Foiler Design

Discussion in 'Sailboats' started by tspeer, Nov 12, 2003.

  1. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    These sections were an exercise to see if blunt-edged sections could be competitive with sharp-edged sections. The CFD results say, "yes," but I've heard anecdotal evidence that they may not work nearly as well as predicted. But I don't have any empirical data to support that.

    I would also consider sharp-edged sections.

    Another approach would be to design something in XFOIL similar to the P3 series, but carry the rooftop portion of the pressure distribution closer to the leading edge. That would make the edges narrower and start to better approximate a sharp trailing edge.
  2. Nik777
    Joined: Aug 2007
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    Nik777 New Member

    Hi Folks!

    I discovered this thread whilst googling on foil moths, and as others have said, this is a goldmine! I'd like to thank everyone who has posted so much information so freely.

    I'd like to contribute initially by summarising what has already been said and offering some ideas based on that.

    Consolidating what has been said here, the goal is for a sailing boat which lifts off onto foils in the lightest wind possible, maintains stable flight thereafter, and which can attain the highest possible top speed.
    Other less-common goals include jumping and safe landing.

    Summarising others again, liftoff occurs when the power produced by the sail(s) is sufficiently larger than the total drag that the boat achieves the speed needed to generate the required lift. It has also been pointed out that this is not a binary switch, but a continuum in which the foil(s) begin to lift, reducing some drag (eg of the hull), allowing further acceleration.

    So liftoff is predicated on maximising lift whilst minimising drag, or put another way: maximising the lift/drag quotient.

    I believe this explains some of the effects that have been observed, such as heeling the boat to windward helping liftoff. When the boat is heeled to windward, it reduces the pressure on the centreboard thereby reducing the drag caused by the centreboard. I don't know if the drag caused by the foil stays constant (the boat speed has) or whether the extra downward pressure on the foil has increased its drag, but it seems most likely that the overall drag has been reduced.

    I also know from a limited amount of waterskiing that the best way to get liftoff with a single ski is to first bend the knees, then straighten them (ie push out and down on the ski), causing the skier to "pop" out of the water.

    So heeling to windward is probably doing something similar as well, although I expect on a smaller relative scale.

    So for liftoff, I think we should focus on a hull design with the following properties:

    1. minimal drag when sailing in low wind strength (high-wind drag is almost irrelevant)
    2. minimum increase in drag, or even a reduction, when heeled to windward

    There are two others which *may* be also be useful:

    3. minimum increase in drag, or even a reduction, when sailed bow-up
    4. some lift when sailed bow-up - particularly when heeled to windward.

    I can see good potential for some or all of these in both skiff and scow forms.

    For stable flight, we want to regulate the lift as speed increases. I don't know enough about the various compromises in terms of drag between trim tabs, pivoting foils and surface-piercing foils, but I would like to look at piercing foils as they represent a very simple design (no moving parts).

    I don't actually agree with this. Moth class rules effectively only prohibit foils which are affixed to the wings, but surface-piercing foils attached to the hull near the sides of a scow would be legal, as would surface-piercing anhedral or dihedral foils attached to a centreboard on a skiff or scow.

    Top-end speed gets back to reducing drag. Again, I don't know the relative compromise of a surface-piercing design, particularly compared to (say) a ladder design where top-speed is attained by lifting all but the bottom-most foils out of the water. However, it seems clear that top speed will be achieved through reducing wetted area.

    On this point, if the boat is being sailed heeled to windward, then we can presumably reduce the area of the centreboard. According to what's been said here, it is best to maintain the span (length), and reduce the width (distance from LE to TE). Perhaps this could even be quite tapered, so there is relatively more area when in displacement mode, and relatively less when flying.

    I'll post more concrete ideas separately.

  3. Nik777
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    Nik777 New Member

    Ok, here's my first thought on a piercing centreboard-mounted foil:

    I think I need to make it beamier still, and perhaps it would be better to have the bottom rung swept forward rather than back (from point of view of the supports). In addition, I would look at making the supports smaller (from LE to TE).


    The reasoning is:

    1. everything is swept back, to reduce drag
    2. the top rung is mildly anhedral, for stability (ref: post by Wardi) and to give a better combination of lift and push to windward
    3. the bottom rung is dihedral to give surface-piercing regulation of ride-height
    4. the backward-sweep should also shift the centre of lift aft, so that during liftoff it is foward of CoG, and then after liftoff it is at CoG - hence auto-leveling after liftoff.
    5. twin side-supports avoid acute angles at joints, and help smooth vortices, by making reduction in lift progressive (can anyone comment on this?)
    However, the increased drag of two supports may be a larger than the benefits of the closed ends.

    I'll work on it some more...

  4. Doug Lord

    Doug Lord Guest

    Surface Piercing Foils + wand?

    Tom, on SA you suggested that surface piercing foils might benefit from a feedback control system like a wand. Since surface piercing foils vary their altitude with speed wouldn't a wand system tend to negate -to some extent- the advantages of surface piercing foils? I may not understand your idea correctly but I would like to...

  5. TTS
    Joined: Jul 2007
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    TTS Senior Member

  6. Doug Lord

    Doug Lord Guest

    Foiling C Class

    Fantastic-Thanks Tom!
  7. TTS
    Joined: Jul 2007
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    Location: New Hampshire

    TTS Senior Member


    Thought you might like this one.

  8. boogie
    Joined: Feb 2004
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    Location: Windy Wellington, New Zealand

    boogie Member

    hydrofoil kite ski

    hi there,

    the kitesurfing guys have discovered racing and in particular course racing this season. one of the guys in san francisco had some promising results with an airchair type hydrofoil, but development seems to go wild in all corners of the globe now.

    here is a link to video clip taken at the currently running kite world cup in st.-peter-ording in germany which has course racing as a discipline now.

    hydrofoil kite skies, one on each foot
  9. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    There are several reasons why a surface piercing foil could benefit from feedback control - performance and ride quality being two big ones.

    If profile drag were the only source of drag, then the optimum strategy for a surface piercing foil would be to operate just below CLmax and fly as high as possible so the minimum amount of wetted area is in the water. But there's induced drag as well as profile drag.

    Where parasite drag is proportional to wetted area and speed squared, induced drag is inversely proportional to span squared and speed squared. With a fully submerged "T" foil, both the span and the area are fixed, so when the foil is producing a constant lift (because lift has to equal the weight), induced drag drops off with speed and parasite drag becomes the dominant contributor at high speed. But that's not necessarily so with a surface piercing foil.

    Some surface piercing foils maintain constant span as the area changes - ladder foils are of this type. But some, like V foils or diagonal dagger boards, reduce in span as well as area as they come out of the water. If the product of span and speed is held constant, then the induced drag will remain constant as speed changes. If the product span times speed gets smaller with speed, then the induced drag will actually increase.

    Lift is proportional to the lift coefficient, wetted area, and speed squared. If the lift is held constant and the lift coefficient is also held constant, then the area has to be inversely proportional to speed squared. If span is the only way to reduce the area, like with a constant chord "V" foil, then span has to go down with speed squared. The span times speed product will drop with speed, and the induced drag will increase with speed. The parasite drag will be constant for the same reasons the lift is constant, so the total drag will increase with speed in this constant-CL scenario.

    But if the lift coefficient is allowed to decrease as the speed increases, then drag trends will be different. If lift coefficient is made inversly proportional to speed squared, then the area will be constant, the flying height will be constant, and the span will be constant. The surface piercing foil's drag trends would then be the same as a fully submerged foil's.

    If the lift coefficient varies inversely proportional to speed, then the area will also vary inversely proportional to speed to maintain constant lift. A constant chord V foil would then have a span that varied inversely proportional with speed, and the speed*span product would be constant. Induced drag would be constant as speed changed. Assuming the parasite drag coefficient is not affected by angle of attack or speed, the parasite drag would increase linearly with speed instead of increasing with the square of the speed like it does with a "T" foil. This could mean a huge reduction in parasite drag at high speed.

    So flying height can have a big impact on the performance of a surface piercing foil. Too low, and parasite drag dominates. Too high, and induced drag dominates. With no control, the flying height, pitch trim, and stability are all coupled together and stability and pitch trim will dictate the performance. With control, the trim can be adjusted for optimum performance.

    Hanno Smits measured increasing drag with speed and decreasing drag with speed in tow-tank tests of his V-foil equipped Flying Dutchman, depending on the foil incidence (lift coefficient), showing that flying too high causes excess drag.

    Feedback control can be used to change the dynamics of the craft. Hydroptere turned its starboard beams into pretzels when it stuffed a foil into a wave and buckled under the sudden increase in lift. A controlled flap could have been used to offset some of the increase in lift due to the increase in immersed area and given the craft time to react in heave or roll to keep the loads within limits. The shock absorbers they've added do much the same thing.

    Just as the waterplane area determines the heave stiffness of a hull, the surface chord determines the heave stiffness of a surface piercing foil. As the waves pass a platforming hydrofoil, the chord times the wave height determines the change in area, and thereby the change in lift and the vertical acceleration of the foil. At high speed, this may be a rougher ride than you'd like. Feedback control from a wand can be used to offset the change in lift from the wave and smooth out the ride.

    So while many designers use surface piercing foils to avoid having a control system, one may want to use surface piercing foils for efficient operation over a wide speed range and optimize performance and stability using a control system.
  10. Doug Lord

    Doug Lord Guest

    Very enlightening.Thanks, Tom!
  11. Doug Lord

    Doug Lord Guest

    Midship Wand

    The Rave multifoiler uses a wand whose pivot is near the back end of the main foils and close to midship. I've had a few hours experience on that boat in varied conditions and some friends have had hundreds of hours sailing the boat.I've got several hundred hours of testing rc models utilizing midship Bradfield wands with not one crash/pitchpole in widely varying conditions. An interesting thing is that the most experienced Rave guys have told me that the boat almost never pitchpoles or flies out of a wave.
    On the other hand Moths with bow wands seem to have trouble with certain specific conditions in waves.
    If you draw a moth flying level in waves in a crest to crest half length equal to the distance between the bow and the daggerboard and compare a midship(board mounted) wand with a bow mounted wand it appears that for(at least) that specific condition the main foil on the boat with the bow mounted wand will enter the wave with the flap calling for maximum lift where the mainfoil on the boat with the midship wand will enter the wave with the wand calling for minimum lift. Thus the boat with the midship wand would be far less likely to burst thru the wave and wind up crashing.
    Considering an upwind speed of 13 knots( 21.9 ft.per sec./6.68m/sec.) and a downwind speed of
    22 knots( 37 ft.sec./ 11.3 m/sec) it seems to me with "normal" wand response that the boat with the bow wand would be far more likely to come out of the wave into air on the other side than would the boat with the midship wand.
    Any comments?
    Have any of the Mothies that have designed their own boats experimented with a midship wand?
  12. bigmac
    Joined: Dec 2007
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    Location: texas

    bigmac New Member

    multiple foils on dagger

    first, thanks and congrats to many fine-minded sailors contributing to this thread! yes, i read the whole thing. At the ripe young age of 51, my mind is making promises my body can't keep and my air-chair is twisting me in ways no longer natural. a foiling moth seems to be the answer for me.

    I'm guessing there may be a reason for little discussion of ladder foils attached to a single dagger. think stearman biplane. for a moth, lots of lift at low speeds (with a drag penalty to be managed) but as speed increases, the top foil clears the water and you ride on the lower foil alone. less drag at high speed and it seems the lower lift of the smaller lower foil would be less sensitive to dynamic changes of forces and therefore ride with more stability. when wind drops, lift drops and the boat lowers until the upper foil is back in the water and you are back in hi-lift mode to keep foiling at lower speeds. in displacement mode, more drag (4 vortices vs. 2, etc.) but we're not doing this to sail in displacement mode. is this tried, trashed, and i'm the last to know or are there possibilities?

    personal opinion: at 200 pounds, am i too big for a moth? obviously i am past the likely age and weight for big trophies. i just want to go fast and fly.

    there was talk many posts back of someone wanting to jump a boat. get an air-chair. nothing like literal seat-of-the-pants intuitive feel for forces involved. imo, much, much bigger than anticipated. from real life boat jumping experience, the following scientific proven formula evolves: hobie 17 + big ego + real big wind + real big wave = a couple of seconds of completely flying boat + snapped forestay bridle on landing + world class pitch pole. wounds heal in a few days but glory lasts forever (and chicks dig scars). best of luck to you and be assured you will love the ride! it's the landing part that is problematic.

    Task one is build a moth. I'm probably many hundreds of miles from the nearest moth (west texas). can some please suggest where to turn for good shared designs and details? some posters have hinted at interest in seeing develoment of ideas into trials. maybe we should hook up? 60,000 sf manufacturing plant with cnc tooling, architects, engineers (offshore structural matlab, ansys, fea types). way overkill for a tiny bit of boat but if you're interested in collaboration, email me.

    thanks guys. look forward to comments on a ladder scheme.
  13. Olav
    Joined: Dec 2003
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    Olav naval architect


    no intention to offend you, but yes there are several reasons why multiple foils of the kind you suggested are not discussed here:

    1. As you mentioned already there will be a huge drag penalty which will be especially severe when you consider that low take-off speed is a key parameter to regatta success within the Moth Class. Lots of drag will slow you down in such a way that you probably do not reach the necessary take-off speed while others are already flying at twice your speed.

    2. A biplane arrangement has been tried out before by Brett Burvill from Perth in 2001 (pics here and here) with rather little success. It was observed that when the upper foil cleared the water, there was a sudden loss in lift which could cause the boat to descent again, immerse the upper foil, raise again,... short: It made the boat oscillate very badly. IMHO the same behaviour is likely to occur in waves when the upper foil ventilates in the troughs and immerses in the crests.

    3. Even when assumed the biplane configuration is working reasonably well when accelerating it will behave like a completely uncontrolled system when only the lower foil is immersed so one would have to maintain the ride height manually (by altering the AOA via trim change) - usually too hard for humans to react fast enough.

    Nevertheless building and sailing a Moth is a good idea and your weight and age shouldn't debar you from this (I know Mothies who are the same weight and older). Also don't feel discouraged to try out new things or things you think might work although others had no success - but the easiest and fastest way to a foiling Moth is to copy the existing working principles.
  14. Doug Lord

    Doug Lord Guest


    Big Mac, in case you don't know about it this is a major resource on practical foil experimentation in the Moth class:
    Australian Moth Class Association; Index page
    also some of the blogs on this website are interesting:
    and,of course:
    My focus is on working on designs that will make monofoiling easier and more interesting for a wider range of people. Such things as buoyancy pods and retractable foils are important to me.
    'Bi-plane' foils are probably not workable in addition to what Olav said because(so far ) I would think a retractable system using them would be very difficult if not impossible.
    But keep thinking and researching!

  15. RVELL
    Joined: Feb 2004
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    RVELL Junior Member

    On the Winged Trimaran tread I just posted this review and its subject may be of interest to Moth designers and builders:

    Review, by Ray Vellinga of the book, Locus of a boat designer by Kotaro Horiuchi,

    What is the “American Dream”? To some it is having a good job, buying a house, starting a family and voting. Others believe the American Dream follow the examples of Alexander Graham Bell and Thomas Edison. Their compulsion to invent things was as strong as their need for money, shelter, family and freedom, however this form of the American Dream may be more appropriately called the International Dream. The inventive spirit is everywhere, not just in America.

    There is no finer example of an innovator and inventor than Kotaro Horiuchi, chief of the Yamaha Research and Development Center during the 1980s, head of Horuichi Laboratory and former director of Yamaha Motor Co. Mr. Horiuchi, like his company, is Japanese, but the R & D centers are in Los Angeles and Minnesota. Little wonder that many of the designs he created have strong appeal to Americans. Yamaha’s market successes are well known: motorcycles, personal watercraft, etc. But the real excitement in this book is about the vehicles he created for special purposes and, for one reason or another, where never mass-marketed and therefore not widely known.

    The versatile Mr. Horiuchi, in a period spanning over 50 years, designed conventional motorboats, sailboats, planes, helicopters, motorcycles, cars, SCUBA propulsion unit and even a river-powered generator. But his greatest efforts were focused on hydrofoil boats, and this book reveals their secrets. His best known hydrofoil is the OU32 that was feature on the Discovery Channel in 1999. This water-jet powered boat carries two persons in tandem and flies at 40mph. The pilot and passenger are belted in and protected by a locking clear transparent canopy. The submerged foils allow the craft to make balanced turns while banking 45 degrees. The result is a 1.4G thrill ride. There are detailed drawings, descriptions, specifications and measurements to help the reader understand how this boat works.

    During the 1990s there was a lot of interest in Japan about setting records with human powered hydrofoils. There were many boats but two stand out: the Cogito and the Super Phoenix. The fastest was the two rider Super Phoenix. It was designed and built by Yamaha engineer, Fumitaka Yokoyama, in Yamaha's facility under the direction of Mr. Horiuchi. In 1998 it flew at 19.51 knots. That is unofficially faster than the 18.5 knots of the MIT speed record holder, Decavitator, a one man hydrofoil. Officially, Super Phoenix was awarded the two man record by the International Human Powered Vehicle Association for the fastest flying start run through a 100 meter course at 18.67 Knots. This speed fell slightly short of their 20 knot goal. Not to worry, Mr. Horiuchi generously details the final 5 modifications that were made on the Super Phoenix to permit it to fly faster, but even after these modifications its old record could not be beaten. Still optimistic, Mr. Horiuchi calculates that by increasing the rider power by 15% and decreasing the drag by 35% an otherwise similar boat would smash all records by reaching 24 knots. Sounds easy enough.

    There are so many valuable projects in this book that one gets the feeling that Mr. Horiuchi, retired as of 1996, would like more than anything to see the abandoned ideas come back to life in the form of viable, profit making products. Many of his best ideas apparently were shelved because of unrelated circumstances, like the Japanese recession or the surge in product liability problems in the 1990s. Of course the biggest market for these ideas is here in the USA and surely it’s no coincidence that this second edition is in English and available to Americans.

    Included in the book is a 36 minute DVD. It is in NTSC format, region free; so it will play on your TV. There are 50 boats, planes and cars -- many of which the reader probably never has seen. Excerpts from the DVD can be seen on Youtube. Search for OU32 or Kotaro Horiuchi.

    The book is important because of the large number of significant projects and the detail in which they are reported. There are very few other good books about hydrofoils and hydrofoil design principles. This one should be in the library of every serious hydrofoil enthusiast and designer. The book is especially valuable to anyone thinking of building a hydrofoil or anyone dreaming of breaking records.

    Locus is a large book, 308 pages of 8” X 11.5” print. It is filled with over 360 fine line drawings and photos. The line drawings in particular are works of art. About 40% of the book is devoted to hydrofoils, 20% for power boats, 10% for rowing shells, 10% two wheel powered vehicles, and 10% for airplanes and helicopters. The book is filled with great designer and engineering ideas with numbers, dimensions and details. This edition is priced at $99.00 plus $7.00 US shipping. Order it through Dr. Mike Timmons, 126 Sunset Drive, Ithaca, NY 14850. Email Phone 607-227-5638.

    P.S. to see this and other relevant hydrofoil videos go to and search for IHSIHS or my name, Ray Vellinga
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