Conclusions about Hull Smoothness

The theory leads to the following conclusions about how much you should do about hull smoothness.

Fair the Hull

To be competitive your hull should be fair. Small undulations over a distance are not significant. Dents and bumps, especially those with sharp edges are more significant, as they will trip the flow from laminar to turbulent.

Polish to 400 Grit for Acceptable Results

Even with a highly polished hull, boats moving faster than several knots will transition to turbulent flow within the first several feet of hull. In turbulent flow, more roughness is acceptable. Grimes says that sanding to 400 grit is adequate if the flow is turbulent.

Polish to 1200-1500 Grit Equivalent for Best Results

The best chance for sustaining laminar flow is with a very smooth (e.g., 1200-1500 grit or greater) hull traveling at low speeds (light air). Pay special attention to the forward part of the hull, since roughness there will trip the flow to turbulent sooner.

For light air and overall peace of mind, polishing to 1200-1500 grit is not outlandish, but only if you have time to do the more important stuff – like practicing.

Other Hull Drag Factors

Waxing and water beading.

Beading of water on the hull has no effect on skin friction drag. The no-slip condition still holds true. If you put wax on just to get beading, you may make the surface rougher. We’ll see more about this in a future article.

Foil Smoothness

The foils (boards and rudder), being narrower, can sustain laminar flow over their entire surface. Foil smoothness is thus more important. We’ll cover this in a separate article.

Do You Have the Right Touch? Thoughts from Bruce Goldsmith

Hull care – how to get smooth and stay that way, you may also like, prevent breakdowns – top tips, updated, dehumidifying your a-scow, mc boards sticking three potential causes, season prep tech tip: clean your pulleys, stabilize the rig for mooring or on-water..., hull care – how to get smooth..., maintenance tips and myths for cleaning and..., leave a comment cancel reply.

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Compass Articles

• February 2, 2024

Semi-Displacement Hulls: Blending Speed and Comfort

Understanding Semi-Displacement Hulls: What They Mean for Your Yachting Experience

Semi-displacement hulls strike an enviable balance between speed and comfort, setting the stage for the modern yachter’s dreams. This architectural marvel uses design principles that allow a vessel to benefit from the efficiencies of both displacement and planing hulls. It’s a testament to the nautical evolution that answers a captain’s dual call for velocity and stability. But how does this translate to your time on the water? A semi-displacement hull offers smoother sailing with moderate fuel consumption, embracing both leisurely cruises and the rush of cutting through the waves. The ride is characterized by a gentle rise instead of a dramatic lift, making it an ideal choice for those seeking a versatile maritime lifestyle. The bow cuts cleanly through water, and the flatter stern section provides that extra push for speed – all without compromising on the sublime feeling of being at sea. With sleek lines and a sophisticated design, yachts boasting semi-displacement hulls encapsulate the very essence of modern seafaring innovation.

The Dynamics of Design: How Semi-Displacement Hulls Enhance Yachting

Moving beyond the basics, the science behind a semi-displacement hull is a fascinating study in naval engineering. At the core of its design is a balance between buoyancy and planing. When at rest or at low speeds, the hull behaves much like a displacement hull, yet as acceleration occurs, hydrodynamic lift comes into play. This translates to a hull that operates efficiently across a range of speeds, allowing for both extended voyages and brisk jaunts alike. The designed underwater profile minimizes resistance, which in turn optimizes performance. For the yachting enthusiast, this means enjoying the pleasure of a swift passage without the jarring experience that often comes with high-speed planing vessels. Such agile maneuvering is not only enjoyable but can be crucial in dynamic sea conditions. The result is a yachting experience that is luxurious, secure, and responsive to the captain’s intent.

Selecting the Right Yacht: What Might a Semi-Displacement Vessel Offer You?

Choosing the right yacht is a deeply personal decision that hinges on several factors including lifestyle, intended use, and design preferences. For semi-displacement hull enthusiasts, the decision is often driven by the desire for versatility. A semi-displacement yacht can be the perfect companion for those who appreciate both the journey and the destination. These vessels are known for their spacious accommodations, allowing guests to enjoy extended stays in utmost comfort. The intelligent design often translates to greater onboard space for amenities, storage, and living quarters. Key considerations when exploring these yachts include onboard technology, craftsmanship, and the level of customization available. Would you like a yacht with advanced navigation systems, opulent interiors, and ample deck space for social activities? A semi-displacement vessel might just be the ideal match for your maritime aspirations, ready to deliver unforgettable experiences across a spectrum of sea conditions.

Fly Yachts: Navigating Your Path to the Perfect Semi-Displacement Yacht

As the conversation around semi-displacement yachts continues to unfold, there exists a beacon within the yacht brokerage industry that stands out for its profound knowledge and commitment to the discerning yachtsman – Fly Yachts. We at Fly Yachts understand the nuances that make semi-displacement vessels both intriguing and valuable to our clients. Our team, consisting of seasoned professionals with a deep love for the ocean, is dedicated to guiding you through the intricacies of yacht selection. Each client’s journey is unique, and we pride ourselves on curating a personalized and enjoyable experience that culminates in not just a transaction, but a lifelong relationship with the sea. Speak to a Fly Yachts team member today, and embark on the voyage towards your perfect yacht with confidence and excitement.

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Speed at which the wavelength of a vessel's bow wave is equal to the waterline length / From Wikipedia, the free encyclopedia

Dear wikiwand ai, let's keep it short by simply answering these key questions:.

Can you list the top facts and stats about Hull speed?

Summarize this article for a 10 year old

Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension (height) increases as well. When hull speed is exceeded, a vessel in displacement mode will appear to be climbing up the back of its bow wave.

From a technical perspective, at hull speed the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Ship drag for a displacement hull increases smoothly with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.

The concept of hull speed is not used in modern naval architecture , where considerations of speed/length ratio or Froude number are considered more helpful.

As a ship moves in the water, it creates standing waves that oppose its movement . This effect increases dramatically in full-formed hulls at a Froude number of about 0.35 (which corresponds to a speed/length ratio (see below for definition) of slightly less than 1.20 knot·ft −½ ) because of the rapid increase of resistance from the transverse wave train. When the Froude number grows to ~0.40 (speed/length ratio ~1.35), the wave-making resistance increases further from the divergent wave train. This trend of increase in wave-making resistance continues up to a Froude number of ~0.45 (speed/length ratio ~1.50), and peaks at a Froude number of ~0.50 (speed/length ratio ~1.70).

This very sharp rise in resistance at speed/length ratio around 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. This led to the concept of hull speed.

Empirical calculation and speed/length ratio

Hull speed can be calculated by the following formula:

If the length of waterline is given in metres and desired hull speed in knots, the coefficient is 2.43 kn·m −½ . The constant may be given as 1.34 to 1.51 knot·ft −½ in imperial units (depending on the source), or 4.50 to 5.07   km·h −1 ·m −½ in metric units, or 1.25 to 1.41   m·s −1 ·m −½ in SI units.

First principles calculation

Because the hull speed is related to the length of the boat and the wavelength of the wave it produces as it moves through water, there is another formula that arrives at the same values for hull speed based on the waterline length.

This equation is the same as the equation used to calculate the speed of surface water waves in deep water. It dramatically simplifies the units on the constant before the radical in the empirical equation, while giving a deeper understanding of the principles at play.

Hull design implications

Wave-making resistance depends on the proportions and shape of the hull: many modern displacement designs can exceed their hull speed even without planing . These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly used by canoes , competitive rowing boats , catamarans , and fast ferries . For example, racing kayaks can exceed hull speed by more than 100% even though they do not plane.

Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing.

Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed.

Semi-displacement hulls are usually intermediate between these two extremes.

• Ship resistance and propulsion
• Wave making resistance
• A simple explanation of hull speed as it relates to heavy and light displacement hulls
• Hull speed chart for use with rowed boats
• On the subject of high speed monohulls , Daniel Savitsky, Professor Emeritus, Davidson Laboratory, Stevens Institute of Technology
• Low Drag Racing Shells

External links

• Converter: knots   > km/h & km/h   > knots

Semi-Displacement Hulls Explained (Illustrated Guide)

If you want to know exactly what a semi-displacement hull is, this article is right for you. Here, I explain simply what it is, how it works, and why it's different from other hulls.

What is a semi-displacement hull? A semi-displacement hull is a hull design that combines features of the displacement and planing hull. It displaces water at low speeds but is able to generate lift at cruising speed. It is more stable than planing hulls, and faster than displacement hulls. It planes at lower speeds than regular planing hulls.

Generally, you'll find this type of hull on motor cruisers, trawlers, and the likes. Sailboats don't really come in this flavor, and I'll explain why below.

On this page:

Semi-displacement hulls explained, maximum hull speed, semi-displacement hull advantages, semi-displacement hull disadvantages, differences between the hull types, who might like this type of hull, sailboats with semi-displacement hulls, examples of boats with semi-displacement hulls.

A semi-displacement hull is a perfect hybrid between displacement hulls and planing hulls. It's the perfect medium speed range hull design. It is also called the semi-planing hull. To summarize briefly:

• A displacement hull lies inside the water, and displaces it as it moves. It holds up the boat using buoyancy.
• A planing hull lies on top of the water as it moves and holds up the boat using lift. This is called planing .
• A semi-displacement hull displaces water at low speeds, but is able to semi-plane at cruising speeds.

All three of these hull types come with their own unique advantages and disadvantages:

• The displacement hull is efficient and very dependable in rough waters.
• The planing hull is extremely fast and agile.

The semi-displacement hull is a bit of both. It's seaworthy and can be relied upon in rough waters, but at the same time, it's a lot faster than displacement hulls. Semi-displacement hulls are perfect for boats that need to be steady and seaworthy but fast at the same time.

Design Features

The most important thing to understand is that generally, planing hulls are fine and flat aft (in the back), and displacement hulls are bulky and round. The semi-displacement hull combines these two design features. It is flat aft but gets bulkier towards the front. The bow (the front), has more of a wedge shape to it, like a Deep-V. The bow has to be able to displace water and lift at the same time. That's why the quality of the design is really important with these boats.

From the front, it looks like a sailboat, from the back, it looks like a powerboat.

The hull shape allows it to semi-plane. Planing simply means it's riding its own bow wave, lifting it out of the water. Planing is a great way to add speed, but it decreases stability.

The semi-displacement hull is heavier than a planing hull but lighter than a displacement hull. Because of its weight, it can't generate enough lift to fully plane. But because of its weight, it's also a lot more stable in rough waters.

How Does a Semi-Displacement Hull Work?

So now we understand the different features of this hull type, let's see how it comes together. At low speeds , the hull acts as a displacement hull, cutting through the water instead of riding on top of it. This makes it stable and reliable in waves. Its weight and keel make sure it handles well in choppy waves and rough weather, which is why it is a great design for coastal cruisers and trawlers and the likes.

At high speeds , the hull acts as a planing hull, riding on top of the water. This makes it a lot faster. Its flat back allows it to plane, as this will lift the front out of the water (but only partially). At the same time, the design of the bow helps it to climb out of the water. Between 12 - 16 knots (cruising speed), the semi-displacement hull will start climbing its own wave, generating lift. This reduces drag (water resistance). This adds a lot to its top speed. Roughly anywhere between 5 - 10 knots. That's a lot.

The semi planes at lower speeds than planing hulls though. This is great because most semis aren't that fast. The reason? The center of gravity is farther forward than planing boats 1 .

The reason the semi-displacement hull is so much faster is that it's able to climb its own bow wave. Regular displacement hulls can't do this. This means they have an upper-speed limit, called the maximum hull speed. The maximum hull speed is a direct correlation between the length of the boat's waterline and the maximum speed.

To learn more about calculating maximum hull speed, and to view examples of different boat lengths and their upper limit , please check out my previous article here .

• faster at cruising speed
• can outrun storms thanks to speed
• larger range
• excellent rough-water boats
• are able to cross oceans
• shallower draft
• less efficient at low speeds than a displacement hull
• uses a lot of fuel at lower speeds
• a bit less comfortable than a displacement hull
• less storage due to flat aft and fine bow
• not as fast as planing hulls

Difference between semi-displacement hulls and displacement hulls

Semi-displacement hulls:

• have finer bow entry
• are flatter aft
• can generate lift, making it faster
• can outrun storms because of their speeds
• have less storage due to flat aft and fine bow

To learn more about displacement hulls , I recommend reading my full guide on it. It explains the pros and cons of displacement hulls .

Difference between semi-displacement hulls and planing hulls

• are narrower than planing hulls
• have a deeper and narrower hull forward, more like a deep V
• are not as fast as planing hulls
• are a lot steadier than planing hulls
• vessels weight is still supported by buoyancy

Yes, that's right. You can cross oceans with a semi-displacement hull. With a planing hull, that's not recommended. However, for those types of journeys, I'd still pick a full displacement hull over a semi every day of the week.

For examples of the most common hull types , please read my Illustrated Guide to Boat Hull Types here .

If you want to have a comfortable ride, even in moderate to heavy chop, and want to have a reliable boat that also has some speed up its sleeve, this is a great hull design for you. With pretty decent speeds, often anywhere up to 20 - 30 knots, this hull can has plenty of thrill to it, while at the same time being comfortable and reliable. It's perhaps one of the most versatile hull designs out there. In my opinion, it offers the perfect fast-paced family cruiser for people who live near the coast and want to take her out for a bluewater spin.

I haven't heard of any sailboats - monohulls - that have semi-displacement hulls. (If you have, please let me know in the comments below.) I think the reason is simply that sailboats can't deliver the power necessary to generate any meaningful lift. Cruising speed for most semi-displacement boats is roughly 15 - 20 knots - that's when lift is generated and it starts to semi-plane. Monohulls can't get up to that speed.

Catamarans can. There are cats out there with wide and flat aft hull sections, enabling them to get into a semi-plane. This is one of the reasons why cats are so much faster than monohulls.

The most famous semi-displacement boat type is the Maine Lobster boat. Other boats that use semi-displacement are trawlers and motor cruisers.

https://www.soundingsonline.com/boats/how-different-hull-types-react-in-rough-water   ↩

Thank you for a well formed explanation—assuming you’re correct. My wife and I are looking at boating as a retirement plan. She asked what the difference is between hull types. I can send her your link.

Stay safe and healthy!

Thank You for an excellent education, I learned much from your article. Is there a chance ou may provide your comments/comparison of the Majesty 140 and theh Benetti Diamond 145? I would so much appreciate your commentary. Most Appreciated !

MacGregor 26M is a semi displacement sail boat :-)

The MacGregor 26X sailboat is also a semi-displacement hull. Factory specs suggest 24 knot top speed under sail or with a 40 hp outboard motor. I have not reached those speeds under sail … more like 14 to 16 knots.

Thank you so much I understand the different types of hulls so much better!

robert salverda

Hi Shawn, I once owned a Lancer 39.5 foot sailboat. It had two Perkins 4-254 85 h.p engines in it. At full throttle it could actually plane, defying all the rules of a displacement hull, so I am assuming it must have had a semi-displacement hull.

Leave a comment

You may also like, the illustrated guide to boat hull types (11 examples).

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

A Complete Guide to Displacement Hulls (Illustrated)

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• This New 131-Foot All-Aluminum Superyacht Lets You Take a Dip at Tip and Tail

The sleek cruiser can cover some 4,000 miles, too.

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Rachel Cormack's Most Recent Stories

This 92-foot yacht has a deck big enough for your chopper.

• This New 131-Foot Catamaran Comes With See-Through Hulls
• Share This Article

ISA Yachts ’ latest model is Unica by name, unique by nature.

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Geared toward indoor-outdoor living, Unica seamlessly connects the interior with the exterior decks and the broader oceanic surroundings. The stern is “a true haven of pleasure,” according to the Italian yard. Across the three aft decks lies a swim platform, an infinity pool with a lounge, and a cockpit with a dining area and plush seating.

Another highlight is the flybridge. Measuring 52 feet from end to end, the sprawling deck includes two sunbathing areas, a lounge, a dining area, a bar, and a round Jacuzzi. ISA says there are also more dining areas, sunbathing spots, and lounges on the main and upper deck.

With a beam of 27 feet and an interior volume of 390 GT, Unica features quite generous living quarters. The main deck includes a light-filled salon, a dining room, and a galley. Also on this deck is an owner’s suite with a lounge, vanity, dual walk-in wardrobes, an en suite, and an office.

There’s more to explore above and below deck, too. The lower deck is outfitted with three VIP suites, one twin, and the crew quarters, while the upper deck is home to a lounge, bar, captain’s cabin, and wheelhouse.

Unica’s performance specs are nothing to sneeze at, either. Powered by two MTU engines, the yacht can reach a maximum speed of 20 knots and a cruising speed of 15 knots. As for range, the cruiser can cover roughly 4,000 miles when traveling at 12 knots.

Unique or not, the Unica 40 is one impressive superyacht.

Click here to see all the photos of the Unica 40.

Rachel Cormack is a digital editor at Robb Report. She cut her teeth writing for HuffPost, Concrete Playground, and several other online publications in Australia, before moving to New York at the…

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Boat Test: 2024 Formula 457 CCS

• By Randy Vance
• March 29, 2024

When I pressed the throttles on the 457 CCS, it didn’t rear up like a ­Lippizan, but rather surged forward like a Kentucky Derby thoroughbred out of the starting gate. Formula’s proprietary double-stepped hull kept the bow down as it stretched forward in a long, loping gate, the two-speed transmissions of the Merc V-12 Verados proving their worth. In turns, sharp and hard-over, the 457 CCS held its ground, carved them smoothly, and without the chine tripping or spinning out. The outboards themselves held their ground, sensing with electronic magic the added load that turns cause. They fed in more power to keep the speed steady through the arc. In port, joystick steering and a bow thruster made docking the 47-footer (LOA with ­engines) a breeze. Even an amateur can do it with confidence.

We weren’t surprised by its performance—every Formula that our test team has ever touched over the years has offered a high-performance ­experience thanks largely to the FAS 3 Tech design and engineering.

In fact, FAS 3 Tech is the technology below deck on the 457. Instead of stringers and bulkheads laid into the hull and storage buckets glued in place, Formula builds a one-piece stringer grid that melds to the hull in a single piece, and includes all fish boxes, dry-storage compartments and machinery access to the bilge, plus ample wire chases for a nearly unlimited array of accessories. The boat is filled with gadgets such as inductive cellphone-charging brackets, electric adjustable seats, bow cocktail table and a full assembly of navigation equipment—just for starters. Once the grid is bonded to the hull, the two pieces become one. The deck is then bonded in, and three become one. Formula perfected this technique decades ago, when competitors were still using wood stringers and ­transoms.

Another exceedingly well-built center-­console is Pursuit’s Sport 428. Pursuit was an early starter in the sport-model realm, bringing the 280 Sport to market in 2008 and the 310 ST in 2011. Pursuit met the needs of fishermen with broader interests than just wetting a line. The style became so popular, Pursuit expanded it with several models, including the 428. Soon the company rebadged the entire line as “Sport,” but all bristle with a fishing backbone while catering to diving and cruising fans. The S428 ($1,339,935 with quad Yamaha XTO 425s) ­offers a platform more on par with Formula’s far-fishier 457 CCF, sistership to this 457 CCS. The Pursuit has dual-row helm seating with a deck galley to double as a rigging station and mezzanine seating that faces the cockpit. For at-anchor R&R, there is a starboard-side fold-down terrace in the gunwale, and for diving and easy dockside entry, a portside boarding door. ­Formula ­offers port and starboard transom doors to a spacious platform too. ­Pursuit’s 428 is just 43 feet, 9 inches with a 13-foot beam compared with the nearly 46 feet length overall and 13-foot-9-inch beam of the Formula. Size, plus the fact that the Formula is powered by Mercury’s new V-12 600 hp outboards versus the four Yamaha 425s on the Pursuit, accounts for much of the $600,000 ­difference in base price between the two.

Both competitors drench passengers in ­luxury seating with a dual lounge ahead of the console and wraparound seating at the bow.

Interior and Accessories

Formula’s length and beam make for a roomy cabin below deck that includes a double berth that converts from a C-shaped lounge, and it’s nestled into a cheerily lit (with recessed LED lighting and overhead and forward portlights) arrangement with a large head compartment with shower, and an abbreviated galley with microwave, coffee maker and fridge. The topside galley also has plentiful drawer storage, a fridge, and there’s a grill on the transom hiding beneath its own hatch, expanding galley features.

There are rod holders, and in place of a livewell in the CCS, there is a cooler. But when fishing is on the agenda, the equipment is there to help ­ensure angling convenience and success.

In addition to enormous deck compartments, there are compartments in all of the coamings for boat hooks, rods and gaffs if you do fish. They tilt outward from beneath the gunwales and tuck back in flush with them, to keep the passageway clear. Lines, fenders and life jackets are accommodated as well. A transom lazarette, or “boot,” can stow more mooring gear, but aboard our tester, this housed a Seabob water scooter—a Formula option painted to match.

It’s important to note that this boat is not only part of a series, and sibling to the even-more-fishing-oriented 457 CCF, but also either version of the boat is eligible for a program called FormulaFlex. FormulaFlex is an exclusive plan offering boat buyers individual personalization in key areas such as graphics, upholstery and electronics. Many of these preferential changes can be had at no additional charge. If a charge is required for a buyer’s request, you participate in FormulaFlex MyWay, allowing you to choose paid-for changes or options. The point is not so much the charge of a fee for the change as it is Formula’s willingness to make changes to ­accommodate buyers’ ­desires in the first place.

Read Next: Formula 387 CCF

While we can wax on about the deck plan, it’s better to report what our test truly revealed. The Formula 457 CCS is indeed akin to a thoroughbred, delivering a combination of performance, luxury, versatility and great opportunities for owner personalization.

How We Tested

• Engines: Triple 600 hp Mercury V-12 Verados
• Drive/Props: Outboard/29-inch-pitch stainless-steel contra-rotating propsets
• Gear Ratio: 1.75:2.5 Fuel Load: 650 gal. Water on Board: 50 gal. Crew Weight: 800 lb.

High Points

• Tempered-glass windshield raises on electric actuators; can be fully open while running.
• Standard Seakeeper 6 gyrostabilizer, bow thruster, and 18,000 Btu air conditioning in the cockpit.
• FormulaFlex and FormulaFlex MyWay programs offer great personalization. 
• Cabin berth has ample room to stretch out, but vertical space allows feet-first entry only.
• The diesel generator is safer than gasoline power, but it requires a second fuel tank. 

Pricing and Specs

Speed, efficiency, operation.

Formula Boats – Decatur, Indiana; formulaboats.com

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Clearing Baltimore's shipping channel won't be easy, will take at least weeks

Scott Neuman

An aerial view of the cargo ship Dali after it ran into and collapsed the Francis Scott Key Bridge on March 26, 2024 in Baltimore, Maryland. Tasos Katopodis/Getty Images hide caption

An aerial view of the cargo ship Dali after it ran into and collapsed the Francis Scott Key Bridge on March 26, 2024 in Baltimore, Maryland.

Days after a massive container ship slammed into Baltimore's Francis Scott Key Bridge, causing the structure to crash into the narrow waterway, clearing debris from the channel so that maritime traffic can resume is an urgent priority.

Loss of ship's power and stiff current may have led to bridge collision, experts say

"It has to be done very quickly," says David Von Schmidt, a naval architect and engineer. "The regional, if not the national economy, cannot afford any longer than that."

The likely first step will be making sure that the Dali, the nearly 1,000-foot container ship that smashed into the bridge early Tuesday morning, doesn't do any more damage, according to Captain John Konrad, CEO of gCaptain , a website that tracks the shipping industry.

Before removing the ship, "They'll get a salvage company in to secure the ship and make sure hazardous materials ... [don't leak] from the containers, no fires, that sort of thing," Konrad says.

The next step is removing "the tangled bridge debris," he says. "Then you got to probably drag the bottom again to make sure you don't have any debris that's going to cause a problem."

Von Schmidt says he assumes that the focus will be on "completely clearing the center span so that there's no restriction in navigation, because right now with that debris, it's restricted navigation."

That means moving in large floating cranes and sending down divers, he says. But first survey boats will need to "map out a grid of the bottom to find where all the debris is" and make a plan for removal, he says.

'A generous man': Baltimore bridge worker helped family, community in Honduras

That means scanning the bottom, Konrad says. "Right now, the [U.S. Army] Corps of Engineers is running a couple sonar boats to get a general idea," he says. "That's going to take time. And once they do that, they're going to have to send divers down with welding, cutting torches, cut sections out, and then they're going to have to bring in a crane barge."

Removing debris could be done in stages to speed up the process, Von Schmidt says. "They might open the channel up in phases specific to the displacement of the vessels," he says. So, shallower draft vessels would be allowed to transit before the deeper draft ones that could snag debris on the bottom.

How long will all that take? "It's weeks and months to remove the debris and reopen the shipping channel," Benjamin Schafer, a professor of civil and systems engineering at Johns Hopkins University, told member station WYPR . "I'd be shocked if it's weeks, but I don't think it'd take a year."

Von Schmidt is a bit more optimistic about a timeframe. "What level of traffic? That remains to be seen," he says. "I think it's very possible that traffic moves in two plus weeks. Possibly, he says, "it'll be wide open for traffic shortly after that."

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Chris Baraniuk

Why the Baltimore Bridge Collapsed So Quickly

Just shy of half past 1 in the morning, the MV Dali , a giant container ship, was sailing gently out of the port of Baltimore when something went terribly wrong. Suddenly, lights all over the 300-meter-long vessel went out. They flicked on again a moment later, but the ship then began to veer to the right, toward one of the massive pylon-like supports on the Francis Scott Key truss bridge—a huge mass of steel and concrete that spans the Patapsco River.

The Dali ’s lights went out a second time. Then the impact came. The ship plowed into the support, with large sections of the bridge’s main truss section instantly snapping apart and falling into the river. It took just 20 seconds or so for the structure to come down.

Now, a major US port is in disarray, and several people who were working on the bridge at the time of its collapse are missing. A rescue operation is underway. President Biden has called the disaster a “terrible accident.” Ship traffic is currently stuck on either side of the crash site, and a major roadway through Baltimore has been cut off.

“It’s a dreadful tragedy and something you hope never to see,” says David Knight, a bridge expert and specialist adviser to the UK’s Institution of Civil Engineers. But commenting on footage of the bridge collapse , he says he is not surprised by the manner in which it crumpled.

Large steel structures may seem invulnerable, but steel, explains Knight, is relatively lightweight for its size. As soon as it is pushed or pulled the wrong way with enough force, it can fold like paper. In this case, the Francis Scott Key Bridge was a “continuous,” or unjointed, bridge that had a 366-meter-long central truss section. (Truss bridges use steel beams, arranged in triangular shapes, to support their load.) The central truss was made up of three horizontal stretches, known as spans, with two sets of supports holding these above the water. It was the third-largest structure of its kind in the world.

Will Knight

Aarian Marshall

“When you take a support away, there is very little in the way of robustness,” says Knight. “It will drag down, as we saw, all three spans.” The separate approach spans remain standing. There is nothing in Knight’s view that immediately suggests any structural problem with the bridge. An engineering firm, Hardesty & Hanover, confirmed to WIRED that it performed an inspection of the bridge in 2019, and that other inspections have been carried out since, but did not provide any additional details on the state of the structure. WIRED has approached H&H for further comment. In June last year, the US Federal Highway Administration rated the condition of the bridge as satisfactory .

The immense force of the container ship impact should not be underestimated, adds Knight. Such vessels require a lot of power and time—perhaps many minutes—to come to a complete stop. The Francis Scott Key Bridge was completed in 1977. In more recent decades, bridge engineers have commonly incorporated defenses to reduce the potential damage by ship strikes when bridges are erected in similar locations, Knight says. These include hydraulic barriers and additional concrete around the base of bridge supports, for instance. However, even with such fortifications in place, heavy strikes can still cause devastating damage.

It is not clear why lights turned off and on again on the Dali , a Singapore-flagged ship built in 2015. “That is an indication of a massive problem,” says Salvatore Mercogliano, a maritime historian at Campbell University in North Carolina and a YouTuber who has analyzed the crash .

At the time of the accident, two pilots—mariners who board a ship to help it navigate particular stretches of water, including in and out of ports—from Baltimore were on board. The Dali was broadcasting its position publicly via the automatic identification system (AIS) and was traveling at a speed of over 8.5 knots. It then slowed to around 6 knots in the moments before the crash, according to AIS data .

Both pilots and all crew members on the Dali are accounted for. There are no reports of injuries, the ship’s management company, Synergy Group, said in a statement on March 26.

ABC News reports that the crew of the vessel made a desperate mayday call in an attempt to warn transport officials that the crash was about to occur. A report from the Cybersecurity and Infrastructure Security Agency, seen by ABC, says the Dali “lost propulsion” and that the crew were aware they had “lost control” of the ship. Maryland governor Wes Moore told reporters that , thanks to the mayday call, officials were able to stem the flow of traffic over the bridge, an intervention that he says “saved lives.”

Mercogliano says it is very difficult for ships of this size to make rapid adjustments to their trajectories. Video footage shows a sudden outpouring of smoke from the vessel’s stack, indicating a change in engine activity of some kind. What is particularly disturbing is that, in this case, the vessel ends up plowing straight into one of the key supports for the bridge, clearly off course. No information as to why this happened has become public.

Photographs of the aftermath show the bow of the ship pinned beneath fallen sections of the bridge . The anchor chain is visible, meaning that at some point the anchor was dropped, though it is not certain whether this happened before or after impact. The chain appears to be at an angle, however, which Mercogliano says could be a sign that it was dropped shortly before the crash and dragged for a brief time.

Lawyer James Turner of Quadrant Chambers in London specializes in, among other things, ship collisions. He says that there would have been no automated systems on board a merchant ship of this kind able to prevent the impact. Information from radar, AIS, and visual observations would have been available to the crew, however.

But data-collecting systems may now reveal exactly what happened. As on airplanes, commercial ships have data and audio recorders on the bridge, which are often a key source of information for investigators post-incident. “The master will hit a button and that ensures that the last two hours of audio recording are preserved, as well as all the data from the various parts of the ship, like the engine and steering and so on,” explains Turner. “That can be downloaded and queried.”

He adds that estimates of the ship’s speed at the time of the incident as recorded by AIS are likely “99.99 percent accurate.”

For now, the focus of responders will be on locating survivors from the fallen bridge. Two people have been rescued, one of whom is in the hospital. Six construction workers remain missing .

The disaster has come at a difficult time for shipping, with drought afflicting the Panama Canal and Houthi attacks striking multiple vessels in the Red Sea in recent months. Somali piracy is on the rise again , also. The grounding of the Ever Given in the Suez Canal is very much still within recent memory—it occurred a mere three years ago.

The Port of Baltimore insists in a statement that it has not been shut down—road vehicles are still operating within the port—however, all ship traffic in and out is suspended until further notice. AIS data reveals around a dozen commercial vessels at anchor outside the port, their entry now blocked by the stricken bridge and the Dali . It will take some time for the US Army Corps of Engineers to remove the steel pieces of the bridge, which present a significant threat to passing vessels, from the river.

“Whatever ships are in the port are now stuck,” says Mercogliano, who notes that Baltimore is an important port in terms of car deliveries and coal exports.

Overall, he argues, maritime operations are extremely safe today, though the volume and velocity of trade mean that when things go wrong it can be especially serious.

“We move goods a lot faster than ever before, and there’s very little margin for error,” he says. “When there is a mistake, the mistakes tend to be very large.”

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The Dali was just starting a 27-day voyage.

The ship had spent two days in Baltimore’s port before setting off.

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By Claire Moses and Jenny Gross

• Published March 26, 2024 Updated March 27, 2024

The Dali was less than 30 minutes into its planned 27-day journey when the ship ran into the Francis Scott Key Bridge on Tuesday.

The ship, which was sailing under the Singaporean flag, was on its way to Sri Lanka and was supposed to arrive there on April 22, according to VesselFinder, a ship tracking website.

The Dali, which is nearly 1,000 feet long, left the Baltimore port around 1 a.m. Eastern on Tuesday. The ship had two pilots onboard, according to a statement by its owners, Grace Ocean Investment. There were 22 crew members on board, the Maritime & Port Authority of Singapore said in a statement. There were no reports of any injuries, Grace Ocean said.

Before heading off on its voyage, the Dali had returned to the United States from Panama on March 19, harboring in New York. It then arrived on Saturday in Baltimore, where it spent two days in the port.

Maersk, the shipping giant, said in a statement on Tuesday that it had chartered the vessel, which was carrying Maersk cargo. No Maersk crew and personnel were onboard, the statement said, adding that the company was monitoring the investigations being carried out by the authorities and by Synergy Group, the company that was operating the vessel.

“We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected,” the Maersk statement said.

The Dali was built in 2015 by the South Korea-based Hyundai Heavy Industries. The following year, the ship was involved in a minor incident when it hit a stone wall at the port of Antwerp . The Dali sustained damage at the time, but no one was injured.

Claire Moses is a reporter for the Express desk in London. More about Claire Moses

Jenny Gross is a reporter for The Times in London covering breaking news and other topics. More about Jenny Gross

Photos, video show collapse of Baltimore's Francis Scott Key Bridge after cargo ship collision

The Francis Scott Key Bridge in Baltimore, Maryland collapsed Tuesday into the Patapsco River after it was struck by a large cargo ship.

The bridge's collapse has prompted huge emergency response, with the Baltimore City Fire Department describing the collapse as a mass-casualty incident, and rescue crews searching for seven people in the river. Maryland Gov. Wes Moore has declared a state of emergency.

Baltimore  Mayor Brendon Scott  said on X that he was aware of the incident and was en route to the bridge. "Emergency personnel are on scene, and efforts are underway," he said.

The 1.6 mile, 4-lane bridge named for the author of the "Star-Spangled Banner," was the second-longest continuous-truss bridge span in the United States and third in the world.

Follow here for live updates → Baltimore's Key Bridge collapses after ship collision; rescue effort underway

Photos show collapsed Francis Scott Key Bridge in Baltimore

Videos show francis scott key bridge's collapse.

The bridge's collapse, which came after it was struck by a container ship, was distributed on social media.

What did the Francis Scott Key Bridge look like before it was hit?

Contributing: Charles Ventura, Thao Nguyen and Susan Miller, USA TODAY .