sailboat foil design

Published on June 2nd, 2017 | by Assoc Editor

Foiling and Foil Shapes, a Beginner’s Guide

Published on June 2nd, 2017 by Assoc Editor -->

by Mark Chisnell, Land Rover BAR The rules covering the design and construction of the team’s America’s Cup Class (ACC) boat have defined many of the parts of the boat, including the hull and crossbeams (together called the platform), and the wing shape and size. What’s left for the team’s designers and engineers to work on is principally the daggerboards and rudders, and the control systems that operate them along with the wingsail.

A lot of the technology that goes into the control systems is hidden well inside the hull, with just glimpses of the HMI (human machine interface) that the sailors use to control the board rake, wing trim and so on. The foils are on full view however, so we thought a beginners guide to ACC foil design would come in useful now the racing is approaching.

Basic Principles The foils use exactly the same scientific principles as an aircraft wing. Just as an aircraft wing will lift a plane up off the ground, the foils of an America’s Cup Class boat will lift it out of the water. Wings are foils too, called aerofoils because they work in air. The foils on the new America’s Cup boats are more accurately called hydrofoils, because they work in water.

The secret to both types of foil is the shape – aerofoils and hydrofoils use a special shape to guide the wind or water around them, and generate the lifting force to get planes and boats up in the air. Of course, the America’s Cup boats also use an aerofoil. The main wingsail works exactly the same way as an aircraft wing, it’s just rotated to stand up straight, rather than lie flat.

While an aircraft needs an engine to push the air over the wing fast enough to generate enough force to lift the aircraft up off the ground, the wingsail on the Cup boat generates force from the wind blowing past it. The harder the wind blows, the more force it makes to push the boat forward. When the boat is going fast enough, the hydrofoils will then be able to create enough force to lift the boat out of the water. This reduces resistance to the forward motion and the boat goes faster still.

There are four hydrofoils on the boat — we count the rudders at the back because they have small wings at the tips called elevators. However, the real power to keep the boat in the air comes from the hydrofoils (the daggerboards, as you will often hear them called by the sailors) and we will concentrate on these.

The L-Foil The L-foil is exactly that; a vertical daggerboard shaft that goes through the hull of the boat, with a single horizontal hydrofoil on the bottom, the whole thing shaped like an ‘L’. If nothing else changes, then the L-foil keeps generating lift as the boat goes faster and so the boat keeps rising, and as it rises, less and less of the daggerboard is in the water.

At the basic level, two things then happen: firstly, the boat starts to slip sideways because there is less of the vertical part of the daggerboard in the water and this makes the boat feel unstable and hard to steer. Then, ultimately, if the boat keeps rising the horizontal part of the board that is doing all the lifting will break the surface. If it does, there will be a catastrophic loss of lift and the boat will come crashing back down.

Aircraft use moving parts on the foils to control the amount of lift – trailing edge flaps — but the rules forbid these on the ACC boats, so to maintain stable flight the sailors change the rake or angle of attack of the whole dagger board (and hence the foil) to the water.

Rake If you rake the board backwards as the boat accelerates, the lift will reduce and the boat will come to an equilibrium at a steady height above the water. This is all well and good until the conditions change, maybe the wind speed goes up or down, or the boat hits some waves. When that happens the rake will need further adjustment to find the new equilibrium… until the next puff or lull when it must change again.

In the big breeze and rough water of San Francisco Bay in the 34th America’s Cup it turned out that these moments of equilibrium didn’t last very long and on occasions barely existed at all. The crew’s ability to generate the hydraulic power to change the board and wing trim was simply overwhelmed; they couldn’t achieve stable flight.

V-foil The solution was what’s called the V-foil, in which the horizontal part of the ‘L’ is angled upwards to form more of a ‘V’ shape (the angle at the bottom of the ‘V’ is called the dihedral – a dihedral of 90 degrees would define an L-foil, less than that is progressively more of a V-foil).

The V-foil uses the same principle as one of the most successful original foiling powerboats. The grand old man of 19th century innovation, Alexander Graham Bell put a couple of 350hp engines on the back of what was called HD-4 and set a new marine world speed record in 1919 of just over 70mph.

HD-4 used three ‘ladders’ of small foils, one at the front, and one each side close to the back. When the boat accelerated it started to lift out of the water, and as it lifted, one by one the ‘rungs’ of the foils would break clear of the water. As they did so the lift would decrease, and unless the boat continued to accelerate the boat would stop rising and settle at an equilibrium.

The V-foil achieves this same effect with a single foil and is used in the commercial application of fast ferries— one runs between Southampton and Cowes on the Isle of Wight, right across the Solent waters where the team train, and has done so (on and off) since 1969 – so V-foils are well understood.

When a boat equipped with a V-foil keeps rising as more lift is generated by faster speeds, both parts of the ‘V’ come out of the water together. Critically, when the ‘horizontal’ section starts to break the surface at the tip, it has the effect of reducing the lift gradually, because it doesn’t all come out of the water together. So the boat comes back down gently, working towards an equilibrium ‘ride height’ of its own accord.

It might be that it doesn’t reach this equilibrium before something else changes, but the V-foil has some inherent stability (unlike the L-foil) that doesn’t require human intervention. The shape provides a feedback mechanism to control the amount of lift and produce a more stable ride at a consistent height above the water. The downside of the V-foil is that it will generate less lift and more drag than the L-foil under the same conditions, because some of the lift generated is pushing sideways rather than up.

So one of the big questions facing the teams at the outset of this campaign was whether or not the sailors could achieve stable flight with an L-foil in the new boats and the new venue. Bermuda was a very different place to San Francisco; the winds were expected to be lighter, the water flatter and it seemed that stable flight should be easier to achieve with an L-foil under human control.

A huge amount of work has gone into foil and control system design and we now know that the answer is yes, they can – all the teams are using L-foils, often with unloaded dihedral angles of greater than 90 degrees. These angles close as the boat sails and the foil is loaded up to become much closer to, or 90 degrees.

Cant Another buzz word for the 35th America’s Cup is the cant. The cant of the board is similar to the rake, except that the bottom of the board is moving sideways across the boat, to and from the centreline, rather than backwards and forwards. When the board is canted outwards (towards the edge of the boat) it creates greater ‘righting moment’ and more power to drive the boat forwards.

Righting Moment When the wind hits a sail it creates the force to move the boat forward but it also creates a force that is trying to tip the boat over. If you have ever seen a dinghy or yacht knocked flat by a big gust of wind then you’ve already got the idea.

It’s considerably simplified, but essentially the more force that can be applied to resist the wind’s effort to tip the boat over, then the faster the boat will go, because more of the wind’s energy can be captured and applied to forward motion. The resisting force is called the righting moment and creating as much righting moment as possible is a fundamental principle of designing fast sailboats. It’s the reason that you see people leaning over the windward side when they are racing, putting bodies as far out on the windward side as possible is creating righting moment.

S-Foil Finally, there’s the question of whether the vertical part of the daggerboard should be straight or ‘S’ shaped. The curve of the S-foil could be used — like the cant — to move the bottom of the board outboard and increase the righting moment. So S-foils are more powerful, but they are also more difficult to use. The curves have to raised up and down through the bearings and internal mechanisms in the hull, and that means a lot of work to keep the friction down and the efficiency high.

Tags: AC35 , America's Cup , foiling , Land Rover BAR , Mark Chisnell

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High-Performance Foils

Shaping better centerboards and rudders

From Issue   July 2020

W hile it’s generally accepted that the right sails and sail trim will determine how close you can sail to the apparent wind, a sailboat’s progress to windward also depends on the lift and drag generated by the centerboard and rudder. How much difference does proper foil shape make over a simple rounded leading edge and tapered trailing edge, anyway?  Foils operating in fluids, whether air or water, are a well-studied topic. C.A. Marchaj, in his book, Sailing Theory and Practice , discusses the theory and gives the results of actual tests of differences in foil planform (side view), cross-section shape, size, and aspect ratio (AR – length to width). Lacking other constraints, an ideal centerboard, daggerboard, or rudder blade should have a reasonably high AR (greater than 2) planform with a streamlined cross-section that has a parabolic leading edge and a thickness of somewhere near 10 percent of the chord width (the distance from leading edge to trailing edge). A thickness of 8 percent produces less drag but stalls sooner; 12 percent has a higher stall angle but produces more drag.

Exactly where the point of maximum thickness should be located is a matter of some debate. Marchaj suggests it should be at 50 percent of the chord width, halfway between the leading edge and trailing edge, but provides no data to back that up. Other sources suggest that the NACA (National Advisory Committee on Aeronautics) symmetric foil sections, originally developed during aircraft research, are actually a good fit for boat foils operating at low speeds in water. A NACA 0010 foil, for example, has a maximum thickness of 10 percent of the width of the foil, located at 30 percent from the leading edge.

Of course, there are many practical reasons why not all keels, centerboards, and rudders have high AR planforms, but the cross section for a foil of any planform should be streamlined. My personal experience of doing it wrong on one boat, and getting it right on another boat, has convinced me that the NACA sections and guidelines above provide good performance.

HORNPIPE, my first sail-and-oar boat, was an 18’ Kurylko Alaska with a standing-lug ketch rig, and sailed well enough to windward in flat water, but lost 10 to 15 degrees of pointing ability as soon as the water got choppy. I knew it wasn’t poor sail trim. Eventually I got looking at the daggerboard and analyzed it. It was only about 2.5 percent of the sail area and its thickness was only about 6 percent of the chord width, neither big enough or thick enough in my view, and in rough water it lost laminar flow and lift. When I designed my 18′ lug-yawl cruiser, FIRE-DRAKE, I gave it a thicker centerboard with a greater fraction of the sail area, about 4 percent. I also gave it a straight quarter chord line (think of the shape of the wing of a Spitfire aircraft) and a moderately high aspect ratio of about 3:1 for the planform area. To get the daggerboard foil shaped accurately and quickly I opted to have it cut on a computer numerical control (CNC) machine. All that was left for me to do was sand, seal and paint, and make an epoxy-lined hole for the pivot pin.

The results have been what I had hoped for. FIRE-DRAKE sails quite well to windward and maintains its performance in rough water. I sailed in the company of a similar boat—with the same length and beam, the same weight, and the same sail plan—along the south half of the Inside Passage, and that boat’s centerboard was shaped by eye. When sailing to windward, FIRE-DRAKE would consistently point higher and walk away in speed. My centerboard even let me continue sailing to windward when my partner gave up and took to the oars.

Although I had the daggerboard shaped with a CNC router, it is possible to shape a high aspect ratio, fully streamlined foil in the home shop. I’ll walk you through my second project, a kick-up rudder blade that I made at home to replace the original one I built for FIRE-DRAKE. I settled on a planform that is one-quarter of an ellipse with an elliptical leading edge and a straight trailing edge. (The shape would move the center of lateral resistance of the boat aft a few inches, and is intended to lighten the weather helm I’d experienced with the original rubber blade.) I drew the new blade with an aspect ratio of 2.5:1, with a length of 30″ (762 mm) and a maximum chord width of 12″ (305 mm), which would increase the lift and reduce the tip vortex drag.

To draw the planform shape of the quarter ellipse you can use an online graphing tool such as Desmos for a full ellipse. If you center the ellipse at zero, you can drag the two axes out until you get the aspect ratio you want. Since the graph has a grid in the background, you can then print out a screen capture of a quarter of the resulting ellipse and scale up the printed image to the actual dimensions required. If you are comfortable with computers, you can download and run Freeship (available for Windows only) which has a “keel and rudder wizard” that accurately generates several different planforms.

The new rudder blade for FIRE-DRAKE has a quarter-ellipse planform. The plywood’s glue lines show the contours that help with shaping the foil.

Obtaining the cross-section profile of a chord of a given width is best left to a computer. For any of the NACA foils, like the 0010 foil I mentioned above, Competition Composites Inc . (CCI) has a very simple and handy calculator . You need enter only the chord width and the maximum thickness and it will generate a table of X-Y coordinates that you can copy and print out. They’ll be your offsets for drawing a pattern for the foil cross-section. If you intend to sheathe your foil with ’glass and epoxy, for example, you can also enter the skin thickness and it will calculate the coordinates for the plywood core.

Now, here’s the tricky bit. If you have a rectangular foil planform, you only have one chord width and therefore one section profile for the entire length of the foil. However, if you have any other planform (e.g., half-ellipse, quarter-ellipse, trapezoidal, straight-chord-quarter-line, etc.), the thickness, which will be one-tenth of the chord width, changes along the length of the foil because the chord width changes.

I used the CCI calculator to generate profile coordinates for three different points along the length of the rudder blade: at the root, at about two-thirds of the way along and at about 90 percent of the way to the end. I chose those points because the chord width for my quarter ellipse planform doesn’t change much for the first half of its length, but it changes more quickly toward the tip. The idea is to shape the foil to these profiles at these points and then taper the foil evenly between them. You can lay out your foil plan directly on to the ply or you can use something thin, like doorskin, to make and fine-tune a template, which is what I did.

I made a blank for my rudder by gluing layers of marine ply with epoxy to the required 1″ thickness. I have found that the plywood, in spite of its cross-grain plies, has sufficient strength for the size of small-boat foils that I have built (though the cross-grain would weaken a long thin foil). Plywood does not warp and has the added advantage over solid wood in that the plies create a kind of contour map that give you graphic visual feedback as to the evenness of your surface once you start shaping the foil. You can make a foil with solid wood or even foam plus a ’glass-and-epoxy skin, but without the plywood laminates as guides, you would have to make more section profile templates to ensure a smooth and accurate shape.

Clamping a 4′ level to the flat part of the rudder blade provides a reference line to gauge how much wood to remove to achieve the foil’s taper.

The next step is to taper the thickness of the laminated foil blank along its full length. Knowing the required thickness at your chosen points, you can draw a pattern for the curve of the taper and half the thickness of the blade stock and measure how much wood you have to remove at each point. I clamped my 4′ aluminum I-beam level to the flat part of the rudder blade above the shaped part, and used a ruler to measure the depth I had to cut to. To remove the wood for this part of the project, I used my #4 Stanley plane. While I have a power hand planer, I didn’t trust myself with it to not take too much off too quickly.

The female half-section template for a given chord for a foil gets its shape from the X-Y coordinates generated by a foil calculator.

I made three female half-section profile templates, one for each of the three points noted above, by plotting out the generated X-Y coordinates on pieces of doorskin and carefully cutting them out. One thing to note is that the CCI calculator generates a profile that has a trailing edge of zero thickness. Obviously, this is not practical to build in wood, and a knife edge is not that critical anyway. I adjusted the trailing edges so that the finished edge would end up about 1/3″ (4mm) thick.

Applying the template to the foil in the works shows the high and low spots as the shaping continues.

Next, I used the profile templates to shape the foil at my three chosen points. I shaped the plywood with a Shinto rasp , regular rasps, and coarse sandpaper. It’s a process of taking some wood off, placing the template, and repeating until you get the section of wood shaped to the templates. Once that is done, I could go to work taking down the wood between the sections, using the ply layers as a guide. I used my block plane, Shinto rasp, and sandpaper for this task. I eyeballed a smooth transition around the tip from the leading edge to the trailing edge.

I sealed the surface of the shaped foil with a couple of coats of epoxy to provide a smooth, hard surface to accept a finish coat of marine epoxy enamel.

Alex Zimmerman is a semi-retired mechanical technologist and former executive. His first boat was an abandoned Chestnut canoe that he fixed up as a teenager and paddled on the waterways of eastern Manitoba and northwestern Ontario. He started his professional career as a maritime engineer in the Canadian Navy, and that triggered his interest in sailing. He didn’t get back into boatbuilding until he moved back to Vancouver Island in the ’90s, where he built a number of sea kayaks that he used to explore the coast. He built his first sail-and-oar boat in the early 2000s and completed his most recent one in 2016. He says he can stop building boats anytime. He is the author of the recently published book, Becoming Coastal .

For further reading on the pros and cons of the variables in foil design, Competition Composites (CCI) has a good discussion . For those of you who want to go into the math, Paul Zander has a good presentation from nearly 20 years ago, and also, for those inclined that way, an updated discussion with a lot more math.

You can share your tips and tricks of the trade with other Small Boats Magazine readers by sending us an email .

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Comments (17)

Is there someone who can craft a good rudder for my boat? I have not the time or the skills to do such. Have a 15′ Delaware Ducker. Love the boat but the rudder is (I think) a disaster. Hardly brings the boat around and doesn’t help much to windward. Flat plywood barn door out of 3/8″ ply with no shape other than outline that barely gets 2″ – 3″ into the water. If it could be a kick-up so much the better. I do a lot of shallow, sand-bottom sailing. Thank you for any help.

I know about the ducker and a foil shaped rudder will help a lot. Some of the modern ones built in glass and cold molded have been fitted with modern dinghy rudders and foil shaped daggerboards. They tack more like a dinghy than like my traditional one where I have to sail it around like a larger boat.

Most any small boat builder in your area should be able to build you a rudder. Lines for a kick up would be a nuisance. I don’t bother with them on a larger boat, just use a pivot bolt with tension. Means I have to shove it down.

You might want to look at the system that Mike Storer has developed for his goat island skiff, a straight foil, easier to shape.

Alex has done a great idea showing us how to work these out using hand tools. We used to do it in my dinghy sailing days by drawing a 30 % line. Make a bunch of parabola templates and hack away. For really long narrow boards before the days of carbon we used to use 5/4 stair tread fir.

Interesting article. When I built my Oughtred JII Yawl eighteen years ago, I did some research on appropriate foil profiles for the centerboard and rudder. It’s a long time ago now, so I don’t remember the exact profiles I chose, but I do believe I picked NACA 0010 for the CB. I picked another profile for the rudder, one that had a steeper stall angle, on the theory that with typical weather helm the rudder meets the flow at a steeper angle than the CB. I really don’t know how valid my theory is. I have no experimental evidence to back it up. Any thoughts?

Andrew, that seems to make sense although I haven’t seen any research to back that up.

Hi Andrew, I remember seeing your beautiful JII; but did not ask about the foils! I wonder how NACA 0010 compares with my approximate foil shape, inspired mainly by guesswork. Are these profiles available? By the way, I do not like plywood foils; CBs break. Friends near here lost their fine old rebuilt Wayfarer; were in danger themselves, when they capsized and the plywood board broke. Too near a rugged lee shore. A class racing boat, by a particular builder – all their plywood centerboards broke.

Iain, I don’t know what your standard foil shape looks like, but a NACA 0010 has a maximum thickness of 10% of the chord distance, occurring at 30% of the distance back from the leading edge. Marchaj believes that the maximum thickness should occur at about 50% of the distance back from the leading edge. Other designers agree, I think. If I understand him correctly, John Welsford uses a foil section that is closer to 50% distance, but I am not sure what thickness of foils he favors or what the exact foil shape is. You can see the shape, and all the requisite numbers for extracting an X-Y plot to reproduce them, for a whole bunch of different foil types, on the Airfoil Tools site. As for plywood foils, I understand your concern. Half the plies are oriented in the wrong direction and don’t provide much in the way of resisting sideways bending moments. However, it has been my experience that this is not a major concern if the foil is thick enough and not too long. The centerboard foil on my latest boat, for example, is ply, but is nearly 2″ (50 mm) at maximum thickness. It’s got over 1,000 nautical miles in four years under the keel by now, including a couple of practice capsizes, with no issues so far.

I don’t know what a standard Wayfarer foil looks like. Is it long and thin?

Very interesting article. I am just in the process of building a Lillistone Flint and have no experience building or knowledge of foil design and performance. I calculated the various ratios and percentages. The AR as per the plans is 2.43, so that looks good. The thickness however is only 4% of chord width. Area of the dagger board is 3.5% of sail area, so probably OK there too. Lillistone does state in the plans that the board can be made thicker if preferred so I think I may do that as 4% is a pretty big departure from the 8 – 10% of the chord width suggested. Any comments? The Flint was featured in this mag a few issues ago.

David, both my experience and authorities who design for a living and/or who have tested these things would suggest that 4% is rather too thin for good lift. I suspect that you might find it works reasonably well in dead flat water but you would lose lift and pointing ability as the water gets more turbulent. If it was me, and I hadn’t yet built the foil and its case, I’d go for one that was at least 10% and maybe even 12%. The additional thickness would also be more robust should you need to stand on it to flip the boat over if you capsize.

Thanks so much for taking the time to reply, Alex. I have decided to use some salvaged King William (King Billy here in Tasmania) pine that was salvaged. I thicknessed it to clean it up a bit and reckon I will eventually get 20 – 22 mm out of it. I plan to laminate 10 pieces into a 304mm board. Hopefully I will get a good result.

Hi David, It highly depends where you sail and how you sail. I have one older (1975) 420er dinghy for fun and local competitions with foiled board and rudder. But then I have 21″ German Jollenkreuzer veteran from 1952 which has both from 1/4″ steel plate, and it works fine. I sail that for pleasure on lakes. It would definitely be more performant (and point better) with both foiled, but I’m surprised how well it performs in its nearly ’70s (comparing to modern GRP boats with foiled boards).

For your job, I would probably stick to the plans. The rudder in this case is more important and can be somehow easily modified (foil). The centerboard I would keep the same, not only to conform the centerboard box (which would need to be sized), but also the overall design. Finally the most important here is what is your building and designing experience, because the worst thing is when something is incorrectly designed and then improperly built (the simple rounded plywood then may work better).

Anyway, I’m also thankful for this article. It reminds me my childhood when I built airplane models and used exactly the same methods used here to create the wings (in much smaller scale).

Thanks for your thoughts. I am going for a thicker foil and risk it (see reply to Alex above). I am well aware of risks in departing from designer’s work but I don’t think I will go far wrong.

Do SUP fins follow the rules for centerboards?

I’m not a SUP guy myself, but my understanding is that the fins are there to assist in tracking. That is, they don’t need to provide lift the way a sailboat keel/CB going to windward does. The thing you would be aiming for in the case of SUP fins is having sufficient lateral area to provide that tracking ability, and then having it streamlined to reduce drag. I would imagine that thinner would be better, although you’d still want a streamlined foil section, as that would produce less drag than a flat plate. The leading edge of a flat plate tends to separate the flow from the sides of the plate, even if that edge is rounded, and separation produces turbulence and drag.

For twenty years, beginning in the early 1970’s, I raced a Lido 14. The boat was pre-owned, and had its original solid wood foils which were in pretty bad shape, and I decided to build new foils. After reading Marchaj’s book, and the Lido 14 Class Rules, I designed a new centerboard and rudder. The NACA-0009 section most closely fit the required class measurements, and I used that profile. I did alter the leading edge of the rudder, making it more rounded, to allow for the fact that the rudder angle of attack is variable, and is more likely to stall.

In fleet racing, and sailing close hauled, the results were astounding, with the boat seeming to sail slightly sideways, relative to other boats. I also began to pay particular attention to the condition of the foil surfaces, as Marchaj writes that the drag on underwater foils is many times greater than the drag on the hull surface. One time we were sailing in an area of submerged trees, and my centerboard lightly brushed a tree branch. I then noticed that the centerboard ‘hummed’ on port tack. Later, when I examined to board, there was a barely visible scratch.

I very much enjoyed the article on high-performance foils in the July 2020 issue of the Small Boat Magazine .

For twenty years, beginning in the early 1970’s, I raced a Lido 14. The boat was pre-owned, and had its original solid wood foils which were in pretty bad shape, and I decided to build new foils. After reading Marchaj’s book, and the Lido 14 Class Rules, I designed a new centerboard, and rudder. The NACA-0009 section most closely fit the required class measurements, and I used that profile. I did alter the leading edge of the rudder, making it more rounded, to allow for the fact that the rudder angle of attack is variable, and is more likely to stall.

In fleet racing, and sailing close hauled, the results were astounding, with the boat seeming to sail slightly sideways to windward, relative to other boats. The boat didn’t seem to point higher, when close hauled, it just didn’t make as much lee way. More benefit on the port tack, a little less on starboard. I did set the centerboard jibe angle to the maximum allowed by class rules. In all honesty, I was probably the only one in the fleet that had read Marchaj. It still took me five years to win the fleet championship. I also began to pay particular attention to the condition of the foil surfaces, as Marchaj writes that the drag on underwater foils is many times greater than the drag on the hull surface. One time we were sailing in an area of submerged trees, and my centerboard lightly brushed a tree branch. I then noticed that the centerboard ‘hummed’ on port tack. Later, when I examined to board, there was a barely visible scratch.

I am a new subscriber to Small Boat Magazine , and look forward to each issue. Keep up the good work.

Was just about to make the centerboard for my Oughted Caledonia Yawl. So I was happy to see this article. But was then disappointed when I calculated my centerboard area to be only 1.9 percent of my total sail area. I briefly thought gee I will make it a little bigger…but then realized the centerboard trunk is already complete and limits that. I don’t plan on racing, so it is what it is.

Mark, I’d be really interested in your results once you launch the boat and do some trials. Theory is one thing, but nothing beats data from real-world results.

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Sailing Foiler Design: Foil Assist and Full Flying

Discussion in ' Hydrodynamics and Aerodynamics ' started by Doug Lord , Dec 9, 2011 .

Doug Lord Flight Ready

I'd like help from everyone interested in foiler design to build this thread into a comprehensive source of references, discussion, ideas etc. I want to copy and paste relevant material by people such as Tom Speer, Greg Ketterman, Dr. Sam Bradfield ,Mark Drela, Steve Clark , Gary Baigent and many others. I'd like to see the thread turn into a resource for all of us as time goes by. Some topics I think would be worthwhile to expand on: 1) Foil Assist vs Full Flying-when, how, why? - 2) Foil Assist in Current Applications: a. DSS b. lifting foils on rudders( National 12, I-14, some cats, etc) c. Multiple foils for foil assist-not full flying-advantages-disadvantages. - 3) Lift formula-pratical formula for calculating lift at various speeds and areas. - 4) Full Flying Foiler Configurations-how and why. - 5) Monofoilers-applications of lifting hydrofoils for full flying or foil assist on monohulls from dinghies to 100' + maxi keelboats -and beyond. - 6) Multifoilers-applications of lifting hydrofoils for full flying or foil assist on multihulls from 12'(or less) to 100 +'. - 7) Lifting hydrofoils in cruising applications. - 8) Lifting foils in Kite sailing and Boardsailing. - 9) Reference books, papers etc. - 10) Veal Heel, its applications and the physics involved. - 11) Foiler "jumping" including how why and especially re-entry-primarily for dinghy, board or kite applications. - 12) Marketing Foilers: how? - 13) Righting Moment from hydrofoils-applications, results, ideas. - 14) History of foiler design. - 15) Building foils. - 16) Foil Systems-advantages, disadvantages, examples etc. -a. Altitude control systems for fully submerged foils. -b. Surface piercing foils. -c. Ladder foils. 17) The future of foiling. 18) Surface piercing foils.  

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Hydrofoils For anyone wanting a practical guide to hydrofoils this is it. It is a book by Ray Vellinga called "Hydrofoils Design Build Fly , published by Peacock Hill Publishing, Gig Harbor Washington ISBN number: 9780982236116 e-mail for Peacock Hill Publishing: [email protected] It is written in a clear ,concise manner and teaches you how to calculate foil area and provides a lot of info on hydrofoil types and control systems including a chapter on the Moth foiler. The formula for lift from page 27: ( get the book-there is more to understand than just this formula) L=Vsquared X S X Cl L = Lift in pounds V = Velocity,ft./sec(1mph=1.47 ft/sec) (1 knot= 1.15mph) S =Surface area of foil projected vertically in square ft. CL =Coeficient of lift, this is a dimensionless number found in tables of wing profiles as in Theory of Wing Sections by Abbott and Doenhoff. For a 63412 section used on a Moth,for instance, the Cl will vary from .9 at takeoff to 0 at top speed with cruise(neutral flap) at about .3 . ========= This is one of the best references for airfoils I've found on line-still learning to use it. It shows the airfoil and also gives Lift coeficient etc. http://www.worldofkrauss.com/foils/784  

Olav naval architect

Doug Lord said: L=Vsquared X S X Cl Click to expand...

hydrofoils Olav said: ↑ 1) Actually I miss the term 0.5 * ρ in that formula, since lift (or drag) are usually non-dimensionalized using a reference area (here: projected wing area) and the stagnation pressure P = 0.5 * ρ * v², which can be derived from the Bernoulli equation. You can leave this out of course, as long as you stick to one density and as long as you use lift (or drag) coefficients that already include this factor - which is not the case with the coefficients from the graphs in "Theory of Wing Sections" (or in any other book on that topic that I know). 2) Also be aware that Abbott and von Doenhoff's graphs start at Reynolds numbers of 3e6, which is about three times the Rn of say a Moth foil at 20 knots. However the book sounds interesting and I'll try to get hold of a copy, so thanks for the hint! Click to expand...

"Because 1/2 p=1, we can simplify the formula to: L=V2 X S X Cl . " Click to expand...

Sailing Hydrofoil Technology Olav, thanks for your comments and pointers! Please don't hesitate to add material, ideas or anything you think is relevant.  

RVELL Junior Member

Olav, Thank you for your thoughts on the lift formula that was simplified for my book, "Hydrofoils: Design, Build, Fly". As you point out, I claim "Because 1/2 p=1, we can simplify the formula to: L=V2 X S X Cl . " You responded: "OK, if you use imperial units (slugs/ft³ for water density) the approximation and simplification by Vellinga makes sense. With metric units this doesn't work." For our friends outside the American measuring system, I thought about putting in the book a factor for converting the formula to metric units. But an important goal I followed was to keep the math as simple as possible. However, I am pleased to respond to you and provide a factor to convert the formula variables from the American system to the Metric system. The Metric formula would be: L= 4.06 X V^2 X S X Cl. The 4.06 is the conversion factor. The variables inserted into the formula will be in kilometers per hour and square meters. The resulting lift will be expressed in kilograms. The coefficient of lift is dimension-less and does not change. The lift formula is taken from the sister science of aerodynamics. Because air is less dense at altitude and denser at sea level, the density is usually included in aircraft lift calculations. Temperature also affects air density. Water varies little in density even with changes in temperature and altitude, like at lake Titicaca, 3800 meter above sea level. One could adjust for salt water vs. sea water, but the difference is small enough that I chose to avoid this unnecessary complication. Ray Vellinga  

Welcome to the forum ,Ray!  

Foiler Design-The Moth The next three posts will give some accurate data(as of 2009 I hope) of three current foilers to give a general idea of what is required for a practical foiler The Moth was the first sailing bi-foiler in history and John Iletts incarnation in 1999 was the first foiling Moth to use a wand as altitude control. Ilett patterned the wand system after Dr. Bradfields system except that he located the wand near the bow-but the princible was identical. The Moth has created a revolution in dinghy sailing with speed that beats all sailboats under 20' in foiling conditions. Pretty incredible for an 11' boat(+gantry). Thanks to Ray Vellinga's new book-"Hydrofoils Design Build Fly" for some information from Table 18-5, page 219. Also see Bill Beavers paper on the Moth below. ================== LOA 11' + rudder gantry-appox. 12.75' -- Beam 7' 2" -- SA : a. main only= 86 sq.ft. -- Weight : a. hull+rig= 66lb.(varies a bit) b. crew= 175lb.(varies from 140-200lb) c. all up sailing weight= 241lb. -- Foils - two fully submerged foils, one on daggerboard ,one on rudder-area and loading varies between boats-see Vellinga's book and "The Foiling Guide" by Adam May(below): a. mainfoil area= 1.09 sq.ft. b. rudder foil area= .84 sq.ft. c. mainfoil loading @ 80% of sailing weight= 176.9 lb./sq.ft. d. rudder foil loading @ 20% of sailing weight= 57.38 lb./sq.ft. e. nominal angle of incidence of mainfoil= 0-1.5 degrees f. nominal angle of incidence of rear foil= 0 degrees( some have adjustable trim) g. mainfoil flap angle= +(down flap)=30 degrees; -(up flap)=15 degrees h. rudder foil flap angle= adjustable by tiller twist grip/ some boats angle the whole rudder instead of using a flap. -- Altitude Control System -As mentioned above most Moths use bow mounted wands, though some also have experimented with twin bow mounted wands and twin midship wands. A single bow mounted wand is to one side of the CL and therefore altitude can change tack to tack. Twin wands eliminate this problem. No experimentation yet reported in manual altitude control. ================= W/SA (weight in pounds divided by SA in sq.ft.)= 2.8 SA/total foil area (both sides-not struts)= 22.28 -----------------  

CSYSPaperFeb09 Beaver paper on Moth.pdf

Motheurogarda05_3041_t.jpg.

Foiler Design: Mirabaud Mirabaud is still the largest bi-foiler in history. Thanks to the generous help I received from Thomas Jundt, designer and owner of the boat this is accurate as of 2009. Awaiting details of the 2012 version with a new taller version of a wing sail and more total sail area. ================== LOA 26' -- Beam 17' -- SA : a.big rig= 355sq.ft. b.small rig=258 sq.ft. -- Weight : a. hull+rig= 374lb. b. crew= 528lb. c. all up sailing weight= 902lb. -- Foils - two fully submerged foils, one on daggerboard ,one on rudder: a. mainfoil area= 3.77 sq.ft. b. rudder foil area= 3.77 sq.ft. c. mainfoil loading*@ 80% of sailing weight= 191.4lb./sq.ft. d. rudder foil loading* @20% of sailing weight= 47.85lb./sq.ft. e. nominal angle of incidence of mainfoil= +.5 degrees f. nominal angle of incidence of rear foil= 0 degrees(trim+1 degree,-2 degrees) g. mainfoil flap angle= +/- 12 degrees h. rudder foil flap angle= +/- 12 degrees *Note from Thomas: a. at take-off(8.5 knots) loading is 50% main foil; 50% rudder foil b. at 23 knots all load on main foil -- Altitude Control System -Mirabaud uses twin wands set about halfway between the bow and the mainfoil and in 2010 will use, for the first time, a manual control system that bypasses the wand for direct crew control of the mainfoil flap particularly in rough conditions. ================= W/SA (weight in pounds divided by SA in sq.ft-big rig)= 2.54 SA/total foil area (both sides-not struts-big rig)= 23.54 -click on image-  

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Foier Design: Rave multifoiler The Rave multifoiler was designed by Dr. Sam Bradfield in the nineties. It uses three hydrofoils with dual independent wands that allow the leeward side to lift up and the windward side to pull down. All the RM for the boat is derived from the foils and not crew movement. Foil loading below is calculated based on a .5lb/sq.ft pressure@ a 10'CE spread evenly between the main foils. Loading goes up considerably as the boat sails in more pressure. ================== LOA 16' -- Beam 16' -- SA : a.main +jib= 195sq.ft. b.main,jib and screecher= 292sq.ft. -- Weight : a. hull+rig= 380lb. b. crew= 175lb. c. all up sailing weight= 555lb. -- Foils - three fully submerged foils,two forward ,one on rudder: a. mainfoil area= each 1.77sq.ft.; total 3.4 sq.ft. b. rudder foil area= 1.77 sq.ft. c. mainfoil loading*@ 80% of sailing weight= 176.48lb./sq.ft. d. rudder foil loading* @20% of sailing weight= 58lb. /sq.ft. e. nominal angle of incidence of mainfoil= +2.5 degrees f. nominal angle of incidence of rear foil= 0 degrees g. mainfoil flap angle= +/- 20 degrees h. rudder foil flap angle=+/- 20 degrees *Note -- Foil loading at .5lb/sq.ft. pressure (approx. 8.69 knots/10mph) / angle of incidence measured from static waterline. Altitude Control System - The Rave uses dual ,independent wands that not only control altitude but control righting moment as well. Some Raves have been successfully sailed/raced with manual altitude control.The boat features retractable foils and also has two "flight" settings to reduce draft where required.The wands retract with the foils and are attached to the foils. ================= W/SA (weight in pounds divided by SA in sq.ft-big rig)= 2.84 SA/total foil area (both sides-not struts-big rig)= 18.36 ==================== Wand system animation: https://docs.google.com/viewer?a=v&...WFpbnxoeWRyb3NhaWx8Z3g6NjU0NDdkNmI0ZDFmOGM2ZA Click on the little arrows at the top left..... Short Wand System Video : http://www.youtube.com/watch?v=yuFwDm8t3IM  

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Rave hydrosail site.jpg.

The Rave , Hobie Trifoiler, Skat(40'Rave) and Dr. Bradfields new 18' Osprey all use dual independent altitude control systems. Here is a description of this type of system by Greg Ketteman, designer of Long Shot and the Hobie Trifoiler: HYDROFOIL SAILBOATS IN GENERAL / HYDROFOIL CONTROLLED STABILITY "Hydrofoil boats can be categorized into two categories; 1) Incidence controlled hydrofoils* and 2) surface piercing hydrofoils. The difference lies in the way the boat maintains the proper altitude above the water surface. A surface piercing hydrofoil boat maintains proper height by varying the amount of foil submerged. The boat raises up as the speed increases and reduces the amount of foil submerged and therefore the lift. The boat finds equilibrium at the proper altitude. An incidence controlled hydrofoil sailboat has a mechanism that controls the angle of attack of the foil to maintain the proper altitude. It is generally believed that surface piercing is simpler, but incidence control is more efficient. In reality, it is the method that works with fewer problems that is simpler. From the beginning it was felt that incidence control was better suited for a sailboat even though most of the existing hydrofoil sailboats were of the surface piercing type. There are many advantages of the incidence controlled foils; however, the most important is what I call the DLA (dynamic leveling affect). This is the increase in righting moment or stability due to the ability of the windward foil to pull down. The DLA has little affect on the low wind performance, but it essentially makes the top speed of the boat limited to the strength of the boat. Conventional boats with a finite amount of righting moment can only extract so much power from the wind, but with the DLA, the righting moment is virtually unlimited. Intuitively many people think that the added drag of the windward foil plus the increased induced drag of the leeward foil would offset the gain in righting moment, but calculations show and practice proves otherwise. The dynamic leveling affect not only produces a dramatic increase in top speed, but is also responsible for all the other key features that this stability provides. The other major advantage of the incidence controlled foils is they are less affected by the waves and other surface affects. Drag and losses associated with the surface are the major reason incidence controlled foils are more efficient. All hydrofoil sailboats have problems with ventilation; however, surface piercing foils have larger problems, because the foils are piercing the surface at a smaller dihedral angle which makes it easier to ventilate." ------ * On the Trifoiler the entire foil was moved to control RM, lift and negative lift hence the term "incidence controlled foils". On the Rave the incidence was generally fixed at +2.5 degrees for the main foils though some owners found a way to decrease the incidence on the windward foil. Lift and negative lift on a Rave foiler is generated by the wand (designed by Dr. Sam Bradfield), a surface sensor(dragging in the water) and attached directly via linkage to a flap on each main foil. The wands are independent just like the trifoiler "incidence controlled" foil sensors. http://www.hobiecat.com/sailing/TriFoiler History Original/Magazine Articles/Multihulls 1990.htm  

Foil Assist Foil Assist is when a hydrofoil or hydrofoils are used to reduce the displacement and wetted surface of a boat without causing the whole boat to fly. Generally, "foil assist" does not require an altitude control system. Foil assist is used on multihulls( Orma 60's, Mod 70's, Banque Populaire V, A Class and C Class cats, Nacra 20, Sea Cart 26, Marstrom 32 and many more) and monohulls( I-14, National 12, Quant 28(DSS), Welbourn 25(Brace,Brace,Brace-DSS), JK 50 cruiser racer(DSS), and more. =============================================== Here's an interesting tidbit from an article on ORMA Tris about the introduction of curved lifting foils a few years ago: full article: http://www.wired.com/wired/archive/14.05/sail_pr.html Most radical of all has been the introduction of curved foils below the hulls that act like wings, producing vertical lift and reducing drag by raising even the leeward hull out of the water. Lift equals speed; speed begets more speed. "Sometimes you are even 100 percent on the foil - the whole boat on one single point," says Vincent Lauriot Prévost, the Brittany-based designer behind seven of the multihulls that competed in the Jacques Vabre. "Six years ago, foils took 30 percent of the displacement, and the boats were sailing at 28 or 29 knots," he says. "This year it's 70 percent, with speeds of up to 39 knots, which means you are sailing on the very sharp edge of a knife. We are pushing the limits of the machines." Flying at 45 miles per hour on a foil means the boats are as precariously balanced as a tightrope walker. If the foil loses lift for any reason - turbulent water, say, or a floating log - bad things happen. Fast. ================== And from an Interveiw about the Mod 70's-replacing the ORMA 60 tris: Excerpt from interview by Lia Ditton with Vincent Prevost on SA frontpage: MOD 70 LD: The Multi One Design or MOD 70 is the so-called new one design, which will replace the ORMA 60 fleet. You must be very excited to be the designers for this project. What new ideas about multihulls were you able to exercise with this class? VLP: The new idea is a very pragmatic one: conceive a multihull, which is the synthesis of 20 years of evolution and development. A new generation of hull shapes, more seaworthy platforms, increased safety factors for structure reliability, all that staying in the same type of power as the last ORMA 60 projects. LD: “A fast, modern design with foils” says your VPLP website. Am I correct in assuming that if these foils are designed to support the same percentage of displacement as an ORMA tri, a single foil could lift up to 70% of the boats weight in "normal" sailing? VLP: Correct. LD: At what point would you say a multihull becomes a ‘foiler’ rather than ‘with foils?’ VLP: Good question! No one has ever asked me that! Maybe a foiler would apply to ‘Geant’ whose main rudder was equipped with a T foil with adjustable angle of attack to get rid of hull buoyancy at the stern. LD: Standardizing and establishing a one design of this size with such a varied design brief (offshore, inshore, full crew, short handed) must have presented a great deal of technical challenges. What sort of design boundaries has the MOD 70 project pushed for you? VLP: The MOD 70 design brief is clearly for offshore races with 6 people on board. Inshore races will happen but it is not the primary goal of these boats. LD: Wave piercing bows seem to be the zeitgeist. Can they be seen as one approach to addressing two different problems: reducing pitch for the purpose of stabilizing airflow over the sail plan and moving reserve buoyancy forward in big seas to reduce the chance of pitch poling? If we assumed a fixed LOA then wouldn't the later of these two benefit by adding reserve volume to the extremes thus allowing a more powerful hull and rig for the length? VLP: These kinds of bows are possible when, as for the MOD 70, hull length is not limited by any box rule. The volume distribution forward of the front beam to counteract pitch poling is distributed more aft and the extra length allows us to have a finer angle of entry, reducing drag when passing through waves. LD: Isn't the hull length of the MOD 70 limited by the box rule? VLP: The idea was to design a boat safer than an ORMA, more in terms of structural reliability than in high speed behavior, notably the longitudinal stability, all of that by keeping the load numbers of an ORMA (same range of rig size, same deck gear, same appendages etc…) Increasing the reliability implies adding structure and weight at the same time. To keep the same load values, we have reduced the beam size to end at the same righting moment. About longitudinal stability, we replaced the bowsprit with a main hull bow extension to reach the same overall length, which corresponds to adding 10 feet. This extra main hull length at the front, combined with the 5 feet increase in the length of the floats adds some important buoyancy forward to delay the time when the boat could capsize. So you are right! I made a shortcut. It is not a box rule, all boats being identical as monotypes. picture: Mod 70-click on image-  

Mod 70 new tri.jpg

Trimaran mod 70 rocker main vs rocker ama.jpg, trimaran- first mod 70.jpg.

Gary Baigent Senior Member

I didn't know that Geant, now Vodafone, had an inverted T rudder setup - she certainly didn't have one when assembled and launched here, maybe the crew has tried it out since. Michel Desjoyeaux was/is always pushing the boundaries (was frustrated that the MOD 70 fleet won't allow him to experiment) so it's not surprising. Gitana X (the ORMA with the X beam design) also had T rudders but that boat was the dog of the fleet, (although designed by a collection of brilliant designers) mostly because she was too heavy, I think. Maybe also, too many superb brains clashing during the design.  

Copy of IMG_1689.JPG

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Technical info from 2010 on Doug Halsey's Broomstick foiler: -- This is the fourth in my series presenting the technical details of as many foilers as possible. So far: 1) Mirabaud post 10 2) Rave post 11 3) Moth post 9 -------------------------- Broomstick -- Video compiled in 2011 here: http://www.boatdesign.net/forums/mu...oil-trimaran-broomstick-41082.html#post513023 This boat is a surface piercing foiler designed and built by Doug Halsey and I thank him for his generous contribution of this information. -- LOA 15' --- -- BEAM max 17' Between tips of "V" foil 13'9" --- SAIL AREA : 1) 108 ft^2 using the usual rig (main + jib) 2) 143 ft^2 using the larger main + same jib (haven't used since 2007) --- WEIGHT : ~210 lbs for usual setup (smaller rig, 7' amas, etc.) + crew=355lb --20 lbs less when I don't use the amas-190lb+crew= 335lb --20 lbs more when I use the larger rig-230lb+crew= 375lb --145 lbs = crew weight --- FOILS : Describing surface-piercing foils is obviously harder than fully-submerged foils, since the area is constantly changing, but maybe this will make sense: -- Dihedral Angle = 60 Deg. -- Chord = 6" -- Section Shape = Conventional, uncambered, untwisted ~NACA 0012 -- Foil Span = 23.3". This is the horizontal projection of the span of 1 foil at takeoff, which I'm defining as the point where the lowest point of the main hull just touches the water, assuming the boat is level in both pitch & heel. At higher speeds, the span of the leeward foil reduces to some fraction of this value, depending on the speed, position of my weight, main-foil incidence setting, aft-foil incidence, etc. Span of the windward foil reduces much more, sometimes almost to 0. -- Foil Area = 23.3" X 6"= 139.8 In^2 = .97 Ft^2 (Again at takeoff-each side) -- Main-Foil Incidence : Currently ~8 1/2 Deg. with respect to the design waterline. This has been changed a few times over the years, each change requiring patching & redrilling bolt holes in the foil brackets (attached to the crossbeam) & the vertical stubs (at the top of each foil). Too small an angle gives insufficient righting moment when hullborne. Too large an angle gives too much bow-down pitch angle when flying. -- Note :I've designed a new set of main foils, with similar planform, but cambered & twisted sections. Also a new deeper aft foil. It will probably be several more months before I sail on foils again though. --- AFT-FOIL INCIDENCE : This angle is adjusted with a screw mechanism & control bar under the tiller. In past years, I would have to luff up, lean over the aft deck & go thru all sorts of contortions to adjust it, so it was usually set at some sort of compromise angle. Last year, I improved it so that I can adjust it (on 1 tack anyway) by twisting the tiller extension. That way I could try to find the "optimum" setting where the boat flies at just the right height for the existing conditions. --- Construction -- Main Hull - planked foam, epoxy glassed -- Ama -stich and glue/ 3mm Okume ply -- Foils - wood,hand shaped and epoxy glasse More info here(scroll to near bottom): http://www.foils.org/gallery/sail.htm Since that time there have been several improvements & speeds over 25 knots have been recorded on 3 separate occasions. ========== Pictures below by Terry Curtiss:  

Broomstick_4.jpg

Broomstick_6.jpg.

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Hydrofoils for Sailboats

• By By Steven Callahan
• Updated: July 29, 2020

Hydrofoils have been providing dynamic lift since fish sprouted fins. And people have been employing foils ever since they first put paddle to water, and certainly since adding keels and rudders to boats. But the modern, flying America’s Cup boats, kiteboards, Moth dinghies, shorthanded offshore thoroughbreds—these are all ­playing in a new world in which the terms “hydrofoils” or “lifting foils” describe those oriented to raise a hull or hulls from the water. In these racing realms, if you ain’t got foils, you ain’t got nothin’.

Lifting foils that allow these boats to sometimes home in on three times the wind speed might appear to be of little interest to cruising sailors, but with such common cruising features as self-steering and autopilots, self-tailing winches, rope clutches, fin keels and faster hull shapes all having been passed down from the racing scene, one must ask, “What promise, if any, do hydrofoils hold?”

Lifted or partially lifted boat patents extend back to 1869, but workable watercraft took roots along with early flight. Italian Enrico Forlanini began experimenting with foils in 1898. In 1906, his 1-ton 60 hp foiler reached 42.5 mph. Alexander Graham Bell’s HD-4 Hydrodrome flew on Bras d’ Or Lake at 70 mph in 1919. And several sailing foiler patents began appearing in the 1950s. Notably, JG Baker’s 26-foot monohull, Monitor, flew at 30-plus mph in 1955. Baker experimented with a number of foil configurations, and at least built, if not used, the first wing mast. The first offshore foiler was likely David Keiper’s flying trimaran, Williwaw , in which he crisscrossed the Pacific in the 1960s.

By the 1980s, numerous speed-trial and foil-enhanced offshore-racing multihulls showed huge promise, and have since evolved into behemoth trimarans clocking 30 to 40 knots continuously for long periods, not to mention the monohulls in the Vendée Globe (and soon the Ocean Race) that are capable of speeds exceeding 30 knots. But as boat designer Rodger Martin once reminded me, “If you want a new idea, look in an old book.” He was right. The fully foiling monohulls that will compete in the 2021 America’s Cup will bring things back full circle to the foiling monohull Monitor .

Fluid Dynamics Primer

Any foil—a wing, sail, keel, rudder or lifting foil—redirects the flow of fluid (air included), creating high- and low-pressure areas on opposite sides of the appendage, while developing lift perpendicular to the foil’s surface.

Advancements in foiling science is due in part to the hundreds of foil shapes that were tested, with tabulated results, by the National Advisory Committee for Aeronautics, the forerunner of the National Aeronautics and Space Administration. For the better part of a century now, aircraft and boat designers have been able to choose from a spectrum of refined foil sections that produce predictable amounts of lift and drag for known speeds of fluid and angles of attack, or the angle at which the foil passes through the fluid. Sections of efficient faster foils, as seen on jets or as we flatten our sails to go upwind or reach high speeds, have smaller nose radii and are thinner, with the thickest section of the foils farther aft, up to nearly halfway toward the trailing edge.

The most efficient foil sections at slow speeds are fatter, with the maximum thickness farther forward, and with larger nose radii, than faster foils. The angle to fluid flow or angle of attack also is greater. We see these slower foils on wings of prop planes and sails when off the wind or in light conditions.

Most sailors are familiar with traditional foils on boats, the teardrop sections of keels that produce lift to weather, reducing leeway, and of rudders, allowing them to steer. Even a flat plate can be a foil, but these tend to be inefficient. Such a shape is prone to fluid separation from the surface, meaning they stall easily, and they maintain poor lift-to-drag ratios. Even keels and rudders are somewhat lift-­compromised because they are ­symmetrical and have to work with fluid coming from either side, whereas lifting foils are more like aircraft wings or propellers, with asymmetrical sections honed for performance in a more stable, fluid flow.

The point is, any foil can be employed at various angles to the surface to prevent leeway, produce increased stability, or help lift the boat out of the water. But those not required to work with fluid flowing from opposite sides can then be honed to maximize lift and minimize drag. Asymmetrical foils were used on boats like Bruce King’s bilgeboarders, including Hawkeye , back in the 1970s. And, designers, including Olin Stephens, had previously employed trim tabs behind keels to improve keel performance.

Sails, which are heeled airfoils, not only drive the boat forward, but they also produce downforce, actually increasing the dynamic displacement of the boat. To counter this and keep the boat sailing more upright, multihull designer Dick Newick first employed slanted asymmetrical hydrofoils in the outer hulls of his small charter trimaran, Lark , in 1962. A portion of the lift developed by the hydrofoil resisted leeway, while a portion worked to actually lift the leeward hull, keeping the boat more upright and reducing dynamic displacement and drag.

Anyone who has ridden on even a foil-stabilized boat will know how riding at least lightly on the waves, and especially above them, beats smashing through them. When boats lift off, everything gets a lot smoother, drag falls away, and the boat accelerates.

Cruising on Foils

But why would a cruiser want to whip over the sea? Wouldn’t this demand an inordinate amount of attention by the crew? Would lifting foils even be applicable to a boat that must have substantial displacement to carry crew and stores? Aren’t cruising-boat hydrofoils an oxymoron?

Maybe, but I believe our boats’ hulls are likely to sprout fins much as fish have as we orient foils to more efficiently resist leeway, add stability, aid steering, reduce drag, increase comfort, allow for shallower draft, and enhance wider ­variations in hull shapes.

Boats have gotten increasingly wide through the years to advance form stability, improve performance (primarily off the wind), and boost interior volume. But the downside is that fat boats tend to slam more upwind. What if you could reduce dynamic displacement of the boat and lift that hull even partially from the water? The result would be less slamming, especially upwind.

At the same time, what about narrower boats that are known for being more seakindly, especially when closehauled, but lack form stability to carry adequate sail area for powering upwind, and tend to roll badly downwind? Or shallow-draft vessels that are lovely for cruising, but again, tend to suffer from reduced stability? Foils can give that stability back.

Looking ahead, boat ­designers might choose to reduce ballast, making up for it with a foil. In short, lifting foils can reduce boat drag and motion while increasing power and performance.

Pitching also does no favors for speed or crew comfort. Foils can come into play here as well. Foils parallel to the sea’s surface resist motion up and down, and a lifted boat skating above chop also is less prone to hobby-horsing through waves. Multihulls have always been particularly susceptible to pitching for a number of reasons, but watching videos of multihulls sailing to weather show an obvious huge advantage that foilers have compared with nonfoilers. Offshore multihulls now routinely employ T-foils on the rudders to control the fore and aft angles of the boat (attitude), a feature easily adaptable to any vessel.

OK, so what’s the cost? Obviously, the more things sticking through the hull, ­especially if they are retractable, the more it’s going to impact the interior. There would be added weight, complexity and cost. Foils also create noise, and there’s susceptibility to damage from hitting stuff. And let’s not forget compromises with shapes, purposes and things not yet imagined.

As for damage, it’s possible to fold the foils back into the hull. Think swinging center- boards or actual fish fins. Daggerboardlike foils can at least employ shock-absorbing systems similar to the daggerboard arrangements found in many multihulls. This includes weak links that are outside the hull, so if a foil is struck, it frees the foil to fold back or to come off before being destroyed or damaging the hull. Or, foils might hang from the deck rather than penetrating the hull, allowing them to kick up (and to be retrofitted to existing boats). These configurations also relieve the interior of intrusions, and keep the noise more removed from it. I have no doubt that numerous talented designers will be exploring all kinds of options and compromises in coming years, finding ways to make foils both practical and more than worth the compromises.

Sailing more upright, ­shallower draft, speed, ­comfort—what’s not to like? Just what is possible? I have a feeling the cruising community is about to find out.

Steven Callahan is a multihull aficionado, boat designer and the author of Adrift , an account of his 76 days spent in a life raft across the Atlantic.

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You are here: Home › Marine › Design Tips & Fabrication Overview › Design Tips

The information found here is a collection of tips to help with your foil design, whether you are starting from scratch or modifying a pre-existing design. This will not make you an expert, but should help you understand some of the tradeoffs in foil design (all designs are a compromise after all, as for any given boat there is no one perfect design for all sailing conditions and sailing styles).

While foil design may in fact be rocket science, don’t be afraid. Yes, there are a number of specialized foil design software programs and books on the subject available for those who are truly interested in the subject. But a conservative design of simple planform and foil section, built smooth and fair, will be fast and work well in all conditions.

I’ve taken a fairly simple approach to these subjects, but if you spot any glaring errors feel free to drop me a line.

Common Subjects

Terminology.

• Foil theory
• Fabrication

Centerboards

• Gybing heads
• angle of attack – the angle the foil faces to the flow of water over it (this varies on rudders as you steer the boat).
• aspect ratio – the comparison of the length to the width of the planform of your rudder or centerboard. High aspect ratio foils are long and skinny while low aspect ratios are short and squat. Boats with higher maximum speeds are more apt to use high aspect ratio foils (think of the fin on a sailboard as the ultimate example of this) while slower boats are more suited to lower aspect ratio foils.
• chord – the distance from the leading edge to the trailing edge.
• ellipse – a ‘stretched’ circle (oval). An important shape in planform design.
• foil section – the cross-sectional view of your rudder or centerboard (Similar to the cross-section of an aircraft wing [airfoil] but nearly always symmetrical from side to side).
• NACA 4 digit series (NACA0008, NACA0010, etc) – the most popular foil sections used in sailboats. The first 2 digits (’00’) indicate that the section is symmetrical, and the last 2 digits (’08’, ’10’ etc) define the maximum width of the section as a percentage of the length of the chord. In other words, a NACA0008 foil is 8% as thick as the foil is wide. The maximum thickness on these foils is found at a distance 30% back from the leading edge.
• planform – the outline of your rudder or centerboard when viewed from the side
• spline – a curved line that is defined by a number of points that it passes through (or close to) – think of flexing a thin piece of wood around those points. There are several different mathematical forms of splines with different properties.

First of all, the obvious: the planform you design must fit within your fleet’s class rules. Some fleets offer complete freedom in this, while others may force you to fit within a few mm tolerance of their official dimensions.

The simplest planform is the simple rectangle, and will give good performance while also being easy to fabricate.

A performance increase can be had by going with an elliptical tip – this can be done with either an elliptical leading edge or trailing edge while keeping the opposite edge straight (or the tip can be fully elliptical). The performance increase comes from the increased lift and reduced drag from this shape – with a rectangular tip, water can ‘escape’ by going around the bottom of the foil rather than across the chord, while with an ellipse** its always a shorter path across the chord than it is around the bottom.

If choosing between an elliptical leading edge / straight trailing edge, or straight leading edge / elliptical trailing edge, the former of these is mechanically stronger (this may not be an issue if your composite layup is sufficiently strong).

A few other notes – when an elliptical foil stalls, it stalls all at once. By comparison, a foil with a squared off tip will have the tip stall first – if the helmsman feels this flutter they may be able to correct in time.

(** If you want to split hairs, what’s really important is that the pressure differential over the foil have an elliptical distribution – this doesn’t necessarily require the planform be semi-elliptical.)

The thickness of a rudder (as a percentage of its chord) will be greater than the thickness of a centerboard. Why? Because thin foil sections stall at lower angles of attack than thick sections. Centerboards may be thin because they always travel in the same direction through the water as the boat hull. But rudders sweep through the water as they steer the boat.

• thin foils give lower drag, but less lift, and they stall at lower angles of attack.
• thick foils have higher drag, but more lift, and they won’t stall until higher angles of attack. Thick foils are also mechanically stronger, all else being equal.

Considerations:

• if you steer aggressively in waves, then consider going for a thicker section.

The NACA0012 section (12% as thick as its chord) is a good compromise.

A final performance tweak is to make the cross section of the rudder thinner at the waterline – this yields another drag reduction. The trade off is one of mechanical strength, as this is a very high load area to begin with.

How big should your rudder be? Rudders contribute to both lift and lateral resistance, and this is taken into account by the boat designer when determining foil sizes. If designing a new rudder for your boat, keep the underwater surface area approximately the same as that of your original rudder (even if changing its shape). If you don’t know this measurement, then a good rule of thumb is to have the surface area of the rudder about 1/3 as large as the surface area of the centerboard.

Aspect ratio: if you’ve ever had the experience of going really fast downwind in your sailboat and had the rudder feeling just ‘go away’ (it feels like there’s nothing back there) then you’re a good candidate for a higher-aspect ratio rudder blade. A high aspect rudder has a more sure feel to it at higher speeds, but with the trade off of losing steerage earlier in light air. High aspect ratio rudders also have to be built stronger – the longer blade has more leverage and is more likely to break if it loads up (if a rudder is going to break, it usually breaks where it enters the headstock).

Balanced or not?: a rudder that’s leading edge is forward of its pivot point is said to be balanced, and requires less effort to steer. Balanced rudders are more common in keelboats, but are also possible in dinghy’s.

The angle of attack is low for a centerboard (its the same as the leeway of the boat… about 3 degrees). This allows a thinner foil section to be chosen than you would use for a rudder. The thinner section gives less drag, and you don’t have to worry about the board stalling the way a rudder will. An 8% thickness-to-chord ratio (like the NACA0008 section) is a good general purpose starting point for your design.

Tapered or not? Some builders taper the absolute thickness of their boards as the chord shortens towards the tip. This maintains the same relative thickness and ideal foil section as the board narrows. Others do not taper, with the result that the relative thickness at the tip is much higher than the thickness at the hull. Why? One reason not to taper is to achieve mechanical strength – remember, you have to be able to stand on your centerboard if you capsize – but this comes at a cost of having a higher drag foil section at the tip. How to decide? If you specify a cloth layup over the core of sufficient strength, then go ahead and taper to maintain your ideal foil section. But if, for example, you are building a board out of mahogany with no covering, then don’t taper it – it won’t be strong enough.

One other consideration – sometimes your class rules will prevent you from having a thick enough board relative to its chord where it exits the hull. If for example your maximum centerboard slot is specified as 30mm, but your board must have a chord of 500mm where it exits the hull, then the most you can achieve is a 6% foil section. In this case keep the board at 30mm thickness until it tapers to the point where the ideal foil section is achieved (375mm chord for an 8% section at 30mm) and then begin tapering it from there.

Stiffness: can a centerboard ever be too stiff? Yes! What happens when you’re sailing along and a gust hits? Answer: the boat heels. A well rigged boat will be optimized for crew weight so that when the gust hits, the mast tip bends off to shed power, and the boat heels less and squirts forward rather than slipping off to the side. In a similar fashion, if your centerboard flexes when the gust hits, this will also reduce the amount of heel and contribute to forward motion. So ideally the stiffness of your centerboard is tuned to your crew weight and the amount of sail power that you can carry. Racing sailboarders are probably at the leading edge of this, selecting their fins according to their weight and aggressiveness.

Gybing Heads

Some boats use what is known as a “gybing head” design. If you look at the following picture, you can see that the head is trapezoidal in cross section, rather than the more typical rectangular.

What this does is allow the centerboard to pivot inside the centerboard trunk (rotating about the point of maximum thickness of the head). The forces causing the board to pivot are (1) the lift being generated by the foil, and (2) the lateral resistance from the foil preventing the boat from slipping sideways through the water. Usually you can view the lift vector as being concentrated at the 25 % chord, while the lateral resistance is more or less evenly distributed across the foil. If the pivot point is greater than 50% back from the leading edge of the foil, then the forces will cause the centerboard to be pressed against the side of the trunk, and it is “gybed” into position. Typical centerboard gybing angles are about 3 degrees. The centerboard only gybes when it is all the way down – as soon as you start to rake it back in the trunk, the fat leading edge of the foil will jam enter the trunk and cause the centerboard to jam in a straight fore-aft position.

Why do you want a gybing centerboard? By the board rotating to windward, you increase the angle of attack of the centerboard with respect to the centerline of the hull, and therefore generate more lift, all else being equal. In practice this means you can steer the boat slightly lower and sail with a fuller jib for increased power while maintaining the same leeway angle as a similar boat with a non-gybing board. That’s the theory – some fleets like them, some don’t (and many one-design classes don’t allow them). Frank Bethwaite in “High Performance Sailing” claims they are a bad idea as the rudder may end up directly in the disrupted flow from the centerboard.

Click  here  for a short overview of the fabrication process.

More to come…

If you want to learn more, there is a wealth of information on the internet. I won’t include the links (I don’t want to maintain links, and that’s what search engines are for!) but some of the pages worth looking for include:

• Design and Construction of Centerboards and Ruddersby Paul Zander
• Aerodynamic Forces on the Hobbyinc page
• Rudder Strength by Richard Hinterhoeller on the Shark class home page
• Emergency Rudder Design and Construction by Paul Kamen
• Foil Design Parameters and Performance tutorial on David Vacanti’s page
• The Totally Free Foil Primer by John Dreese
• 5o5 Fins by Bransford Eck on the Int’l 5o5 class page

Hardcover references on my bookshelf include:

Aero-hydrodynamics of Sailing by C.A. Marchaj

High Performance Sailing by Frank Bethwaite

Marine Composites by Eric Greene Associates

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Hydrofoil Design Basics [2023]

• November 15, 2023
• Hydrofoil Basics

Are you ready to take your hydrofoil boarding to the next level? Understanding the basics of hydrofoil design is essential for maximizing your performance on the water. In this comprehensive guide, we’ll dive deep into the world of hydrofoil design, covering everything from the different types of hydrofoil designs to the configuration of a hydrofoil. So grab your board and let’s get started!

Table of Contents

Quick answer, quick tips and facts, types of hydrofoil designs, configuration of a hydrofoil, choosing the right hydrofoil design, hydrofoil design for beginners.

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Hydrofoil design is a crucial aspect of hydrofoil boarding. The design of a hydrofoil consists of several components, including the foil head, foil mast, fuselage, front wing, and stabilizer. These components work together to provide lift, stability, and maneuverability on the water. The type of hydrofoil design you choose will depend on your skill level, riding style, and personal preferences.

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Before we dive deeper into hydrofoil design, here are some quick tips and facts to keep in mind:

• Hydrofoils are made up of several components, including the foil head, foil mast, fuselage, front wing, and stabilizer.
• Many companies offer modular hydrofoil systems, allowing riders to mix and match components for their preferred setup.
• Longer mast lengths provide more leeway for overfoiling and offer stability, while shorter masts offer zippier turns.
• Longer fuselages help with earlier takeoffs and stability, while shorter fuselages are better for maneuverability.
• There are two main types of hydrofoil wings: low aspect and high aspect. Each type has different characteristics in terms of lift, speed, and stability.
• Most hydrofoil wings are made of pre-preg carbon, which is known for its high performance. However, some brands offer alternative materials such as G-10.
• The size of the front wing surface area plays a crucial role in the performance of a hydrofoil. Larger front wings are recommended for average weight riders in moderate breeze, while smaller stature riders can get away with smaller wings.

Hydrofoil boarding has gained immense popularity in recent years, thanks to its ability to provide an exhilarating and unique riding experience. But what exactly is a hydrofoil, and how does it work?

A hydrofoil is a wing-like structure that is attached to the bottom of a surfboard, kiteboard, or stand-up paddleboard. When the board is in motion, the hydrofoil generates lift, lifting the board out of the water and reducing drag. This allows riders to glide smoothly above the water’s surface, providing a sensation of flying.

The design of a hydrofoil plays a crucial role in its performance. Each component of the hydrofoil, from the foil head to the stabilizer, is carefully engineered to optimize lift, stability, and maneuverability. Let’s take a closer look at the different types of hydrofoil designs and their configurations.

Hydrofoils come in various designs, each offering unique characteristics and performance capabilities. Here are some of the most common types of hydrofoil designs:

Low Aspect Hydrofoils : Low aspect hydrofoils have a wider wing span and a thicker profile. They provide excellent lift and stability, making them ideal for beginners and riders looking for a forgiving and easy-to-control hydrofoil.

High Aspect Hydrofoils : High aspect hydrofoils have a narrower wing span and a thinner profile. They offer higher speeds and increased maneuverability, making them popular among experienced riders who prioritize performance.

Modular Hydrofoils : Many hydrofoil manufacturers offer modular hydrofoil systems, allowing riders to customize their hydrofoil setup. These systems typically consist of interchangeable components, such as different front wings, stabilizers, and fuselages. This flexibility allows riders to fine-tune their hydrofoil to suit their riding style and conditions.

Surf-Specific Hydrofoils : Surf-specific hydrofoils are designed specifically for riding waves. They often feature larger front wings and stabilizers to provide maximum lift and stability in the surf. These hydrofoils are popular among surfers who want to take their wave riding to new heights.

Kiteboarding Hydrofoils : Kiteboarding hydrofoils are designed to provide maximum lift and efficiency for kiteboarders. They typically have larger front wings and shorter masts to generate lift at lower speeds. Kiteboarding hydrofoils allow riders to ride in lighter wind conditions and perform impressive jumps and tricks.

Wing Foiling Hydrofoils : Wing foiling hydrofoils are designed specifically for wing foiling, a relatively new water sport that combines elements of windsurfing, kiteboarding, and hydrofoiling. These hydrofoils are optimized for low-speed lift and maneuverability, allowing riders to glide effortlessly across the water using a handheld wing.

Each type of hydrofoil design has its own advantages and disadvantages. It’s important to consider your skill level, riding style, and the conditions you’ll be riding in when choosing a hydrofoil design.

To understand hydrofoil design better, let’s take a closer look at the configuration of a hydrofoil. A typical hydrofoil consists of the following components:

Foil Head : The foil head is the front part of the hydrofoil that attaches to the board. It houses the front wing and is responsible for generating lift.

Foil Mast : The foil mast is the vertical strut that connects the foil head to the board. It provides stability and allows the hydrofoil to move through the water smoothly.

Fuselage : The fuselage is the main body of the hydrofoil that connects the foil head to the stabilizer. It provides structural support and stability.

Front Wing : The front wing is the main lifting surface of the hydrofoil. It generates lift as water flows over it, allowing the board to rise above the water’s surface.

Stabilizer : The stabilizer is a smaller wing located at the rear of the hydrofoil. It provides stability and helps control the pitch and yaw of the hydrofoil.

Each component of the hydrofoil is carefully designed to optimize performance and provide the rider with a smooth and controlled riding experience. The size, shape, and material of each component can have a significant impact on the hydrofoil’s performance characteristics.

Choosing the right hydrofoil design is crucial for maximizing your performance on the water. Here are some factors to consider when selecting a hydrofoil design:

Skill Level : Beginners may prefer low aspect hydrofoils, which offer stability and forgiveness. Experienced riders may opt for high aspect hydrofoils, which provide higher speeds and increased maneuverability.

Riding Style : Consider your preferred riding style. If you enjoy riding waves, a surf-specific hydrofoil may be the best choice. If you’re into kiteboarding, a kiteboarding-specific hydrofoil will provide the lift and efficiency you need.

Conditions : The conditions you’ll be riding in can also influence your hydrofoil design choice. If you’ll be riding in light wind conditions, a kiteboarding hydrofoil with a larger front wing may be ideal. If you’ll be riding in choppy waters, a hydrofoil with a longer fuselage can provide stability.

Modularity : Consider whether you want the flexibility to customize your hydrofoil setup. Modular hydrofoils allow you to mix and match components to fine-tune your hydrofoil to your preferences.

Brand and User Reviews : Research different brands and read user reviews to get an idea of the performance and quality of different hydrofoil designs. Look for brands with a good reputation and positive feedback from riders.

Remember, the best hydrofoil design for you will depend on your individual preferences and riding style. It’s always a good idea to try out different hydrofoils before making a purchase to see which design suits you best.

If you’re new to hydrofoil boarding, choosing the right hydrofoil design is essential for a smooth learning experience. Here are some tips for beginners:

Start with a Low Aspect Hydrofoil : Low aspect hydrofoils are generally more forgiving and easier to control, making them ideal for beginners. They provide stability and lift at lower speeds, allowing you to focus on mastering the basics of hydrofoil boarding.

Consider a Longer Mast : A longer mast can provide additional stability and make it easier to maintain balance on the hydrofoil. However, keep in mind that longer masts may limit your maneuverability, so it’s important to find the right balance for your skill level.

Take Lessons : Consider taking lessons from a qualified instructor to learn the proper techniques and safety guidelines for hydrofoil boarding. A professional instructor can help you choose the right hydrofoil design and provide valuable tips to accelerate your learning curve.

Practice in Calm Conditions : Start practicing in calm, flat water conditions to build your confidence and get a feel for the hydrofoil. As you progress, you can gradually venture into more challenging conditions.

Remember, learning to hydrofoil takes time and practice. Be patient with yourself and enjoy the journey of mastering this exciting water sport.

What is the most efficient hydrofoil design?

The most efficient hydrofoil design depends on various factors, including riding style, skill level, and personal preferences. High aspect hydrofoils are generally considered more efficient in terms of speed and maneuverability, but they require more skill to control. Low aspect hydrofoils offer stability and forgiveness, making them a popular choice for beginners and riders looking for a more relaxed riding experience.

What are the different types of hydrofoil designs?

There are several types of hydrofoil designs, including low aspect hydrofoils, high aspect hydrofoils, modular hydrofoils, surf-specific hydrofoils, kiteboarding hydrofoils, and wing foiling hydrofoils. Each type offers unique characteristics and performance capabilities, allowing riders to choose a design that suits their riding style and preferences.

What is the configuration of a hydrofoil?

The configuration of a hydrofoil includes the foil head, foil mast, fuselage, front wing, and stabilizer. These components work together to provide lift, stability, and maneuverability on the water. The size, shape, and material of each component can have a significant impact on the hydrofoil’s performance characteristics.

How do you hydrofoil for beginners?

Hydrofoiling for beginners requires patience, practice, and the right equipment. Here are some tips for beginners:

• Start with a low aspect hydrofoil for stability and forgiveness.
• Consider a longer mast for additional stability.
• Take lessons from a qualified instructor to learn the proper techniques and safety guidelines.
• Practice in calm, flat water conditions to build confidence and develop your skills gradually.

By following these tips and investing time in practice, you’ll be on your way to becoming a proficient hydrofoil boarder.

Read more about “How Easy is Hydrofoiling? …”

Understanding the basics of hydrofoil design is essential for maximizing your performance on the water. Whether you’re a beginner or an experienced rider, choosing the right hydrofoil design can make a significant difference in your riding experience. Consider factors such as skill level, riding style, and conditions when selecting a hydrofoil design. Remember to take lessons and practice regularly to improve your skills and enjoy the exhilarating world of hydrofoil boarding.

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• Hydrofoil Basics: Hydrofoil Basics
• Hydrofoil History: Hydrofoil History
• Advanced Hydrofoiling Techniques: Advanced Hydrofoiling Techniques
• Hydrofoil Equipment Reviews: Hydrofoil Equipment Reviews
• Hydrofoil Boat: All You Need to Know 2023: Hydrofoil Boat: All You Need to Know 2023
• McConks windSUP/windsurf/wing surf/wing foil guide – hydrofoil types …

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Can you surf waves with efoil [2024] 🌊.

• March 15, 2024

Can You Surf on a Foil Board? [2024] 🏄‍♂️

Can you hydrofoil on flat water [2024] ✅, leave a reply cancel reply.

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MAIN FUNCTIONAL REQUIREMENT:   Lift the boat’s hull outside the water.

DESIGN PARAMETER:   Hydrofoil (It is a foil or wing under water used to lift the boat’s hull until it is totally outside the water.)

GEOMETRY/STRUCTURE:

EXPLANATION OF HOW IT WORKS/ IS USED:

1. At low speeds the hull (body of ship) sits in the water and the hydrofoils are totally submerged in the water.

2. As the boat’s speed increases, the hydrofoils create lift.

3. At a certain speed, the lift produced by the hydrofoils equals the sum of of the boat and cargo weights. Therefore the hull comes out of the water.

4. Instead of having an increase in drag with increasing speed because the hull is lifted out of the water (contrary to what happens in traditional boats due to pressure drag), the hydrofoils provide a more efficient way of cruising. Decreasing the drag contributes to the better use of the power needed for the movement of the boat.

DOMINANT PHYSICS:

How is the lift produced - Fluid Dynamics.

For the purpose of this project two explanations will be presented in a general and basic way. These theories are the application of Bernoulli’s Equation and Euler’s Equation for Streamline Curvature Effect.

Bernoulli’s Equation:  Po = P 1 + � r v 1 � + r gy 1 = P 2 + � r v 2 � + r gy 2

  This equation applies to flows along a stream line which can be modeled as : inviscid, incompressible, steady, irrotational and for which the body forces are conservative. Also the difference on the height of the foil (the distance from the bottom section to the upper one) is small enough so that the difference r gy 2 - r gy 1 is negligible compared to the difference of the rest of the terms. What is left is that the pressure plus one half the density times the velocity squared equals a constant (the stagnation pressure) . As the speed along these streamlines increases ,the pressure drops (this will become important shortly) .   The fluid that moves over the upper surface of the foil moves faster than the fluid on the bottom. This is due in part to visous effects which lead to formation of vertices at the end of the foil . In order to conserve angular momentum caused by the counter-clockwise rotation of the vortices, there has to be an equal but opposite momentum exchange to the vortex at the trailing edge of the foil. This leads to circulation of the fluid around the foil. The vector summation of the velocities results on a higher speed on the top surface and a lower speed on the bottom surface. Applying this to Bernoulli’s it is observed that, as the foil cuts through fluid, the change in velocity produces the pressure drop needed for the lift. As it is presented in the diagram, the resulting or net force (force= (pressure)(area)) is upward.

This explanation can be enriched with the Principle of Conservation of Momentum. (Momentum = (mass)(velocity)) If the velocity of a particle with an initial momentum is increased, then there is a reactant momentum equal in magnitude and opposite in direction to the difference of the momentums. (See diagram). (Mi = Mf + D M) Euler’s Equation: d(p+ r gy)/dn = r v�/R

Here again, the term referring to the height is assumed negligible compared to the other terms in the equation. This equation says that as you go further from the center of the radius of curvature of a streamline, the pressure on the streamlines increases. The upper surface of the foil is closer to the center of curvature of the streamlines , therefore there will be a lower pressure than the ambient pressure above the foil. The difference between the pressure on the top surface and the ambient pressure at the bottom surface will produce a net pressure that will cause the lift.(See diagram.)

Angle of Attack:

As it has been presented, lift comes from the dynamics of the fluid in the area surrounding the foil. But the lift can be optimized by positioning the hydrofoil at an angle (relative to the incoming fluid flow) called the angle of attack (See diagram). The goal is to optimize the lift to drag ratio. This ratio depends on the shape of the foil, which in this case is considered to be a thin foil. With a small angle of attack, the lift increases rapidly while the drag increases at a small rate. After an angle of ~ 10� the lift increases slowly until ~ 15� where it reaches a maximum.  After ~15� stall can set in. When the angle of attack is 3� to 4� the ratio of lift:drag is at it’s maximum. So the foil is more efficient at those angles (3�and 4�) with lift to drag ratios of ~ 20 to 25:1

LIMITING PHYSICS:

At first, people can think that stalling is likely to be a problem in hydrofoils as it is in airfoils, but surprisingly it is not. A steep angle of attack is not needed in the design of the hydrofoil. On the contrary, small angles of attack are used on hydrofoils to optimize the lift to drag ratio as explained before.

What is a primary concern is the design of the foil, the struts/supports, and their positioning. All these features have to be taken in consideration.  So the features are designed to produce a minimum speed that will lift the boat of certain weight and keep it foilborne.

One problem that a hydrofoil craft can experience is the height of the waves being greater than the struts. Also, if the craft is traveling faster than the waves, the foils could break to the surface and outside of the water, resulting in a loss of lift and a negative angle of attack when the foil dives into the next wave, making the craft crash into the sea. Engineers have designed hydrofoils to minimize these limitations and better the ship’s performance.

PLOTS/GRAPHS/TABLES:

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SOME HYDROFOILS AND THEIR USE:

Hydrofoils have become very popular. They are used in various kind of sea traveling, from military use to watersports. The high speed, smooth cruise and better turns delivered by hydrofoils have been used in military ships. Sailing has also adopted the hydrofoils to gain more speed. They enable new inventions that can satisfy people’s desire to challenge danger , like the sky ski. It is a water ski with a hydrofoil attached which permits people to fly above the water surface. Every day more hydrofoils are used, and in the future, they may be the dominate method of sea traveling.

REFERENCES/MORE INFORMATION:

See also on this site: Airfoil , Sailboats

Alexander, Alan, James Grogono, and Donald Nigg; Hydrofoil Sailing . Juanita Kalerghi: London, 1972.

Bertin, John and Michael Smith; Aerodynamics for Engineers, Third Ediotion . Prentice Hall: New Jersey, 1998.

Hook, Cristopher and A.C. Kermode; Hydrofoils . Pitman Paperbags: London, 1967.

The International Hydrofoil Society’s Web Page: http://www.erols.com/foiler/index.html

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How Hydrofoils Work: Physics and Limitations

Hydrofoils are underwater wings that allow boats to glide over the surface of the water at high speeds, reducing drag and increasing efficiency. Hydrofoil technology is used in a wide range of applications, from racing sailboats to passenger ferries and military vessels. In this article, we’ll explore the physics behind how hydrofoils work and some of the limitations of the technology.

The Dominant Physics

Hydrofoils work on the principles of fluid dynamics, specifically Bernoulli’s principle and Newton’s third law of motion . When water flows over the curved surface of a hydrofoil, it creates areas of high and low pressure around the foil. The higher pressure under the foil creates lift, which causes the boat to rise out of the water.

This lift is proportional to the speed of the boat and the size and shape of the hydrofoil. As the boat’s speed increases, the hydrofoil generates more lift, reducing drag and allowing the boat to go faster with less power. The hydrofoil can also be adjusted to change the angle of attack, which affects the lift generated and the stability of the boat.

Euler’s Equation and Hydrofoils

Euler’s equation is a fundamental fluid dynamics equation that describes the motion of a fluid in terms of pressure, velocity and density. This equation can be used to model the behaviour of fluid flow around the curved surfaces of a hydrofoil. By using Euler’s equation, designers can optimise the shape and placement of the foils for maximum lift and stability.

The Angle of Attack

The angle of attack is the angle between the chord line of the foil and the direction of the water flow. It is an important factor in the performance and stability of hydrofoils as it affects the lift generated by the foils. At an angle of attack above the maximum lift coefficient, the lift generated by the foil begins to decrease and the foil may stall, leading to instability. Control of the angle of attack, by trim or other means, is a critical aspect of hydrofoil design and operation.

Limiting Physics of Hydrofoils

There are several physical limitations that can affect the performance and stability of hydrofoils. Some of the common limitations of hydrofoils include

• Cavitation Cavitation occurs when the pressure on the surface of the foil drops so low that water vapour bubbles form and then immediately collapse, causing localised erosion or damage. This can reduce the lift generated by the film and cause instability in the hydrofoil.
• Wave interference Hydrofoils can be affected by waves and wave interference which can cause instability or loss of lift. Large waves can also cause the foil to “porpoise” or bounce up and down uncontrollably.
• Foil design The shape, size and placement of the foils can significantly affect the performance and stability of the boat. Improper foil design can result in reduced lift, excessive drag or instability.
• Weight and balance Hydrofoils must be properly balanced and not to heavy, as excess weight can reduce the lift generated by the foils and increase drag, limiting the performance and efficiency of the hydrofoil.
• Human error Finally, poor piloting or decision-making can lead to accidents or loss of control, with serious consequences for crew and passenger safety.
• “Hydrofoil.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., www.britannica.com/technology/hydrofoil .
• Cummings, R. M., & Schultz, W. W. (2014). Computational Fluid Dynamics: An Introduction for Engineers. Springer.
• Korobkin, A. A. (2008). Hydrodynamics of High-Speed Marine Vehicles. Cambridge University Press.
• Newman, J. N. (1977). Marine Hydrodynamics. The MIT Press.

Hydrofoils are underwater wings that allow boats to glide above the surface of the water, reducing drag and increasing efficiency. They work based on fluid dynamics principles, generating lift as water flows over the curved surface of the foil. Designers use equations like Euler’s equation to optimize the shape and placement of the foils for maximum lift and stability. The angle of attack is an important factor in the performance and stability of hydrofoils. However, there are several limiting physics that can affect their performance and stability, including cavitation, wave interference, and improper foil design.

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For the DIYers or simply the curious a nice webapp for parametric foil wing design https://www.winghopper.com/web_app

cool, thanks for sharing

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• Yachting World
• Digital Edition

Sailing the new foiling Birdyfish dinghy

• March 24, 2022

The BirdyFish is a dinghy that aims to make foiling accessible to sailors of all levels. François Tregouet took it for a test sail to see if it really does bring foiling within reach

Based on an engineering school graduation project, three young Frenchmen have created the BirdyFish, a dinghy that aims to make foiling accessible to everyone.

BirdyFish is the story of a sailing start-up, something western France does well. Near Nantes, Jean-Baptiste Morin, Pierre Rhimbault and Alban Satgé were not yet 25 years old when they started out to create a new class of boat. Foiling designs fascinated them, from the Mini Transat small yachts to IMOCA 60s , but they wanted to make flying on the water easier.

Ambitious but realistic, the trio focused on the business and management of the project, and turned to specialists for key areas. Naval architect Etienne Bertrand, well known for his work on the Mini, was commissioned to design the BirdyFish, whose beamy bow is reminiscent of the 6.5m Mini ocean crossing scows.

With their personal savings and a €10,000 grant from a foundation for young entrepreneurs, they built a prototype. Three years later they’re on the verge of finalising a €250,000 fundraising operation and moving to 600m² premises to scale up to mass production.

While the hulls are subcontracted a few kilometres south, the foils are built in Nantes, in-house, a key factor in controlling the quality of these essential parts.

Under gennaker the flying experience starts with less than 10 knots of wind. Crew position determines trim. Photo: BirdyFish

Foil design was entrusted to a master of the art, Jean Baptiste Behm. With their J-shape, the foils create maximum lift when fully deployed but remain quite simple to retract. The carbon foils weigh 10kg each and are symmetrical, so can be used on either side of the boat. This simplifies production and after sales service.

Also with simplicity in mind, the position of the foils is fixed, without any adjustment. Rudder rake can be adjusted but to enable good control of the helm the rudder profile is not extreme.

A longer rudder chord means a little more drag but also more tolerance, and less risk of losing control. Sailing at low speeds with a smaller rudder surface area would give no feel to the tiller, making it much more difficult for beginners. The development of the Birdyfish, and refining that balance, took more than two years.

The hull is made of a glass-polyester sandwich, weighs 93kg and has four watertight zones making it unsinkable. The mast is aluminium and, once the foot is unpinned, can be dropped backwards with a crewmember controlling the descent with a halyard in hand.

The maximum speed recorded by a BirdyFish is 22 knots (like a Class 40 but 40 times cheaper!). Photo: MULTImedia/François Tregouet

The BirdyFish is limited to three sails to keep it simple. There’s no trapeze either, the righting moment being entrusted to the foils. This is also a safety choice because, at high speed, any fall could be dangerous. Two crewmembers sit on the gunwale, or move slightly outboard upwind with their feet in the straps.

A major evolution in the development of the BirdyFish means there is also no longer a daggerboard. The first boats sold played their part as pioneers, but it turns out that the foils, although symmetrical, generate more anti-leeway effect than expected. Removal of the daggerboard and its box made the Birdyfish’s cockpit even simpler.

Each foil weighs just 10kg (22lb) and they are symmetrical on port and starboard. Photo: MULTImedia – François Tregouet

Officially, the BirdyFish will fly in upwards of 12 knots of wind. But with a trained crew, the boat can take off from 8 knots. Finding the right angle, producing just the right amount of power at the right time to get the hull out of the water will show the difference between a novice crew and one that already has a few hours of flying experience.

Rusty from decades of cruising on non-foiling boats (and having long forgotten my 420 and Mini years), for me trying the BirdyFish felt like a good test of its genuine accessibility to all. I returned to the shore reassured about my abilities – but more importantly blown away by the extraordinary sensations of flight.

Off the beach at La Rochelle the BirdyFish flew very fast at about 15 knots in a wind oscillating between 10-15 knots. High speed means constant vigilance is essential: the transparent Mylar window in the jib helps with visibility.

There are no more lines at the mast foot than on a classic dinghy: simple. Fittings are attached so as not to compromise watertight zones. Photo: MULTImedia – François Tregouet

As there are no flaps to adjust the boat’s trim it’s the crew’s position that needs to be tweaked. Sitting on the windward side of the boat, well wedged against the shroud, I was quietly enjoying the stability of the boat – its tolerant foils and T-shaped rudder do a remarkable job – when Jean-Baptiste Morin handed over the helm.

I was instructed to use as little helm angle as possible, instead using the mainsheet as an accelerator first, then as a damper. Very quickly, the miracle happened, and we took off! Soon we were flying, perfectly dry just above the chop.

I quickly learned that to remain foiling requires being very sensitive to movement. Rudder correction angles must be as small as possible. There’s no question of moving the whole tiller, instead Jean-Baptiste advised I keep my tiller hand close to my body and only make small movements using just the span of my fingers.

When transporting or storing, the self-regulating J-foils fit inside the cockpit. Photo: MULTImedia – François Tregouet

Course deviations must also be very limited. I estimate that the maximum allowed is more or less 2° around the true wind direction without trimming the sails. The penalty for overdoing it is immediate: the BirdyFish touches down, either softly or more brutally.

Tacking and gybing are not difficult, but it takes a little more experience to complete them in ‘flight’ mode. To demonstrate, Jean-Baptiste retook control of the boat for a series of foiling gybes. Crouching at the front of the cockpit, my role was limited to managing the Solent sheet and above all to hold on, as the rate of turn is brutal, a reminder of the extraordinary performance achieved.

The foil housing is slightly smaller than its well. Foil rake setting is fixed by four bolts: no brain work required. Photo: MULTImedia – François Tregouet

Only twice have I reached 18 knots at the helm of a sailing boat on my first try, and those were on a Gunboat 68 and an 80ft Ultim trimaran – very different budgets to the €18,840 standard BirdyFish.

Three essential options do raise the bill to €21,200 – it’s difficult to do without the jib furler for manoeuvres, a Code 0 with furler offers light airs performance, and a launching trailer is essential for the 135kg (297lb) whole package – but if you want to go foiling for a reasonable budget and with little experience, the BirdyFish rocket hits the target.

BirdyFish specifications

LOA: 4.70m / 15ft 5in Beam: 1.90m / 6ft 3in Draught: 0.90m / 2ft 11in Displacement: 135kg / 297lb Upwind sail area: 13.5m² / 140ft² Downwind sail area: 24.5m² / 258ft²

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Hydrofoil Design - How are Hydrofoils Structured?

• Categories : Marine machinery, engines & controls
• Tags : Marine engineering

Introduction

We have already studied about conventional ship design and hull strength of ships . Now we will study about a type of boat which uses a totally different hull design. Hydrofoil boats are the boats that use the technique of hydrodynamics using hydrofoils (wing like structures under the boats) for locomotion. This technique is used for the efficient use of engines that power the boat. We have already learnt about the generic aspects of hydrofoil sailboats in another article here, so in this article we will focus on their working principle and design.

Hydrofoil Design

Hydrofoils are wing-like structures which are mounted on struts below the hull of the sailing structures, which can vary from small boats to big passenger boats. The unique geometry of the hydrofoils allow the sailing structures (small boats or passenger boats that use hydrofoils) to use the power supplied by the engine efficiently. There are many variations of the hydrofoil that has been developed over the years.

In normal boats, the water in front of the hull is pushed out of the way. The surface area of the boat in contact with water is more and this requires more energy to push the boat forward. In hydrofoils, this is not the case. When the boat is in rest, the hydrofoils are fully submerged in water and the entire hull is in contact with the water. When the boat starts moving and the speed gradually increases, there is a lift experienced by the boat which is due to the use of hydrofoils. Thus there is a reduce in drag as the hull of the boat is lifted and the energy required to power the boat reduces. This results in increase of the speed of the boat.

Hydrofoil Physics

Why the lift is experienced.

In a practical perspective, water is never still. It has waves, so there are crests and troughs. The hydrofoils are lifted in crests. When there are troughs, the hydrofoils leave water, so they land with a crash. This might be dangerous as it might topple or the hydrofoils might crash.

Problems like ventilation is a major problem faced when designing hydrofoils. Ventilation occursat the air-water interface. This happens when the hydrofoil keeps lifting and it meets the air-water interface and at that point, the air is sucked down the lifting surface of the foil and this results in crashing of the boat, as air is much less denser than water and the lifting process stops once the air-water interface is met.

Also the angle of attack should be kept in mind while designing hydrofoils. The angle of attack is adjusted by sensors and other electronic components in modern day hydrofoils.

Configurations:

Some of the configurations used are the V shape and the inverted T-shape. Each configuration has its own advantages and disadvantages. In some configurations ladder foils are also used which are relatively more stable in high seas.

12 foiling boats for sailors of all levels

• April 15, 2022
• No Comments

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Birdyfish – 4,7 m – For foiling beginners

Flo1 – 4.25 m

NACRA F20 – 6,20 m – Like America’s Cup cats

Foiling Dinghy – 3,86 m – Small and versatile

Moth – 3,35 m – The foiling cult dinghy

Peacoq 14 – 4.70 m – Like a flying Fireball

Persico 69F – 6,9 m – The trendy boat

Skeeta – 3,35 m – Easy for anyone

Stunt S9 – 4,16 m – The Italian Foiling cat

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THE FLYING YACHT The new era of sailing begins

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With a novel 740 hp hydrostatic propulsion system and ENATA's custom torpedoes, the FOILER continues to revolutionise the way you explore the seas.

The hydro-foiling system enables the boat to fly 1.5 metres above the water, providing an unmatched experience where speed and reactivity are the centrepieces.

Experience a unique sense of tranquillity and comfort at high speeds. Anchored in the water by four powerful foils, the FOILER is both extremely stable and immune to wave interference. Driving the FOILER is surprisingly easy and effortless, and the subtle luxury will make any sailing session a relaxing and memorable experience.

Voyage into luxury in complete serenity as you experience something new. The FOILER glides above the water, while her retractable foils and futuristic design deliver a unique presence and bold extravagance. FOILER will create a little drama in your life and turn heads - but who wouldn't want that?

Don't you feel the urge to fly?

It isn't a dream anymore. For the new generation of sailors, we are building your FOILER.

Design and build a boat from tin foil that can hold as many pennies as possible before sinking.

In this activity, you’ll build a simple boat using tin foil. Then you’ll test well it floats by adding pennies, or other small things. to see if it sinks.

All you need is tinfoil, some coins, and a container filled with water.

This page will print, but looks a little funky. Click the button for a PDF version which looks a bit better. This is a stop-gap while we work on a better solution!

What you’ll need

• Scissors (or you can tear the foil)
• Pennies (or something heavy to sink your boat)
• Shallow plastic container or a sink that can hold water (or the bath!)

30min to an hour

Suitable for…

Age 3 and up.

Safety notes

You know your children better than anyone, and you should judge whether they’re ready for this activity. You might want to think in particular about:

• Supervision: the activity involves pennies, so there’s a choke hazard.

Use the scissors to cut the foil into a square.  If you don’t have scissors, you can carefully tear it into a square. The foil squares can be any size you like, and you don’t need to be exact. Later, you might try making another bigger or smaller boat.

Fold the edges of the foil to form a boat shape. If you want to try out different shapes, you could use different types of boats . Or you could draw pictures of boats before you start building.

Carefully half-fill your container with water and put it on a towel to soak up any spillages. If you’re making a bigger boat you could float it in the sink or bath.

Gently, put your boat in the water. Well done if it floats!

Now add pennies (or other coins) one-by-one in the boat until it sinks! Make sure you count how many pennies it takes for the boat to sink, this will help you work out how good it is.

You could write down your results in a table like the one in the picture.

Completely sunk?!

Can you hold more coins with a different boat design?

Keep a record of your results using your table.

How does it work?

When your boat was floating on the water there was one force pulling it down due to gravity (the weight) and another pushing it up called buoyancy.  To work out whether your boat floats we need to think about how heavy it is and its shape.

To start with your boat was light but it got heavier as you added pennies. So how could it hold so many pennies? The shape of the boat is important, a shape which contains lots of empty space (like a boat) will be good at floating because it’s able to push more water out of the way. This makes a bigger buoyancy force keeping the boat from sinking. But when more pennies are added, the weight of the boat becomes bigger than the buoyancy force and the boat sinks.

Things to discuss

Ask questions to get your child thinking about why objects sink or float:

• How many pennies was your boat able to hold? Did it matter how or where you placed the pennies in your boat?
• After testing your boat, did you make any changes to the shape of your boat?
• What shapes seemed to work the best?
• What could you change to make your boat hold more coins before sinking?

Other things to try

Your first boat was made of tin foil. Now try:

• Making a boat from a different material, paper, cardboard, plastic
• Making an origami boat – here’s a good video to show you how

You can test these other boats in same way, by adding pennies or small objects. Record your results so you can work out which is the best boat.

Watch the story together

Sit down and watch the story ‘Who sank the boat’. If you can, find somewhere comfortable and watch it together.

Careers link – naval architect

Naval architects design, engineer and manufacture boats, ships, oil rigs… they care very  much about what sorts of things float. They also sometimes care about the sorts of things that  sink  – naval architects design submarines, too!

Attributes:

Naval architects are curious, organised and creative.

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