sailboat length to beam ratio

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Calculations

Sail area / displacement ratio.

A measure of the power of the sails relative to the weight of the boat. The higher the number, the higher the performance, but the harder the boat will be to handle. This ratio is a "non-dimensional" value that facilitates comparisons between boats of different types and sizes. Read more.

SA/D = SA ÷ (D ÷ 64) 2/3

• SA : Sail area in square feet, derived by adding the mainsail area to 100% of the foretriangle area (the lateral area above the deck between the mast and the forestay).
• D : Displacement in pounds.

Ballast / Displacement Ratio

A measure of the stability of a boat's hull that suggests how well a monohull will stand up to its sails. The ballast displacement ratio indicates how much of the weight of a boat is placed for maximum stability against capsizing and is an indicator of stiffness and resistance to capsize.

Ballast / Displacement * 100

Displacement / Length Ratio

A measure of the weight of the boat relative to it's length at the waterline. The higher a boat’s D/L ratio, the more easily it will carry a load and the more comfortable its motion will be. The lower a boat's ratio is, the less power it takes to drive the boat to its nominal hull speed or beyond. Read more.

D/L = (D ÷ 2240) ÷ (0.01 x LWL)³

• D: Displacement of the boat in pounds.
• LWL: Waterline length in feet

Comfort Ratio

This ratio assess how quickly and abruptly a boat’s hull reacts to waves in a significant seaway, these being the elements of a boat’s motion most likely to cause seasickness. Read more.

Comfort ratio = D ÷ (.65 x (.7 LWL + .3 LOA) x Beam 1.33 )

• D: Displacement of the boat in pounds
• LOA: Length overall in feet
• Beam: Width of boat at the widest point in feet

Capsize Screening Formula

This formula attempts to indicate whether a given boat might be too wide and light to readily right itself after being overturned in extreme conditions. Read more.

CSV = Beam ÷ ³√(D / 64)

The theoretical maximum speed that a displacement hull can move efficiently through the water is determined by it's waterline length and displacement. It may be unable to reach this speed if the boat is underpowered or heavily loaded, though it may exceed this speed given enough power. Read more.

Classic hull speed formula:

Hull Speed = 1.34 x √LWL

Max Speed/Length ratio = 8.26 ÷ Displacement/Length ratio .311 Hull Speed = Max Speed/Length ratio x √LWL

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What is the L/B ratio?

Choosing the right beam for a multihull

QUESTION:   When assessing or designing a trimaran or catamaran, what guidance can you give to guide the choice of beam ?             

                    Lech K:  Gdansk, PL

ANSWER: An interesting question as we do see quite a variation on existing boats.

First, let’s call the Overall Beam to Length ratio B/L and the individual hull length to hull beam, L/b .

Here are a few basics to consider as inputs to your overall beam choice.

*    More beam gives more transverse stability, permitting a powerful rig to drive a boat faster, but also,       excessive beam tends to lower diagonal stability so increasing pitch-poling.    More beam also tends to allow more  fore & aft pitching.

*    More beam requires stronger connecting beams (called akas on trimarans), aggravated by the two hulls potentially being now be in different waves

*    More beam can be a problem in a marina where space is increasingly limited

*    Folding trimarans can be limited in beam due to geometric space when folded, such as:     

Transverse folding system (Farrier etc) are limited by how far down the hulls can be managed when folded.  

Swing-arm folding is limited by the overall length increase when folded.

          Hinge & latch systems are limited by what height and weight can be lifted  

*    Less beam allows a boat to heel more, thereby reducing sail exposure to side wind.

*   Less beam brings the hulls closer together, reducing beam strength requirements and weight, but potentially adding to resistance from hull-flow interaction.

*   Hulls with a high L/b ratio can be closer together than hulls with a low L/b ratio if overall stability permits.

*   As smaller boats need proportionally more displacement due to crew and structural weight, they cannot have a very high L/b ratio as they then have insufficient displacement.

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So what do all these points finally lead to ?      Well, let’s see.

For Catamarans , the sweet spot seems to be with a L/B of 2 to 2.1.

If the beam is excessively increased, pitching and reduced diagonal stability (see dwg) start to become an issue and when such boats are lengthened to make their L/B slightly above 2, they generally become faster and have less negative issues ... but over about 2.3, their relatively lower transverse stability then starts to kick back.  

If the beam is decreased, stability drops quickly and one may start to also add wave interference between the hulls unless the boat is very light with slim hulls.

Of course, this is a simplification of things as top weight, windage, wing clearance, center of gravity, sail plan, etc etc .. all have their effects, though individually less than the important L/B ratio.

Let me give you an example of how other design criteria can move things from what may initially seem the ideal.  

Beam also has a huge effect on stability.   But the designer Jan Gougeon (then of West Systems) was an inventive guy, so he approached this design in a non-conventional way.   To achieve his first criteria .. “a fast non demountable weekend catamaran”, he needed to address the obvious lack of stability in other ways.  A low rig could work with low weight, that would then allow very slim, fast hulls.   Then he added water ballast to help keep the windward side down …. and finally, a masthead float to prevent the boat from turning turtle, where she would stay like virtually all other multihulls do IF that happens.  In this case, it was rather often as unfortunately, most sailors were not ready to adapt to this new way of sailing and with capsizes happening too quickly for most, only a dozen or so were sold.  But I did get to try the boat and felt the concept did work in the sense that the boat IS fast and also comfortable & dry, as with such long, narrow hulls, there is very little disturbance of the surface water so spray is minimal and even if the hulls are pretty close, they are too slim to create any significant cross-hull wake- interference. 

To keep the rig low (mast is shorter than the boat), she uses 2 foresails that can be furled up fast.   Those that still own one have learned to understand them and can enjoy their merits … but this is not a boat with reserve stability for sudden gusts, so you need to sail this boat more like a race dinghy and also reduce sail early.  This further means that sailing at night when you cannot see squall warnings in the sky is best avoided unless the stars are truly out for you.

But it IS an example of thinking WAY outside the box .. even if the result is not for everyone.   So ‘sweetspot L/B ratios’ do not necessarily mean they give the only solution .. just that you, as a designer, also need to work differently around the rest of the design to solve the issues you might create if you are well outside the norm.    

The lesson here is:  If you choose to go outside the norm, fully understand the implications and work around them.  You cannot ignore them and still expect success. If the designer failed at all with this radical G32 design, it was in not sufficiently educating new owners of the different sailing nuances needed to keep the boat on its feet.

For Trimarans, my studies and observations show that the preferred B/L ratio changes with boat size. 

To some degree, the same effect on diagonal stability (as for Cats) will occur with excessive beam, but with a trimaran, the two hulls in the water will be closer so it’s also important to allow for good flow between them.  So as very large racing tris can have slimmer hulls due to great length and low weight, they can have proportionally lower B/L ratios than smaller boats that need proportionally fatter central hulls just to support the displacement they need.  After all, we cannot just change things in proportion, because the weight of things (such as crew, structure etc) will not automatically get smaller for a smaller boat .. in fact it proportionally and typically, gets greater!    So smaller multihulls can often be harder to design than larger ones, where you have more space and volume to work with.    The above observations led me to plot data from good boats and create this simple little formula that fits their B/L curve pretty well.

Here is what the curve gives as a recommended B/L ratio for a sailing trimaran

                            (Sailing Trimaran) B/L ratio  = 1.48 ÷ (L  ^ 0.21)        [ Length L in feet ].

While this may initially look complex to calculate for some, it’s very easy with the right help.  Download the Mobi Calculator on your phone or tablet.  You can then add the expression x n to your basic calculator by first hitting the 3 dots [ ... ] that brings you to the Scientific Options, and then   clicking on [ x n ] that will add this feature to your basic calculator.    You can now enter the formula exactly as written, typing 1.48 ÷ (  your L value , and then x n and finally 0.21 and the closing bracket ) and then ‘ = ‘.

If you enter say L = 17 , it will give you a B/L ratio of 0.816, closely matching a W17 , while for an L of 100 ft, it will give you a B/L of only 0.562, closely matching a big ocean racing tri like Sodeb’O.

While of course you can go outside the calculated ratio, IMHO you should have a very good reason and specific justification for a deviation of more than 15% either way.   Use the list at the beginning of this article to justify your change.

For both Tris and Cats, there may be other factors that will change your design, but this gives a good starting and target point that’s based on both practical and justified design needs.

Enjoy …. playing with figures is fun ;)

mike … march 2022

ADDED NOTE ... re MOTOR MULTIS

As noted, the above ratios refer to Sailing Craft.  Without the heeling force of a sail, pure motor-tris and cats are not bound by the same needs.

A motor catamaran can have less beam, with a clean flow between the hulls now taking prominence over high beam for sailing stability, so L/B ratios of 2.5 to 3 are now more appropriate.

Hulls may need to be asymmetrical with a straighter side on the inside to avoid unfavorable hull wave interaction between them.

For a powered trimaran , overall beam should also be reduced or the motion will become uncomfortable.  (With a sailing tri, the boat is heeled with one ama out, but with a motor tri, all three hulls are immersed so wave action on the boat would be too severe if the boat is too wide) .    Amas (pontoons) now need to be narrow but deep, as a slow gentle roll of slightly greater amplitude, is more comfortable than a short quick one.  These amas (now only 40-50% of the main hull length), seem best with their center about 60-70% aft of the main hull length and need to be of fine section and relatively deep with the connecting bridge arched high above any waves, so that neither ama or aka-bridge will slam when re-entering a wave.  L/b ratios of all 3 hulls can be at their most efficient, namely 13-16 at the waterline.   The amas are now more like permanent training wheels and with a much longer central hull and no heeling force from sails, diagonal stability is no longer something to consider.    Overall beam will depend on maintaining a clean flow between the main hull and the shorter slim amas, that need to extend well down into the water, so that motion is acceptable in waves.   Typical overall B/L ratios might now be down around 0.4, becoming even less as boat design gets bigger, provided the center of gravity is kept low.

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Catamaran Beam to Length Ratios Explained: For Beginners

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Starting my sailing career something that struck me was the wast number of weird words and strange terminology, no longer was a rope just a rope, it’s a halyard or a sheet. In this article, I will explain one concept that is important to understand for anyone trying to buy a boat or for someone who wants to better understand the limitations of the vessel they already have.

Catamaran beam-to-length ratios are mathematical representations of the difference between the length of a sailing vessel and its width. There are multiple beam to length ratios, some impacts stability (Bcl/Lwl), and the amount of sail the vessel is able to carry. Others are used to calculate exterior space (B/L). In general, a narrow boat will be less stable but weigh less and cheaper to build.

Most modern catamarans have a beam to length ratio of >50%. You can easily calculate this on your own by following the steps below. But first, let’s check out some more terminology to make sure we really understand this ratio.

Table of Contents

Nautical Terminology

No matter how much you love the ocean, you will have limited success if you are unfamiliar with the words that go with adventuring out on it. I need to clarify some nomenclature before we delve into the ins and outs of ratios and catamarans (and monohulls).

• Beam overall (Boa): is the width of a boat at its widest point. The wider a ship’s beam, the more interior and exterior space. this allows for more gear and and better living accomodations.
• Draft: sometimes spelled “draught,” is the measure of how deep your vessel “sits” in the water. Catamarans have shallower drafts than monohulls, meaning they can sail in shallower waters and some can even be sail all the way up onto the beach, called beaching a cat .
• Catamaran: is a boat with twin hulls positioned parallel to each other. This design lends stability to the craft, and since there are two hulls, each can be narrower than a monohull without giving up stability. 
• Monohulls: boats with one hull. They derive their stability from a heavy keel and a wide hull, in comparison to a catamaran with two thin hulls separated far apart.
• Length over all (Loa): is measured from the aft to the bows including all gear such as bowsprits etc. To be compared with Length on waterline LWL.
• Length on waterline (Lwl) is the boats length measured on the surface of the water.

If you want to better understand catamaran construction and the impact of hull shape on performance and safety I suggest you read the book Catamarans; The complete guide for cruisers . It has helped me to better understand multihull dynamics in a more structured way than just googling. Gabo

Different Beam to Length Ratios

Hull centerline beam to waterline length (bcl/lwl) :.

The distance between the centerlines of the hulls divided by the waterline length on one hull is a good indicator of performance. It measures the points of the boat that interacts with the water. A higher ratio will give a higher resistance to capsizing and a lower ratio will increase drag due to wave interactions under the bridge deck.

Compared to the beam overall to length overall (Boa/Loa) that more or less only gives you an understanding of whether or not the boat will fit in a certain slip or what you will pay for a canal passage.

Hull Fineness Ratio (HFR)

Hull Fineness Ratio (HFR) is another name for Hull length-to-beam ratio . This is basically the same as the ratio mentioned above but only measures one of the hulls instead of the entire boat. And “fineness,” essentially, means “thinness.” Most cats have a ratio between 8:8 and 10:1 .

Boat Overall Beam (Boa) to Length Overall (Loa)

These are the exterior measurements of the boat. This ratio will not offer much other information than estimating marina fees and general boat size. To understand catamaran stability the two above ratios are much better since they show how the boat interacts with the water. It is in theory possible to have a very high Boa/Loa ratio but still have a boat that is very unstable due to having a low Bcl/Lwl ratio.

General Rules When Calculating Ratios

Ratios are exercises in long division. Since you remember your rules from math in school, you know that the order of the numbers in the equation makes a difference. 

Make sure you divide Beam by Length (B/L) and not the opposite!

If you mix them up you will get the wrong result and you might assess the stability of the boat incorrectly. And remember to stick to either meter or feet.

The formula looks like this:

B/L = Beam (in ft or meter) to length (in ft or meter) ratio

But how do you measure and from where to where? With those questions in mind, we add even more terminology to all this ciphering.

If, for instance, you have a bowsprit (the railing at the bow that extends past the deck), including this in your length measurement will skew your ratio. The extra length added by the largely cosmetic feature will not contribute to the stability or lack thereof of the craft, mainly because it does not touch the water.

So we look, then, at the measurements at the waterline .

Why Ratios Matters

If your Bcl/Lwl is too low, you will have an unstable craft. Adding a sail to the mix makes it even more so – if you have a ridiculous ratio of something like 1:18, wind in the sails at the correct angle will very likely capsize it. A wave of moderate size could do it, too.

If you want to know why catamarans capsize i suggest you read my other article ! Gabo

But a 1:1 Bcl/Lwl will make for a floating square with the maneuvering ability of a brick. A floating brick, sure, but it’s still a brick. This ratio is something you only see on really fast racing trimarans, since trimarans lift the windward hull the actual ratio when turning is half of that.

The fineness of a hull determines its speed and stability, which means that with every increase to one of those factors comes a decrease in the other. 3:1 seems to be the Goldilocks Zone for most monohulls. But since catamarans have two hulls separated wide apart the cat will be able to have thinner hulls while still maintaining high stability, a ratio around 8-12:1 is common on catamaran cruisers.

Final Thoughts

Casual sailors may never calculate Bcl/Lwl, B/L, or hull fineness ratio. But if you’re looking to buy a boat and want to better understand its sailing capabilities then these numbers will give you the ability to objectively compare different boats.

Speed and stability are the main factors governed by these ratios, and a change in one of them changes the other in the opposite direction. Generally speaking, the wider the beam, the more stable a ship is.

• Boat Building: Catamaran Design Guide – Catamarans Guide
• Marine Link: What Hull Shape Is Best?
• MB Marsh Marine Design: Length-beam ratio
• Multihull Dynamics: Six Kinds of Cats and Two Kinds of Trisi
• Ocean Navigator: Beam and Draft

Owner of CatamaranFreedom.com. A minimalist that has lived in a caravan in Sweden, 35ft Monohull in the Bahamas, and right now in his self-built Van. He just started the next adventure, to circumnavigate the world on a Catamaran!

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Beam and Draft

For many voyagers, trying to define of the "ideal" voyaging boat is one of the sport’s greatest debates. It is far easier said than done in that there are a large number of factors to be taken into consideration, many of them contradictory.

As a result, every boat is the result of a series of compromises that will differ according to the priorities of the person driving the decision-making process. At one extreme, performance under sail may be the overriding concern; at another, gunkholing in shallow anchorages may be the primary interest. These differing priorities should (if the yacht designer does his or her job) result in very different boats.

When exploring design choices, we can look at a number of commonly quoted numeric parameters that are often used to compare boats and their implications. One excellent place to start is with beam and draft calculations.Contemporary boat trends

Almost all voyaging boats, including world-girdling boats, spend the majority of their time either anchored out, on a mooring, or secured to a dock. At such times the boat is little more than a floating condominium. It is natural to want to make it as comfortable a floating home as possible. This, in turn, calls for space, and as a result yacht designers and boat builders are always under pressure to create as much volume as possible in any given design.

Volume nowadays typically translates into a wide beam, carried as far aft as possible, with high freeboard. The boat owner is sometimes going to want to be able to take this floating home into relatively shoal anchorages. This requires a shallow draft. To get a beamy boat with little draft, the boat must have a flat bottom. Even though this boat will probably not spend much of its life at sea, the builder and owner are still going to want it to perform reasonably well. A couple of keys to maximizing performance are to keep the overall weight, and thus the displacement, as low as possible (lightweight construction), and to minimize wetted surface area by using the minimum keel area necessary to achieve reasonable upwind performance (a fin keel), together with the minimum rudder size and supporting structure necessary to maintain control (a smallish spade rudder).

The kind of boat that is taking shape should be familiar; it can be seen at every major boat show. There is nothing wrong with this boat; it is built to fit a certain formula that is market driven, and by and large it does an excellent job of fitting this formula.

When it comes to voyaging boats, and indeed any boat that may be used offshore, we have to add at least one more criterion to the mix. This is the ability to safely deliver the crew, together with all stores and belongings, to its chosen destination in the worst conditions that might be encountered, and to do this at an acceptable speed and with as little discomfort as possible.

Among other things, this translates into a boat that is reasonably fast but with an easy motion at sea (a seakindly boat), that tracks well and has a light helm, that is stiff enough to carry sufficient sail area to keep moving to windward in heavy weather, and that has, in an extreme situation, the ability to claw off a lee shore under sail alone in heavy seas and gale-force winds. It must, of course, be built strongly enough to survive the gale.Form stability

Just about any boat can be pushed to windward in smooth water, but when things start to get rough it requires a great deal more power to counteract the boat’s windage and motion. Power requires sail area. Sail area requires a stiff boat- i.e, one that resists heeling: all the sail area in the world won’t do a bit of good if the boat rolls over and lies on its side!

One way to achieve stiffness is to increase beam. As the boat heels, the immersed volume shifts rapidly to leeward, keeping the boat more-or-less upright. This is known as form stability. A lightweight, beamy boat generally has excellent form stability. However, when the going gets tough the wide, flat sections, combined with the relatively light weight, are not only likely to make it pound and roll uncomfortably, but also will have a tendency to cause its keel to stall out. As it stalls out, if the boat has a relatively shallow draft and minimal lateral surface area in the keel and rudder, it will offer little resistance to making leeway. If it also has high freeboard, the windage will simply exacerbate problems. In other words, many of those features designed to improve comfort at the dock or on the hook, and to ensure a sprightly performance in relatively protected waters, can become a handicap. A less extreme design approach is needed. The first thing to reconsider is the wide beam.Length-to-beam ratio

The "beaminess" of a boat can be quantified by calculating its length-to-beam ratio – a number obtained by dividing the length by the beam. Often the length overall (LOA) – although in this case it should not include a protruding bow pulpit – and the maximum beam (Bmax) are used, although I prefer to use the waterline length (abbreviated to LWL) and waterline beam (BWL). Note that the two different formulas produce quite different values, so when making comparisons between boats it is essential to see that the same methodology is used to derive the numbers. For example, our Pacific Seacraft 40 has a LOA (excluding the bow pulpit) of 40.33 feet and a Bmax of 12.42 feet, giving a length-to-beam ratio using these numbers of 40.33/12.42 = 3.25 (note that the inverse ratio is sometimes given by dividing the beam by the length, in which case we get a beam-to-length ratio of 12.42/40.33 = 0.308). But if we use the waterline length (LWL) and waterline beam (BWL), we get a waterline length-to-beam ratio of 31.25/11.33 = 2.76.

As noted, for comparison purposes it is preferable to use the LWL and BWL to derive a waterline length-to-beam ratio, but unfortunately, although the waterline length is commonly published, the waterline beam is almost never published. As a result, yacht designer Roger Marshall, in The Complete Guide To Choosing A Voyaging Sailboat (published by International Marine, 1999) suggests that a way to use available data is to work with the waterline length and Bmax x 0.9, which will approximate the waterline beam on many boats (note, however, that when looking at a range of boats, I found this factor varied from as low as 0.75 to as high as 1.00, so this is a pretty crude approximation). When we apply these numbers to the Pacific Seacraft 40, we get: LWL/(Bmax x 0.9) = 31.25/(12.42 x 0.9) = 2.80. This is pretty close to the actual waterline length-to-beam ratio (2.76). Lower length-to-beam ratios indicate proportionately more beam; higher ratios less beam. A higher ratio is desirable both in terms of windward performance in difficult conditions, and also as an indicator of handling characteristics and seakindly behavior.Beam and stability However, this is not the whole picture. Beam affects stability on a cubic basis, which is to say that any increase in beam has a disproportionate effect on stability. If the length-to-beam ratio is kept constant, as length increases, the increase in beam needed to maintain a constant ratio produces a disproportionate increase in stability. For example, a 36′ LWL boat with a 3:1 ratio will have a 12′ waterline beam while a 48′ boat with the same ratio will have a 16′ beam; the 48′ boat will be considerably stiffer, even though it has the same ratio. What this means is that if two boats have the same length-to-beam ratios, the one with the longer waterline is likely to have greater stability and sail-carrying ability, and better performance to windward. Or, put another way, as length increases the syme relative sail-carrying ability can be maintained with a proportionately narrower beam and thus a higher length-to-beam ratio. As a result, to improve stability and sail-carrying ability, shorter boats need proportionately more beam, resulting in lower length-to-beam ratios. Consequently, there is no absolute length-to-beam ratio “magic number” that can be used for comparing boats; length must also be taken into account: the shorter a boat’s waterline length, the lower its length-to-beam ratio is likely to be. Nevertheless, when looking at the 35-foot to 45-foot boat range (the “norm” for offshore voyaging these days), for a comfortable offshore voyager I like to see a waterline length-to-beam ratio of 3.00 or higher (using LWL/[Bmax x 0.9]). Shorter boats may have a lower ratio; longer boats should have a higher ratio. Looking at a sampling of contemporary European and American boats (see table on page 88), we see that the only two boats below 40 feet LOA that have a ratio of over 3.00 are the Alerion Express 38 and the Shannon 39. At 40-feet and above, many of those boats that follow the current fashion of short overhangs, which maximizes the waterline length, have ratios of 3.0 and higher, whereas more traditional voyaging boats, with longer overhangs, for the most part do not. Our Pacific Seacraft 40, for example, has a waterline length-to-beam of 2.80. This is the price that has to be paid for its long overhangs combined with the beam necessary to provide a more spacious interior as compared to voyaging boat designs of a generation ago. Many older, but nonetheless highly successful, voyaging boats in this same size range have waterline length-to-beam of 3.0 and above (based on LWL/[Bmax x 0.9]). Steve Dashew, the designer of the Deerfoot and Sundeer series of boats, has taken the length-to-beam ratio to extremes. His boats commonly have ratios of 4:1, 5:1 and up. This is all to the good except that, because of the relatively narrow beam, in order to establish a reasonable interior volume, the boat has to get longer and the costs start to soar. He writes in the second edition of the Offshore Voyaging Encyclopedia that he and Linda, his wife and partner, decided to see just how small a boat they could design that would contain what they felt to be their minimum requirements for just the two of them, including accommodating a couple of guests for a week or two a year. They arrived at 56 feet in length! Unfortunately, however desirable it may be, such a boat is beyond the budget of most of us, not only up front but also in terms of mooring or dockage fees, gear replacement costs, maintenance, and so on.Keel types A narrower beam results in less form stability, which can translate into greater heeling when on the wind. To counteract this tendency to heel it’s necessary to put a lot of weight down low. In its extreme form this results in the 14-foot fin keels, with massive lead bulbs, seen on some narrow racing boats. Clearly, such a keel is not practical on a voyaging boat, but the principle is the same – to get as much weight as possible as low as possible. How low is primarily a function of where the boat is intended to sail. In general, a six-foot draft is acceptable, still allowing access to most of the world’s finest voyaging grounds. However, a boat specifically intended for voyaging in shoal areas such as the Bahamas might be designed with less draft, whereas one intended for Pacific voyaging might have a deeper draft. A voyager/racer, with an emphasis on the racing side of things, is likely to exceed six feet, trading access to some voyaging grounds for improved performance when racing. For a given draft, the use of a bulb keel keeps the weight as low as possible. A wing keel does the same, but needs to be carefully designed if it is not to foul lines and seaweed, or get stuck in the mud in a grounding. (A wing keel has a shape much like a Bruce anchor. Wing keels originated as a rule-beating device in the America’s Cup, and have since become something of a fad. I doubt that any advantage over a bulb keel outweighs the disadvantages in a voyaging environment.) On our new boat we chose a bulb-keel option, with a draft of five feet two inches, as opposed to the standard deep-keel of six feet one inch. We get a significantly reduced draft with a small loss of windward performance. The advent of bulb and wing keel types has pretty much put paid to the old debate as to whether it is preferable to have internal or external ballast: the bulb or wing must be external (it’s hard to mold them into fiberglass). Clearly, lead, with its great density, should always be used as the ballast material (as opposed to iron, which is sometimes used to save cost yet it’s only a little more than 60% of the density of lead). We’ve hardly started looking at the process of choosing an “ideal” voyaging boat, and already we are beginning to sense that there are a complicated series of trade-offs between, for example, beam and draft, interior accommodation and windward ability, and comfort on the hook and at sea. Based on my own experience, which is primarily bluewater voyaging, if I were to settle on two numbers that provide an acceptable beam and draft middle ground for 35- to 45-foot voyaging boats, it would be a waterline length-to-beam ratio of 3.0 or higher, and a draft of six feet or less. Longer boats should have a higher waterline length-to-beam ratio, and may require more draft.

By Ocean Navigator

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How Sailboats Measure Up

• By By Jeremy McGeary
• Updated: October 17, 2012

Sailboats by the Numbers

Boat reviewers rely on numbers to describe some of the key attributes of their subjects, such as length, beam, draft, and displacement. And while judgments on interior layouts and decor are subjective, these figures describing dimensions are not. There are, however, other numbers commonly cited in spec boxes that can prove more elusive, since they attempt to put a numerical value on how a sailboat might be expected to perform while under way. The commonly used ratios are sail area to displacement (SA/D), displacement to length (D/L), and ballast to displacement (B/D). And though they’re so commonly used that a certain amount of dogma has accrued around them, these figures can, in fact, be misleading, or at least misunderstood. And the result is that a boat can be assigned attributes based on numerical values that don’t take into account how sailboat design has changed over the past several decades.

Here, then, is a look at those ratios, what they attempt to describe, and how they should be interpreted when you go off exploring new and used models. (Click to page 2 for a more in-depth explanation.)

Sail Area/Displacement (SA/D)** An automobile buff seeking a high-performance ride looks for a high power-to-weight ratio and compares the horsepower/curb-weight ratios of different cars. For a sailboat, the SA/D provides the same metric. The horsepower comes from the wind on the sails and is proportional to the sail area; a boat’s weight is its displacement (in pounds, kilograms, or tons).

Initially, the SA/D only really gives a measure of potential acceleration rates (in case any physicists are reading this), but since displacement is a key factor in the resistance a boat encounters when moving through the water, SA/D also has a bearing on potential maximum speed.

The traditional calculation for SA/D compares sail area in square feet to displacement in cubic feet. In the formula, displacement in pounds is divided by 64 (the density of seawater) to obtain cubic feet, which are in turn converted to square feet to make the ratio unit-free.

On a spreadsheet, the formula would be S/(D/64) (2/3).

Nominally, the higher the SA/D, the more lively the boat’s sailing performance. The vessel will accelerate more quickly and have the potential for higher speed. But to be able to compare boats with any degree of precision (or fairness), we have to use similar numbers. The displacement must be in the same condition, either light ship (nothing on board) or fully loaded, and the sail-area measurement must reflect the normal working sail plan. Racing boats have measurement certificates from which these numbers can be reliably extracted. The specifications provided in cruising-boat brochures might not be consistent between builders, but we have to assume they are.

Boats measured in the 1970s and the 1980s for racing under the International Offshore Rule for the most part had SA/Ds between 16 and 17, based on the sum of the mainsail triangle (M = P E/2) and 100-percent foretriangle area (100%FT = I J/2). The measurement system favored small mainsails and large headsails, and since designers of cruising boats stuck close to the IOR sail plan, the IOR value for SA/D became the yardstick. An SA/D above 17 said “fast boat,” and anything below 16 said “slow boat.”

After the IOR fell out of favor, cruising-boat design drifted away from raceboat design, and sail plans began to change. Today, many boats are designed with large mainsails and small jibs, and most builders publish a “total sail area” number that includes the standard jib (often as small as 105 percent) and the roach in the mainsail (which is significantly greater on modern boats with full-battened mainsails than on IOR boats).

These builder-supplied numbers are more readily comparable against competing models, but using them in the SA/D formula makes the boats look “faster” than older models. This is a false comparison, because the sail area used for the older boats doesn’t include the extra area in, say, a 150-percent genoa.

The table “Sailboats by the Numbers” (see page 79) illustrates this. It shows SA/Ds calculated for a selection of modern boats and boats from past eras, all about the same length, using different numbers for sail area. For each model, it shows five SA/Ds. SA/D 1 is calculated using the sail area provided by the builder. SA/D 2 is calculated using M (P E/2) and 100% FT (I J/2). SA/D 3 is calculated using M + 105% jib. SA/D 4 is calculated using M + 135% jib. SA/D 5 is calculated using M + 150% jib. The only SA/D that includes mainsail roach is SA/D 1.

Let’s look at some examples. The 1997 Beneteau Oceanis 411 has a published sail area of 697 square feet on a displacement of 17,196 pounds. That gives an SA/D 1 of 16.7 (the same as SA/D 2), which for decades was considered very respectable for a cruising boat.

In 2012, the current Beneteau Oceanis 41 has a published sail area of 902 square feet (453 mainsail + 449 jib) and a published displacement of 18,624 pounds, to give an SA/D 1 of 20.5. Wow! Super-high performance! But this is for the standard sail area, with the 449-square-foot jib (just about 100% FT and typical of the trend today toward smaller jibs that tack easily). Plug in the calculation using I, J, P, and E and SA/D 2 drops to 18.9 because it doesn’t include mainsail roach, which is about 16 percent of the total published mainsail area.

Go back to the 1997 model, tack on a standard-for-the-day 135-percent genoa, and the SA/D 4 becomes 20.7. (If we added in mainsail roach, typically about 11 percent of base mainsail area before full-battened sails, we’d have 21.4.) The 1997 boat has essentially the same horsepower as the 2012 model.

Looking at current models from other builders, the SA/Ds based on published numbers hover around 20, suggesting that designers agree on the horsepower a cruising sailboat needs to generate adequate performance to windward without frightening anyone.

The two boats in our chart that don’t at first appear to fit this model are the Hunter 39 and the Catalina 385, but they’re not really so far apart.

The Hunter’s SA/D 2 is 16.1. Its standard jib is 110 percent (327 square feet), and the rest of the published sail area is in the mainsail—664 square feet, of which 37 percent is roach!

Catalina is a little more traditional in its thinking. If you add the standard 135-percent genoa, the SA/D becomes 21.2—right in the ballpark. (It’s still there at 19.7 with a 120-percent genoa.)

The table shows that, for boats targeted at the “performance cruising” market, the SA/D numbers using actual sail area lie consistently around the 20 mark. To go above that number, you have to be able to fly that sail area without reefing as soon as the wind ripples the surface. To do that, you have to elevate stability—with broad beam, lightweight (i.e., expensive) construction, deep bulb keels, and fewer creature comforts.

Displacement/Length (D/L)** While sailboat builders and buyers are interested in displacement in terms of weight, naval architects view it as volume; they’re creating three-dimensional shapes. When working in feet, to get a displacement in pounds, they multiply cubic feet by 64, the density in pounds per cubic foot of seawater. (Freshwater boats displace more volume because the density of fresh water is only 62.4.) The D/L ratio is therefore a measure of immersed volume per unit of length—how tubby the hull is below the waterline.

According to conventional wisdom and empirical studies, the lower the D/L, the higher the performance potential. This is mainly due to wavemaking resistance being lower for slender hulls than for tubby hulls.

In the D/L formula, displacement in pounds is divided by 2,240 to convert it to tons to bring the values to manageable numbers, so D/L is displacement in tons divided by .01LWL (in feet) cubed.

In a spreadsheet, the formula would be D/(2240*(.01L)3), where D is the displacement in pounds and L is LWL in feet.

In the early days of fiberglass boats, the Cruising Club of America rule was the principal dictator of boat shapes. Because it was a waterline rule, designers kept waterlines short to keep ratings low and relied on long stern overhangs immersing to add “sailing length” when the boats heeled. Carbon fiber was available only to NASA, and boats had full interiors, so “light displacement” wasn’t really in the cards. A D/L of 300 was considered dashing, even risky. Many still-popular designs from the 1970s and 1980s have D/Ls as high as 400; see the Bounty II.

Fast-forward 40 years. Boats now have plumb bows and plumb sterns and waterlines almost as long as their LOAs—there are no rating penalties on a cruising boat. The boats’ weights haven’t changed much because, although builders try to save weight to save cost, the boats are so much bigger. The hull and deck surface areas are greater, and all that extra internal volume can be filled with furniture. The effect on D/L ratios has been drastic—just look at the table. A D/L ratio above 200 today describes a heffalump.

But do these lower D/Ls actually buy you any more speed? Yes and no.

Yes : Because speed is proportional to the square root of the waterline length. Today’s 40-footer has a much longer waterline than yesterday’s and ought to sail as fast as yesterday’s 50-footer. It might also benefit from reduced resistance due to a smaller cross-sectional area, but it also might have greater wetted-surface drag due to the longer immersed length. When sailing downwind in waves, though, the lower-D/L boat will surf more readily.

No : Because, as we saw above, the power-to-weight ratios (SA/D) of modern boats aren’t effectively any higher, and certainly aren’t in the realm that would allow our cruising sailboats to climb out of the displacement zone and plane. In most conditions, the lower-D/L boat is still trapped in its wave.

In the days of the IOR, a D/L of 250 was still pretty racy; see the 1978 Catalina 38. Today, even a D/L as low as 150 doesn’t make a boat a speedster if it can’t carry the sail area to make it so. To compete at a level with a Volvo 70, look for a D/L of about 40 and an SA/D of 65.

Ballast/Displacement (B/D)** The ballast/displacement ratio is simply the ballast weight divided by the boat’s total displacement. Since ballast is there to give the boat stability, it’s easy to jump to the conclusion that the higher the B/D, the stiffer the boat.

However, B/D doesn’t take into account the location of the ballast.

Take a boat that has a total displacement of 20,000 pounds and put its 8,000 pounds of ballast in the bilge. Now take the same boat and put the 8,000 pounds of ballast 4 feet deeper in a bulb at the bottom of a deep fin keel. Same ballast ratio (0.4), but very different stability.

When looking at B/D, therefore, we must ask about the configuration of the keel: How low is the ballast?

Stability analysis is complex and involves beam, hull cross-section, and length, among other factors, of which B/D is just one.

Since the late 1990s, builders of sailboats intended for sale in the European Union have been required to provide stability data, including a curve of righting arm at angles of heel from 0 to 180 degrees—far more information than anyone can divine from a B/D number and a much more useful measure of a boat’s inclination to stay upside down in the unlikely event (the way most people use their boats) that it exceeds its limit of positive stability.

CW contributing editor Jeremy McGeary is a seasoned yacht designer who’s worked in the naval-architecture offices of David Pedrick, Rodger Martin, and Yves-Marie Tanton and as a staff designer for Camper & Nicholson.

To read the related article, How To: Measure Sail Area, click here.

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Catamaran Design Formulas

• Post author By Rick
• Post date June 29, 2010
• 10 Comments on Catamaran Design Formulas

Part 2: W ith permission from Terho Halme – Naval Architect

While Part 1 showcased design comments from Richard Woods , this second webpage on catamaran design is from a paper on “How to dimension a sailing catamaran”, written by the Finnish boat designer, Terho Halme. I found his paper easy to follow and all the Catamaran hull design equations were in one place.  Terho was kind enough to grant permission to reproduce his work here.

Below are basic equations and parameters of catamaran design, courtesy of Terho Halme. There are also a few references from ISO boat standards. The first step of catamaran design is to decide the length of the boat and her purpose. Then we’ll try to optimize other dimensions, to give her decent performance. All dimensions on this page are metric, linear dimensions are in meters (m), areas are in square meters (m2), displacement volumes in cubic meters (m3), masses (displacement, weight) are in kilograms (kg), forces in Newton’s (N), powers in kilowatts (kW) and speeds in knots. 

Please see our catamarans for sale by owner page if you are looking for great deals on affordable catamarans sold directly by their owners.

Length, Draft and Beam

There are two major dimensions of a boat hull: The length of the hull L H  and length of waterline L WL  . The following consist of arbitrary values to illustrate a calculated example. 

L H  = 12.20      L WL  = 12.00

After deciding how big a boat we want we next enter the length/beam ratio of each hull, L BR . Heavy boats have low value and light racers high value. L BR  below “8” leads to increased wave making and this should be avoided. Lower values increase loading capacity. Normal L BR  for a cruiser is somewhere between 9 and 12. L BR  has a definitive effect on boat displacement estimate.  

• Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

10 replies on “Catamaran Design Formulas”

Im working though these formuals to help in the conversion of a cat from diesel to electric. Range, Speed, effect of extra weight on the boat….. Im having a bit of trouble with the B_TR. First off what is it? You don’t call it out as to what it is anywhere that i could find. Second its listed as B TR = B WL / T c but then directly after that you have T c = B WL / B TR. these two equasion are circular….

Yes, I noted the same thing. I guess that TR means resistance.

I am new here and very intetested to continue the discussion! I believe that TR had to be looked at as in Btr (small letter = underscore). B = beam, t= draft and r (I believe) = ratio! As in Lbr, here it is Btr = Beam to draft ratio! This goes along with the further elaboration on the subject! Let me know if I am wrong! Regards PETER

I posted the author’s contact info. You have to contact him as he’s not going to answer here. – Rick

Thank you these formulas as I am planning a catamaran hull/ house boat. The planned length will be about thirty six ft. In length. This will help me in this new venture.

You have to ask the author. His link was above. https://www.facebook.com/terho.halme

I understood everything, accept nothing makes sense from Cm=Am/Tc*Bwl. Almost all equations from here on after is basically the answer to the dividend being divided into itself, which gives a constant answer of “1”. What am I missing? I contacted the original author on Facebook, but due to Facebook regulations, he’s bound never to receive it.

Hi Brian, B WL is the maximum hull breadth at the waterline and Tc is the maximum draft.

The equation B TW = B WL/Tc can be rearranged by multiplying both sides of the equation by Tc:

B TW * Tc = Tc * B WL / Tc

On the right hand side the Tc on the top is divided by the Tc on the bottom so the equal 1 and can both be crossed out.

Then divide both sides by B TW:

Cross out that B TW when it is on the top and the bottom and you get the new equation:

Tc = B WL/ B TW

Thank you all for this very useful article

Parfait j aimerais participer à une formation en ligne (perfect I would like to participate in an online training)

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Sailboat Specifications 101: Explained For Beginners

As a newbie to sailing, the sailboat specifics can be overwhelming. Taking time to familiarize yourself with the measurements and vocabulary associated with boats will allow you to be more informed about boats and know which type is right for which activity. You’ll be a better sailor and boater with this knowledge.

Table of Contents

LOA – Length Overall

Length Overall (LOA) is the most common measurement used to describe the size of a sailboat. It refers to the total length of the vessel, from the tip of the bow (front) to the aft end of the stern (back).

LOA is typically measured in feet or meters. This measurement can be useful when comparing boats of similar types, as it gives you an idea of the overall size.

LOD-length on deck

LOD, or Length on Deck, is the measurement of the boat from the tip of the bow to the stern along the deck.

This length does not include any spars, bowsprits, antennas, etc. that are mounted above the main deck.

The difference between LOD and LOA (length overall) is that LOA takes into account any protrusions such as spars and bowsprits. LOD may be shorter than LOA sometimes.

LWL – Load Waterline Length

The LWL or Load Waterline Length is the measurement of the length of a boat at the point where it touches the water.

It is the length of the boat that makes contact with the water and is often shorter than the overall length (LOA) due to the curvature of the hull.

The LWL plays an important role in determining the performance of a sailboat; for example, a longer LWL can help increase stability and reduce drag, allowing the boat to move more efficiently through the water.

The LWL also affects the size of the sail area needed to power the boat. As such, boats with a longer LWL will require larger sails to generate adequate power, while boats with a shorter LWL may need smaller sails.

Beam – The width of the boat

The beam of a sailboat is the maximum width of the hull and is an important measurement for sailing performance.

A wider beam provides more stability on the water and increases the overall sail area. Having a larger sail area will help to increase speed and maneuverability in windy conditions.

It’s important to consider the beam of the boat when deciding what type of sails to use. A boat with a wider beam will require bigger sails, while a boat with a narrower beam will require smaller sails.

Draft – The depth of the boat in the water

Draft measures the distance from the waterline to the lowest point of the boat’s hull when it is fully loaded.

This is important because it affects the boat’s maneuverability, stability, and performance in different sea conditions.

It also affects the sail area of the boat, since a greater draft can provide more stability and lift, allowing for larger sails to be used. Shallow draft boats tend to be able to get into shallower waters than those with deeper drafts.

Full keel vs. modified keel vs. fin keel

The three main types of keels are full, modified, and fin keels.

Full keels are the oldest and most traditional type of keel. They are typically found on heavier displacement boats such as cruisers and larger sailboats.

Full keels provide more stability due to their size and weight, but also create more drag, which can slow down the boat.

Modified keels are a hybrid between full and fin keels. They are often used on boats with moderate displacement, meaning they have a moderate amount of weight.

Modified keels provide a good balance of stability and speed due to their shape and size.

Finally, fin keels are usually found on lighter displacement boats such as racing and performance sailboats.

Fin keels have the least amount of drag, allowing the boat to move faster, but they are not as stable as full or modified keels.

Displacement – The weight of the boat

The displacement of a boat refers to the total weight of the boat, including all of the materials used to construct it. It is usually measured in either metric tonnes or long tons.

The type of displacement your boat has will depend on its size and purpose, with light displacement boats usually being used for day sailing and racing, while moderate and heavy displacement boats are better suited for coastal and ocean cruising.

Light displacement boats are typically quite lightweight, with a hull weight of around 2 tonnes and a total weight of 4 tonnes or less.

These boats are often very fast and agile but can have limited load-carrying capacity due to their light construction.

Moderate displacement boats typically weigh between 4 and 10 tonnes, with a hull weight ranging from 3 to 8 tonnes.

These boats are best suited for coastal cruising and are usually made from heavier materials than light displacement boats. This makes them able to carry a greater load and handle rougher seas with more confidence.

Heavy displacement boats weigh more than 10 tonnes, with a hull weight of up to 15 tonnes.

These boats are built for long-distance ocean cruising and are designed to be sturdy and reliable even in heavy weather. As such, they are usually made from stronger materials than other types of boats and have a much larger load-carrying capacity.

D/L or DLR ratio- Displacement to length ratio

Displacement to length ratio (DLR) is a calculation used to measure the size of a sailboat.

It is determined by dividing the displacement (the weight of the boat) by the waterline length (the length of the boat that is in contact with the water when it is afloat).

The result of this calculation, also known as the DLR, can be used to compare different types of boats or to determine which type of sailboat is most suitable for specific conditions.

The formula for calculating the displacement-to-length ratio is: DLR = (Displacement/2240)/(0.01xLWL)^3 Displacement in pounds, LWL is Waterline Length in feet

Generally, sailboats with higher DLRs tend to have a more rounded hull shape and are better suited to deep-water sailing in heavy weather conditions.

Sailboats with lower DLRs tend to have a more slender hull shape and are better suited to shallow water sailing in light weather conditions.

Ballast is the weight of the boat that is not part of the boat’s structure. This weight can come from a lead, water, or other materials, and it is located in the bottom of the boat to help keep it stable in the water.

The amount of ballast affects the sail area, as more ballast will lower the sail area while decreasing ballast will increase the sail area.

This is because when there is more ballast in the boat, it will be pushed down into the water which reduces the area that a sail can reach. On the other hand, decreasing ballast will allow a sail to extend further.

Ballast is also important for maneuverability and stability; too much ballast and the boat will be sluggish and difficult to turn, while too little ballast could cause the boat to be unstable and even capsize.

Balancing the amount of ballast is key to achieving optimal performance for any type of sailboat.

CSF-Capsize screening formula

The capsize screening formula is a calculation that provides a good indication of the stability of a sailboat. It is the ratio of a boat’s displacement (weight) to its Beam (width).

Capsize ratio formula: Beam / ((Displacement/64.2)1/3) The beam is in feet. Displacement is in pounds

A good capsize ratio is generally considered to be between 1.33 and 2.0, although this can vary depending on the type of boat and its purpose.

A lower capsize ratio indicates that the boat is more stable, as it will be less likely to tip over in strong winds or waves. A higher capsize ratio indicates that the boat is more prone to capsizing.

Motion comfort ratio

Motion comfort ratio (also referred to as “Ted Brewer” ratio) is a measure of the overall stability of a sailboat.

Generally, a boat with a motion comfort ratio greater than 40 is considered stable and a boat with a motion comfort ratio less than 30 is considered unstable.

A boat with a motion comfort ratio between 30-40 is considered moderately stable. The higher the motion comfort ratio, the more comfortable the boat will be in rough waters.

Ted Brewer’s CR formula is: Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x Beam 1.333 ).

For instance, a boat with an LWL of 35 ft and a displacement of 10,000 lbs would have a motion comfort ratio of 37.5. This would indicate that the boat is moderately stable and should provide an adequate level of comfort in rough waters.

The motion comfort ratio was developed by Ted Brewer and has been used for many years as an indication of a boat’s stability.

It is important to keep in mind, however, that this ratio alone cannot give an accurate picture of how stable a boat is. Other factors such as hull type and keel type should also be taken into account when assessing a boat’s stability.

Ballast to displacement ratio

The ballast-to-displacement ratio is a measure of how much ballast is needed in relation to the weight of the boat.

The higher the ballast-to-displacement ratio, the more stable the boat will be and the less likely it will be to capsize.

the ballast-to-displacement ratio is important for ensuring the boat is adequately balanced and has good performance when sailing.

It is especially important for boats that have large sail areas, as larger sail areas require more ballast to keep the boat steady.

When considering a boat’s ballast-to-displacement ratio, keep in mind that a ratio of 40-50% is generally considered to be optimal. Any higher than that may be too much, while any lower may not be enough.

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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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Length-beam ratio

L/B = length divided by beam.

Units: Dimensionless.

Usually, the waterline dimensions L WL and B WL are used for monohulls, or for a single hull of a multihull.

What it's used for

Performance.

Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a given length.

If a boat can plane, smaller L/B often suggests more efficient performance at low planing speeds. The balance generally tilts in favour of high L/B for fast boats.

Typical ranges of L/B are:

2 to 4 - Small to mid-size planing powerboats.

3 to 4 - Most small to mid-size sailboats and motor yachts, the longer ones generally having higher L/B. Some "skimming dish" racing sailboats also have L/B in this range; their wide beam gives them more initial stability so that they can fly larger sails.

4 to 6 - Fairly long and lean for a monohull. Some large, efficient long-range cruisers fall in this range, along with many racing monohulls.

6 to 10 - Large freighters; main hulls of cruising trimarans; a few very portly cruising catamarans; the lightest and slimmest of large sailing monohulls.

10 to 16 - Fast cruising cats and tris; a few racing multihulls.

Over 16 - Racing multihulls. Such high L/B is conducive to very light, low-drag hulls for race boats, but makes it very hard to get enough room inside the hulls for equipment or living space.

Living Space

If a boat is going to spend most of its time in a marina or at anchor, relatively low L/B implies a larger, more spacious interior and increased carrying capacity when compared to slimmer competitors of the same length. For a boat that must entertain guests at the dock but will rarely be used in rough weather or at high speeds, this may be a significant advantage. The slimmer boat, though, will generally have the advantage when fuel is expensive or when the weather picks up.

Topic: 

• Boat Design

Principles of Yacht Design Sen

Its Loa/Bmax is thus 3.25. For an Lwl in the light condition of 9.85 m this corresponds very well with the median line of Fig 5.32. In fact, for a new design the hull is slightly narrow, since new hulls are often a bit beamier than the median according to the figure. The data in the statistical analysis of this section may be considered representative of the yacht fleets in Europe and the United States in the early 1990s, and may therefore represent an average of design trends in the 1980s and to some extent in the 1970s also.

Length of water tin e/draft (LyyL/T)

LWI/T is plotted versus Lwl in Fig 5.jj. Obviously, this ratio increases with length as well. A larger yacht has a larger ratio, ie a smaller relative draft. In fact, beam is a better scaling factor than length for the draft of a sailing yacht, and a good approximation is BMAX = 1.6 - T, which is valid more or less for all lengths. This relation corresponds very well to the median line in Fig 5.33. The upper and lower limits in this case are 15% from the median line.

Fig 5.33 Length/draft ratio

The choice of draft for a cruising yacht is a trade-off between performance and practical advantages, like the possibilities of entering more shallow water areas, ease of handling ashore etc, while for a racing yacht draft is penalized to cancel the performance advantage. The YD-40 has an Lwl of 9.85 m and a draft of 2.04 m in the light condition. This yields an Lwl/T of 4.83. According to Fig 5.33, the median for this size is 5.2, which yields a draft of 1.89. The extra 0.15 m will give the YD-40 an edge upwind, consistent with the desire to create a fast cruiser/racer.

Length of Since most modern yachts have fin keels it is possible in most cases to water line/canoe body define the canoe body draft. This seems to scale very well with length, draft (Lwl/TJ as can be seen in Fig 5.34. A typical value of Lw,/Tc is 18 for a medium displacement yacht. The ultra light dinghy type racing

machines may reach values up to 26, while heavy displacement, narrow hulls may have as small an Lwl/Tc as 12. For the ultra light hulls data are available only for large waterline lengths. The YD-40 has an Lwl/Tc close to the medium.

As explained above the length/displacement ratio is a very important

Fig 5.34 Length/hull draft ratio

Length/displacement ratio (Lwl/V'a)

quantity for the resistance of the yacht at high speeds. To enable the yacht to exceed a Froude number of about 0.45, ratios above about 5.7 are required. In Fig 5.35 the length/displacement ratio is plotted versus waterline length.

Since beam and draft do not increase linearly with length, displacement

Length/displacemenl ra tio

increases slightly slower than length cubed. In fact, with the same assumptions as above, the displacement increases as (length)7/?, which means that the length/displacement ratio increases as (length)2/». Increasing the length by a factor of two increases the ratio by 17%. The increase is not quite as fast in the statistical data, as may be seen in Fig 5.35.

As was the case in the length/beam ratio the spread is asymmetric. The lower limit in this case is some 12% below the median line, while the upper limit is put 20% above the median. There "are.- however, certain kinds of hulls outside the limits. Thus, some extreme ultra light yachts have considerably higher ratios, and since the statistics are based mainly on yachts which may participate in some kind of racing (performance handicapping system, IMS or 10R), some heavy cruising yachts may have been missed.

The length/displacement ratio is, of course, quite different between a racer and a cruiser, since the equipment required for comfortable living on board is rather heavy. In the case of the YD-40 we have tried to create a cruiser/racer with full comfort. Its length/displacement ratio is 5.16, which is close to the median for a 10 m Lwl yacht.

The overhangs of modern hulls have decreased as compared to hulls designed before the 1960s. To a certain extent this is a matter of fashion, but there is also an attempt to reduce the longitudinal gyradius as much as possible for a given (effective) waterline length. The inverse slope of the transom is another effect of this effort.

A medium value of Loa/Lwl for modern yachts is 1.23 with a spread of 0.15 up and down. There is no discernible trend with hull length . The YD-40 is very close to the median: L0A/LW, is 1.22.

Freeboard height It is a well-known fact that the relative freeboard height decreases with hull length. Obviously this is due to the requirements of the accommodation. Even on very small yachts headroom for moderately tall people is required. The trend is shown in Fig 5.36, which shows the freeboard forward versus the waterline length. No upper and lower limits arc given, since the statistical basis for this graph is smaller than for the others above (only about 50 yachts).

A typical value of freeboard forward/freeboard aft is 1.3. As compared to older yachts this is lower, so modern yachts have a more horizontal sheer line. Both the forward and aft freeboards are higher however, and the camber of the sheer line, the 'spring', is smaller. The YD-40 is representative of modern cruiser/racers and has somewhat higher freeboards than the statistical mean value, which is influenced to a certain extent by some older designs. The freeboard forward/ waterline length is 0.144, while the mean value is 0.138 for this size of hull, and the ratio of the two freeboards is 1.22.

Length overall/length of waterline

Ballast ratio

The ballast ratio, ic the ratio of keel weieht to total weieht, varies

considerably on modern yachts. A good average value is 0.45 and most yachts lie within the range 0.35-0.55 (see Fig 5.37). There docs not seem

Fig 5.36 Freeboard forward/ length ratio

Continue reading here: Keel and Rudder Design

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Readers' Questions

What length boat is considered a yacht?

There is no specific length that officially categorizes a boat as a yacht. However, generally, a boat is considered a yacht when it is at least 33 feet (10 meters) in length.

What length is considered a yacht?

A yacht is generally considered to be a recreational watercraft that is at least 40 feet (12 meters) in length.

What is beam length on a boat?

The beam length of a boat can vary, depending on the size and type of boat. For example, the beam length of a small sailboat may be 9-10 feet, while a large powerboat or yacht may have a beam length of up to 25 feet or more.

How to design ship's draft boatbuilding?

Select a hull type: The first step in designing a ship's draft is to select a hull type. Common hull types for shipbuilding include displacement hulls, semi-displacement hulls, planing hulls, and catamaran hulls. Choose a draft: The draft is the depth of the waterline below the surface of the water. Generally, the deeper the draft, the larger the ship and the more cargo it can carry. Calculate the stability: Stability is an important factor in ship design, as it impacts the ship's center of gravity and its ability to resist leaning or tipping. Calculate the trim: The trim is the angle at which the bow and stern of the ship sit relative to the waterline. It is important for ships to have the proper trim to ensure both stability and performance. Calculate the position of the center of buoyancy: The center of buoyancy is the point at which the ship maintains buoyancy and stability in the water. The position of the center of buoyancy must be calculated in order to effectively design the ship's draft. Calculate the displacement: The displacement is the amount of water displaced by the ship when it is in the water. The displacement of a ship is directly related to its draft and is used to calculate its total weight. Calculate the resistance: Resistance is the amount of force that a ship experiences when it moves through the water. The resistance must be taken into account to ensure that the ship is able to move efficiently through the water. Calculate the manoeuvrability: Manoeuvrability is the ability of the ship to change direction quickly and smoothly. The manoeuvrability of a ship is directly related to its draft, so it is important to consider when designing a ship's draft.

What is the average draft ratio of a yacht?

The average draft ratio of a yacht can vary significantly depending on the type and size of the yacht. Generally, it will range from approximately 1.5 feet to 8 feet.

Sail Calculator

Go Directly To The Sail Calculator Here

What Carl’s Sail Calculator Does:

Physicist and sailor Carl Adler developed this online Sail Calculator for comparing sailboats  and its database has grown over a number of years to almost 3000 boats. It should be one of the first places you go on the Web if you want to know the vital statistics about a sailboat, including Length Overall (LOA) , Length on the Waterline (LWL) , Displacement and Sail Area .

The Sail Calculator will also give you valuable performance numbers for any vessel in its database or any numbers you enter, including the Displacement / LWL ratio, Theoretical Limiting Hull Speed, Sail Area / Displacement ratio, Length to Beam ratio, Motion Comfort value, Capsize Screening value, sailing category and Pounds per inch immersion value .

Naval architects use these values when they design a new boat, and from them you can determine a conventional displacement hull boat’s purpose and predict its performance. Note that planing hulls, catamarans and hydrofoil vessels are not defined in the same way. Here’s what the performance numbers mean:

Displacement/LWL ratio – Heavy boats (D/L above about 300) will carry big loads but require plenty of power to drive. Light boats (D/L below about 150) are generally quicker and more responsive but are affected by loading. Most boats have moderate displacement and they compromise the conflicting virtues of the extreme designs. Contemporary racing boats often have D/L ratios well below 100.

Hull Speed – A conventional hull, which moves through the water rather than rising atop it and planing across the surface, is limited in speed by length of the waves it produces; long waves travel faster. This wave length can be calculated and the top speed of the hull predicted. Long boats make long waves.

Sail Area / Displacement ratio – The SA/D ratio is like the power/weight ratio of an automobile. A high SA/D ratio (> about 18) indicates a powerful rig, while a low ratio indicates a more docile boat.

Length / Beam ratio – A long, narrow hull with limited interior space is easier to drive than a short, fat one with plentiful capacity. Compare L/B ratios to gain insight into the purpose of the boat.

Motion Comfort value – Not as widely used as the previous numbers, the Motion Comfort value tries to predict whether a boat has a quick, motion through the waves or a slow, easy motion. Note that some people get more seasick with a slowly rolling motion than a quick, jerky one. Your mileage will vary.

Capsize Screening number – Developed after the Fastnet Race tragedy, the Capsize Screening number is a quick way to judge if a boat is seaworthy. Values below 2.0 are desirable for offshore yachts. Do not put too much faith in the exact number, as it is an approximation only.

Pounds / square inch Immersion – When you load a boat, it sinks deeper into the water. This Immersion value indicates the weight carrying capacity of a vessel.

There is also a Prop Sizing section which will calculate the optimum propeller to use on any displacement-hull boat, based on noted naval architect Dave Gerr’s formulas.

 To The Sail Calculator

19 Comments on “ Sail Calculator ”

I corrected it. Thanks!

S2 7.3 specs from factory brochure (visible at boatbrochure DOT com SLASH products SLASH s2-7-3-meter-brochure the free preview is pretty legible)

LWL is 18.5 not 18.73 Beam is 8.0 not 8.5 Displ is 3250 not 3373 S.A. is 255, not 261

Thank you so much for your work maintaining this web page; it is tremendously valuable resource that I refer to often!

Tom, The Colgate 26 has a sail area different from that published on your calculator. It’s listed as 338 SF per https://www.colgate26.com/specifications/

Chris, Thanks for the note; I’m glad you find the Sail Calculator useful. I’ll change the value on the database on the next update, since your source is probably more accurate. I rarely know what the sources are when a user submits data, so there are definitely errors in there. It’s possible that one of the numbers is based on the 100% foretriangle measurement and the other is with a larger jib, which could be either the working jib or a Genoa. I get this question from time to time and probably should add something to the description about it (www.tomdove/blog/sail-calculator/). –Tom

Hi Tom, thanks for Carl’s calculator alive. I have a Tayana 48 DS and from their website, I get a different sail area. 1316 sq ft vs 1048.

Regards, Chris

Hi Tom, Looking at your specs the Marieholm 26 literature does not match what is posted. There were 3 versions of this boat built with the Marieholm being the middle one. The Folkboat website shows this: loa 25.83, lwl 19.83, beam 7.17, s.a. 280 sq. ft., draft 4′, disp. 4740, ballast 2750. The 1st model was built from wood, the last (3rd) model is called the Nordic Folkboat built from fiberglas but made with lapstrack design to look like wood. It was heavier in weight than the other 2 with less s.a.. Google “Folkboats Around the World” and the info is there on the main page. Hope this helps. Like to see values once new info is inserted. Wish I could figure it myself but not sure how to. Thank you, Sam

Thanks for catching that. I’ll correct it on the next update. — Tom

The Goletta Oceanica De Biot 39 is missing a decimal point in the LWL so it is throwing off all of the calculations.

David, There’s no simple answer to that. If you enjoy sailing the boat, it’s a good one. When you put the numbers into SailCalculator, it will return some basic information that can be very useful, but note that small, lightweight boats like the American 23 are sensitive to loading. The working displacement is actually the “Light Ship” displacement plus the weight of the average crew. Try adding the weights of you and your crew in SailCalculator and see how that affects the performance numbers. The people I have known who have the American 23 like it. It looks like a nice, stable daysailer. Enjoy! — Tom

I have American sailboat 23 ft. sailboat with a displacement 3500 lbs my keel is a 1000 lbs with a beam 7ft and 11 inch just wondering how good is this boat for sailing thanks

Mark, Very interesting. I can see why the Length/Beam ratio at the waterline would be the defining characteristic for hull speed. That can be an evasive number, I think. Multis with very narrow hulls will sink deeper into the water quickly as the boat is loaded, so the LWL/BWL could change dramatically. It seems that you’d have to be careful about specifying the displacement that produces a specific LWL/BWL ratio, don’t you think? Is there an issue of one hull being submerged more than the other when the boat is under sail? This seems especially important in trimarans, which often have one hull flying and the other deeply submerged, but a long, narrow cat would have some of the same response to a breeze. Keep me posted on your thoughts. I think you’ve hit on a key element here. — Tom

Hello, I’m a mechanical engineer and experienced multihull sailor that has long thought multihulls need a better performance parameter for comparison so sales guys can’t hoodwink people! I have some graduate school education from Dr. Marshall Tulin (UCSB) who has published many works regarding high-speed displacement mode for long slender hulls for naval/military applications and I think this work is very applicable to sailing multihulls. The critical parameter as far as hull drag for catamarans is really L/B at the waterline since other parameters as far as hull form go (prismatic coefficient) are generally within a narrow range. It has the benefit of implying displacement and waterline length as well, since a heavy boat must be either fat, or long to carry the displacement. As a result, I’ve been working on a parameter that includes both sail area and L/B at the waterline for performance comparisons. The trouble is Schionning is one of the few designers that cites L/B in all of his designs but it would be an easy measurement to take dockside, when the true displacement isn’t known.

Steve, I’ve never seen that formula but would love to have it. The speed of a multihull is largely a factor of the hull shapes, and most multis are not limited by the “Displacement Hull Speed” that determines the maximum speed of most monohulls. The hulls are generally long and narrow and do not create the speed-limiting waves. There are exceptions, and I think any formula that predicts the speed of a cat or tri would have to incorporate prismatic coefficient (“sharpness”). Most boats are not speed-limited by their sail area.

I’m looking for a formula that predicts potential performance of a cruising catamaran, in teh same way that SA/D does for monohulls. I saw teh formula years ago – it uses sail area and the second power (i.e., the square) of a factor, but I don’t recall anything else. Can you help me? -Steve

Charlie, Thank you for the compliment. I enjoy running the site and meeting so many people who love sailing. Good luck on your boat search; there are many good deals on used, mid-size cruising boats available now in the U.S. because builders flooded the market with 40-footers a few years ago. Now that so many Baby Boomers have finished their lifetime sailing adventures, the boats are for sale.

I’ve just been introduced to your site by a good friend from the US. Im looking for a retirement live aboard that can take me around the world. He gave me a potted history and speaks very highly of Carl Alder in this site in general. What a great tool. I’ll be flying to the states to view some boats that otherwise wouldn’t have even been on my Radar. Thank you Tom, Thank you Carl (Thank you Harvey).

Thanks for keeping Carl’s program alive, Tom. I sent him hundreds of small boat specs over the years and found quite a few errors from other inputs that Carl tried to correct. Between his poor vision, a lot of incorrect input (especially the difference between LOD versus LOA for most people) and the vague info from boat builders it was a long process. People should have supported him with far more donations, he was a good guy. Les Hall, San Antonio

Thanks for catching that, Paul. Yes, that would be a mighty powerful boat. It appears that the displacement should have been 14,500, so I corrected that. The SA/D ratio still looks a bit high, but I don’t know what the submitter used as a source. Enjoy the site and please send any other corrections you see. — Tom

Cavalier 37, LWL=30, Sail Area to Displacement=2314.05 Cant possibly be correct Great calculator, thanks for keeping it available. Cheers

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For IEEE Members

Ieee spectrum, follow ieee spectrum, support ieee spectrum, enjoy more free content and benefits by creating an account, saving articles to read later requires an ieee spectrum account, the institute content is only available for members, downloading full pdf issues is exclusive for ieee members, downloading this e-book is exclusive for ieee members, access to spectrum 's digital edition is exclusive for ieee members, following topics is a feature exclusive for ieee members, adding your response to an article requires an ieee spectrum account, create an account to access more content and features on ieee spectrum , including the ability to save articles to read later, download spectrum collections, and participate in conversations with readers and editors. for more exclusive content and features, consider joining ieee ., join the world’s largest professional organization devoted to engineering and applied sciences and get access to all of spectrum’s articles, archives, pdf downloads, and other benefits. learn more →, join the world’s largest professional organization devoted to engineering and applied sciences and get access to this e-book plus all of ieee spectrum’s articles, archives, pdf downloads, and other benefits. learn more →, access thousands of articles — completely free, create an account and get exclusive content and features: save articles, download collections, and talk to tech insiders — all free for full access and benefits, join ieee as a paying member., boats should be sleek—but only up to a point, the length-to-beam ratio has risen over the centuries, but there are still practical limits.

In comparison with Moore's Law , the nonsilicon world's progress can seem rather glacial. Indeed, some designs made of wood or metal came up against their functional limits generations ago

The length-to-beam ratio (LBR) of large oceangoing vessels offers an excellent example of such technological maturity. This ratio is simply the quotient of a ship's length and breadth, both measured at the waterline; you can think of it simply as the expression of a vessel's sleekness. A high LBR favors speed but restricts maneuverability as well as cargo hold and cabin design. These considerations, together with the properties of shipbuilders' materials, have limited the LBR ratio of large vessels to single digits.

If all you have is a rough wickerwork over which you stretch thick animal skins, you get a man-size, circular or slightly oval coracle —a riverboat or lake boat that has been used since antiquity from Wales to Tibet. Such a craft has an LBR close to 1, so it's no vessel for crossing an ocean, but in 1974 an adventurer did paddle one across the English Channel.

Building with wood allows for sleeker designs, but only up to a point. The LBR of ancient and medieval commercial wooden sailing ships increased slowly. Roman vessels transporting wheat from Egypt to Italy had an LBR of about 3; ratios of 3.4 to 4.5 were typical for Viking ships , whose lower freeboard—the distance between the waterline and the main deck of a ship—and much smaller carrying capacity made them even less comfortable

The Santa María , a small carrack captained by Christopher Columbus in 1492, had an LBR of 3.45. With high prows and poops, some small carracks had a nearly semicircular profile. Caravels , used on the European voyages of discovery during the following two centuries, had similar dimensions, but multidecked galleons were sleeker: The Golden Hind , which Francis Drake used to circumnavigate Earth between 1577 and 1580, had an LBR of 5.1.

Little changed over the following 250 years. Packet sailing ships, the mainstays of European emigration to the United States before the Civil War, had an LBR of less than 4. In 1851, Donald McKay crowned his career designing sleek clippers by launching the Flying Cloud , whose LBR of 5.4 had reached the practical limit of nonreinforced wood; beyond that ratio, the hulls would simply break.

A high LBR favors speed but restricts maneuverability as well as cargo hold and cabin design. These considerations, together with the properties of shipbuilders' materials, have limited the ratio of large vessels to single digits.

But by that time wooden hulls were on the way out. In 1845 the SS Great Britain (designed by Isambard Kingdom Brunel , at that time the country's most famous engineer) was the first iron vessel to cross the Atlantic—it had an LBR of 6.4. Then inexpensive steel became available (thanks to Bessemer process converters), inducing Lloyd's of London to accept its use as an insurable material in 1877. In 1881, the Concord Line's SS Servia , the first large trans-Atlantic steel-hulled liner, had an LBR of 9.9. Dimensions of future steel liners clustered close around that ratio: 9.6, for the RMS Titanic (launched in 1912); 9.3, for the SS United States (1951); and 8.9 for the SS France (1960, two years after the Boeing 707 began the rapid elimination of trans-Atlantic passenger ships).

Huge container ships , today's most important commercial vessels, have relatively low LBRs in order to accommodate packed rows of standard steel container units. The MSC Gülsün (launched in 2019) the world's largest, with a capacity of 23,756 container units, is 1,312 feet (399.9 meters) long and 202 feet (61.5 meters) wide; hence its LBR is only 6.5. The Symphony of the Seas (2018) , the world's largest cruise ship, is only about 10 percent shorter, but its narrower beam gives it an LBR of 7.6.

Of course, there are much sleeker vessels around, but they are designed for speed, not to carry massive loads of goods or passengers. Each demi-hull of a catamaran has an LBR of about 10 to 12, and in a trimaran, whose center hull has no inherent stability (that feature is supplied by the outriggers), the LBR can exceed 17.

This article appears in the August 2021 print issue as "A Boat Can Indeed Be Too Long and Too Skinny."

Vaclav Smil writes Numbers Don’t Lie, IEEE Spectrum 's column devoted to the quantitative analysis of the material world. Smil does interdisciplinary research focused primarily on energy, technical innovation, environmental and population change, food and nutrition, and on historical aspects of these developments. He has published 40 books and nearly 500 papers on these topics. He is a distinguished professor emeritus at the University of Manitoba and a Fellow of the Royal Society of Canada (Science Academy). In 2010 he was named by Foreign Policy as one of the top 100 global thinkers , in 2013 he was appointed as a Member of the Order of Canada , and in 2015 he received an OPEC Award for research on energy. He has also worked as a consultant for many U.S., EU and international institutions, has been an invited speaker in more than 400 conferences and workshops and has lectured at many universities in North America, Europe, and Asia (particularly in Japan).

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Comparing trimarans & catamarans.

Trimarans tend to be more performance oriented than catamarans. In part, this is because it’s easier to design a folding trimaran, and as a result Farrier, Corsair, and Dragonfly trimarans had a disproportionate share of the market.

In spite of this and in spite of the fact that many are raced aggressively in windy conditions, capsizes are few, certainly fewer than in equivalent performance catamaran classes.  But when they do go over, they do so in different ways.

Trimarans have greater beam than catamarans, making them considerably more resistant to capsize by wind alone, whether gusts or sustained wind. They heel sooner and more than catamaran, giving more warning that they are over powered. 

Waves are a different matter. The amas are generally much finer, designed for low resistance when sailing deeply immersed to windward. As a result, trimarans are more susceptible to broach and capsize when broad reaching at high speed or when caught on the beam by a large breaking wave.

In the first case, the boat is sailing fast and overtaking waves. You surf down a nice steep one, into the backside of the next one, the ama buries up to the beam and the boat slows down. The apparent wind increases, the following wave lifts the transom, and the boat slews into a broach. If all sail is instantly eased, the boat will generally come back down, even from scary levels of heel, but not always.

In the second case a large wave breaks under the boat, pulling the leeward ama down and rolling the boat. Catamarans, on the other hand, are more likely to slide sideways when hit by a breaking wave, particularly if the keels are shallow (or raised in the case of daggerboards), because the hulls are too big to be forced under. They simply get dragged to leeward, alerting the crew that it is time to start bearing off the wind.

Another place the numbers leave us short is ama design. In the 70s and 80s, most catamarans were designed with considerable flare in the bow, like other boats of the period. This will keep the bow from burying, right? Nope. When a hull is skinny it can always be driven through a wave, and wide flare causes a rapid increase in drag once submerged, causing the boat to slow and possibly pitchpole.

Hobie Cat sailors know this well. More modern designs either eliminate or minimize this flare, making for more predictable behavior in rough conditions. A classic case is the evolution of Ian Farrier’s designs from bows that flare above the waterline to a wave-piercing shape with little flare, no deck flange, increased forward volume, and reduced rocker (see photos page 18). After more than two decades of designing multihulls, Farrier saw clear advantages of the new bow form. The F-22 is a little faster, but more importantly, it is less prone to broach or pitchpole, allowing it to be driven harder.

Beam and Stability

The stability index goes up with beam. Why isn’t more beam always better? Because as beam increases, a pitchpole off the wind becomes more likely, both under sail and under bare poles. (The optimum length-to-beam ratios is 1.7:1 – 2.2:1 for cats and 1.2:1-1.8:1 for trimarans.) Again, hull shape and buoyancy also play critical roles in averting a pitchpole, so beam alone shouldn’t be regarded as a determining factor.

Drogues and Chutes

While monohull sailors circle the globe without ever needing their drogues and sea anchors, multihulls are more likely to use them. In part, this is because strategies such as heaving to and lying a hull don’t work for multihulls. Moderate beam seas cause an uncomfortable snap-roll, and sailing or laying ahull in a multihull is poor seamanship in beam seas.

Fortunately, drogues work better with multihulls. The boats are lighter, reducing loads. They rise over the waves, like a raft. Dangerous surfing, and the risk of pitchpole and broach that comes with it, is eliminated.  There’s no deep keel to trip over to the side and the broad beam increases the lever arm, reducing yawing to a bare minimum. 

Speed-limiting drogues are often used by delivery skippers simply to ease the motion and take some work off the autopilot. By keeping her head down, a wind-only capsize becomes extremely unlikely, and rolling stops, making for an easy ride. A properly sized drogue will keep her moving at 4-6 knots, but will not allow surfing, and by extension, pitch poling. 

For more information on speed limiting drogues, see “ How Much Drag is a Drogue? ” PS , September 2016.

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Average Sailboat Length

Last Updated by

Daniel Wade

August 30, 2022

Sailboat length is an important factor for boat performance, comfort, and classification.

The average overall length (LOA) of common production monohull sailboats is 30 feet, and the average LWL is about 25 feet. The average length of production multi-hull sailboats is between 40 and 44 feet.

In this article, we’ll cover the average overall length and waterline length of production sailboats. Additionally, we’ll examine the relationship between hull speed, performance, and length. We’ll also provide examples of average lengths by type, including multi-hull designs.

We sourced the information used in this article from sailboat design guides and from popular sailboat manufacturer specifications.

Table of contents

‍ Average Sailboat LOA (Length Overall)

The average sailboat LOA is about 30 feet, depending on the type of sailboat. Racing sailboats of the same displacement tend to be longer and narrower, while bluewater cruising sailboats tend to be shorter and wider.

LOA, or length overall, is a measurement of the sailboat between its longest hull points. This is usually the measurement associated with the simple term ‘length.’ For example, an Islander 32 has an LOA of 32 feet, and a Cal 20 has an LOA of 20 feet. This is also the measurement used when referring to a vessel as a “30-foot sailboat” and whatnot.

Typically, LOA does not include the length of hull additions such as bowsprits and rigging. Also, it doesn’t consider any modifications such as outboard motors and fishing equipment.

As far as performance is concerned, LOA is actually not the most important measurement. Calculations such as hull speed and displacement, which are critical marine specifications, are made using LWL or length at the waterline.

Average Sailboat LWL (Waterline Length)

Waterline length is measured from the longest points of the submerged portion of the hull. This measurement is usually about 15% to 20% shorter than the LOA, as most sailboats have a sloped bow and stern that gets larger the further up the hull you go.

However, this is not always the case. Vessels with a ‘ram bow’ or reverse-sloped stern could have a waterline length that’s actually longer than the LOA. However, these unusual design elements are not common on popular production sailboats.

Average Catamaran Length

Catamarans, foot-for-foot, are actually much larger than monohulls. These vessels have two separate hulls, which themselves are nearly large enough to be separate sailboats. As a result, the average size of a comparable catamaran is often smaller, but large catamarans remain popular.

The average length of a production catamaran is about 40 to 50 feet overall. The LOA to LWL ratio of most catamarans is much less significant than monohulls, as the effects of hydrodynamics on catamarans are very different. For example, a 30-foot monohull probably has a 25-foot LWL, whereas a 30-foot catamaran could also have a 30-foot LWL.

Average Trimaran Length

Common production trimarans and catamarans both have a typical LOA of about 40 to 50 feet, though some popular 30 to 35-foot models exist. The LOA and LWL ratio differences between trimarans and catamarans are minimal, though many Trimarans have a much more substantial center hull.

Multi-Hull Waterline Length Variations

Multi-hull sailboats are designed to take advantage of the hydrodynamic lift effect. This phenomenon lifts the hulls of the vessel out of the water as they increase in speed, thus reducing drag and increasing speed and efficiency. The faster the multi-hull goes, the easier it is to gain even more speed.

 Depending on the shape of the hulls, multi-hull sailboats can have a dynamically changing LWL in practice. That said, multi-hull LWL specifications are fixed at their non-moving point.

How Sailboat Length Impacts Speed

Sailboat speed and length are closely related. However, the relationship only has a significant impact on the waterline length of monohull designs.

Sailboat length/speed ratios are calculated as hull speed. For monohull designs, the hull speed calculation is (HS = 1.34 x √LWL). This determines the typical maximum speed of a boat based on its waterline length.

But why are sailboat speeds capped due to length? It has to do with wake. When a displacement hull moves through the water, it generates waves at the bow and the stern. These waves cause drag—which the vessel can overcome until a certain speed is reached.

At hull speed, the bow and stern waves ‘sync’ and begin working together against the boat. Once this harmonic interference begins, the effect increases and counteracts any additional wind power that the sails supply.

If you’ve ever seen a sailboat hull speed table, it’s evident that longer vessels can achieve higher speeds. This rule is a great way to measure the peak performance of monohulls by length, but it’s almost useless for multi-hulls.

Multi-hull Length to Speed Ratios

Multi-hull sailboats generate completely different kinds of hull waves, so the hull speed formula used for monohulls doesn’t do much good. This is why multi-hull sailboats of the same length can almost always outrun monohulls-even if they displace twice as much.

Why Are Some Sailboats Faster Than Their Hull Speed?

Modern hull designs don’t always conform to traditional hydrodynamic standards. The hull speed calculation was developed as a general tool for classical sailboat designs with full-displacement hulls.

One way to beat hull speed limitations is planing, but sailboats usually can’t achieve full plane. Modern materials have allowed designers to create sailboats with shorter lengths that exceed their hull speed. This is why hull shape is such an important factor when choosing a design.

The types of monohull sailboats that ‘beat’ their hull speed limitations are semi-displacement hulls, canoe hulls, and other hull types with a shallow draft (excluding a long and thin fin keel, bulb keel, or bilge keels).

Average Sailboat Length by Type

Here, we’ll provide a quick breakdown of average sailboat lengths by type. The smallest class of sailboats, called dinghies, rarely exceed 12 or 15 feet in length.

Other small sailboats, such as single-person racers, typically range in size from 12 feet to 18 feet. Examples include famous racers like the Sunfish and the Laser.

Average Small Sailboat Length

Small open-top displacement vessels, which were once common for fishing and used as workboats, range in length from 15 feet to 25 feet. Open-top sailboats with keels or centerboards rarely exceed 25 feet in length as there’s simply too much usable space to avoid installing a cabin.

Average Trailerable Sailboat Length

Trailerable sailboats, displacement-keel or otherwise, range in length from 15 feet to 26 feet. Sure, you may see a 28 or 30-foot sailboat on a large trailer, but that doesn’t make it a ‘trailerable’ design.

Trailerable sailboats are designed to be transported practically using normal cars, so their length and overall size are limited.

Average Coastal Cruising Sailboat Length

Small cruising sailboats range in size from 20 feet to 29 feet. These vessels are minimally capable of coastal cruising. That said, this is the size range where the smallest practical offshore cruising vessels live.

Famous compact blue water cruisers like the Flicka 20, Amigo 22, and the Dana 24 are capable and fall well within the coastal cruiser length category.

Average Cruising Sailboat Length

The sweet spot of common fiberglass production cruisers is between 30 and 40 feet, with the majority ranging from 30 feet to 35 feet. These are the most common sailboats of their type, as they usually offer the most capability and comfort for the most reasonable price.

These 30 to 35-foot cruisers are usually offshore-capable and small enough for local cruising. Plus, their cabins are large enough for a liveaboard couple or a vacation with friends and family.

Average Bluewater Cruising Sailboat Length

Serious Bluewater cruisers are designed for safe, fast, and comfortable ocean passages. These vessels range from 35 to 45 feet in length, and the majority are over 40 feet long.

Several well-known Bluewater cruisers range between 38 and 40 feet. These are spacious and comfortable vessels with fast hull speeds and acceptable handling characteristics.

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I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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