Foiler Design

The tips presented here will be those that I know work and that have been extensively tested on the water and in testing of two F3 hydrofoils side by side and Dr. Bradfields modified Flyer? hydrofoil. These foilers use fully submerged foils as opposed to surface piercing foils and the major advantage (in rc models) seems to be the fact that they generate their own stability. The more wind pressure ;the more stable the boat is.The F3 has been sailed in winds well over 20mph and the Flyer? in over 30.Fairly accurately ESTIMATED boat speed of the F3 in 16-22mph of wind=20mph.In 5-7mph of wind boat speed was ESTIMATED at 10-12 mph.
No other multihull available anywhere has this kind of automatic hands off stability system. On “normal” multihulls (F48,mini 40 and multiONE) immense concentration is needed just to keep the boat upright in medium to heavy air. The Bradfield type foiler ,like the F3, virtualy eliminates all those hassles and allows the skipper to just enjoy the speed without the capsize worry. A boat like this can still be capsized if it is set up incorrectly(set up should be a one time thing) or if too much sail is used for the conditions but I can supply a sail area guide that will eliminate that problem. And if you use the same rig as the F3, Flyer or X3 you won’t need multiple rigs since those rigs reef thru out the windrange . Another thing:the F3 type foiler can carry its “A”(unreefed) rig in higher winds than any other multihull because of the automatic stabitiy system but at some point sail will have to be reduced.I have a guide to knowing exactly when…
Foiling occurs on the F3 in 5-6mph of wind.Most of this is based on the F3; some ideas relating to the X3 retractable foiler are presented though that boat has not been tested yet.
Design Facts:

  1. the quarter chord of the two main foils should be located so that they carry 80% of the weight and the rudder t-foil carries 20% of the weight. This generally results in a rig further forward than is “normal” on an rc multihull because the quarter chord of the vertical fin is, by necessity, in the same place. That is also the location of the CLR of the boat for balance(weather vs. lee helm)considerations.
    2)A single cross arm is all that is necessary but it must be exceptionally stiff torsionally.
  2. the center line of the outboard foils is ideally located about 1.07 times hull length apart centered on the main hull athwhartship. In the F48 class this is not possible and results in the hydrofoil for the f48 having to do slightly more work than that for the F3. In the multiONE Class while loa is one meter BOA is 48". The wider the boat the less the loading of the foils.In many tests the location of the vertical fins so far outboard has not affected tacking ability in the least; a foiler on foils will tack at least as fast as a fast monohull…
  3. The automatic altitude control system depends on two"wands"which are straight or curved thin carbon tubes or rods that drag in the water. Before foiling the water pressure pushes them aft forcing the flap on the hydrofoil down to the high lift position. A spring or shock cord is attached to the wand so that as the boat rises the bottom of the wand moves forward neutralizing the flap on the foil or even creating negative lift.This is how the boat develops its own righting moment: the diferential lift required to create and maintain the exact right amount of lift AND righting moment is created by the movement of the wands connected to flaps on the two main forward hydrofoils: it is 100% automatic requiring no radio control!
  4. The F3 will foil in very light air but because of the automatic altitude control system there is a lot of drag when not on foils.The “wands” can be removed in light air drmatically reducing drag since the flap is not then being automatically pushed into a high lift position. Or this type of foiler can be designed to use retractable foils which has not been tested in the water but since the foil system is identical and the rotation system so simple it should work but as yet this remains untested. F3 type foilers have been tested with no wands for light air.F3 One Design races would not be held in less than foiling conditions but F48,mini 40 & multiONE races will be held in very light conditions so it could pay if you consider an F48 size foiler to go ahead with a retractable system.
  5. the main foils are installed as previously mentioned with a +2.5 degree angle of incidence relative to the flight waterline. The rear t-foil is set at 0 degrees.
    Mainfoil area should be determined as follows:For a weight of 8 pounds the two main foils will lift 80% or 6.4lbs.; 3.2 pounds each. They require an area of 17 sq.inches to do that or .188lbs per square inch. Note that this is just to make calculation simple and does not reflect the ACTUAL loading of each foil since ,in developing righting moment, they are much more highly loaded. A Foiler should be designed with approximate 200 sq. in. of sail area per pound of weight. Of that sail area the MAIN foils should carry 49 of sail area per of foil area.(Divide Sail Area by 49 to get mainfoil area=both foils; then divide in half for the area of one foil.Try to achieve the loading and sailarea for the foils and displacement of the boat.
    In summary: A).188lb.s per sq. inch loading of the main foils only -just using 80% of the boats weight. B) 49 sq. in. of sail area per sq. in of main foil area. This can be varied a little and is applicable from a multiONE to a two meter.C) this will produce a boat that will take off in light air.(5-6mph)
    The rudder t-foil should be 50% of the total of both mainfoils or slightly less.(Equivalent to one main foil works well)
  6. The amas on a foiler the size of the F3(LOA 56") are 36" LOA with an 18/1 beam to length ratio and total buoyancy of 7 pounds. They are not capable of flying the main hull and do not need to be.Amas for an F48,mini 40 should be about the same size due to the proportionately narrower beam. For the multiONE they can be directly scaled down.
    8)On the F3 the foils fit in trunks in each ama and that requires that not only the tube comprising the cross arm be exceptionally stiff torsionally but the joint between the ama and the cross arm needs to be exceptionally stiff. On the retractable design (untested as yet) the foils are mounted within carbon tubes that fit within bearings into another carbon tube allowing the whole foil/altitude control mechanism to rotate.Viewed from forward the foils rotate like airplane propellers toward the main hull untill they are vertical. The lower portion of each foil then acts as lateral resistance for the boat in light air.The mounting of the rotational part of the the foil system allows the foil to have a zero degree angle of incidence when the foil is vertical. While I haven’t tested this on the water yet I’m confident that it will work since it is so simple.The one downside may be the area of foil still in the water. The idea is to eliminate any need for a daggerboard when the foils are retracted by using the foils themselves as lateral resistance when they aren’t being used for lift.

You don’t need a retractable system to enjoy sailing a foiler as described above; it takes off in such light air that you’ll be foiling most of the time anyway. The only benefit to a retractable system is in light air or transitional foiling conditions(under 6mph wind) and the only benefit is in a class like the F48,mini 40 or multiONE in light air racing.
------For anyone interested I can answer any questions at: and provide sketches of the wand system but the basics are here-you can design a foiler with the information provided above…

Doug Lord
–High Technology Sailing/Racing

I see where you are again referencing the X-3 Multihull in your post. Does this mean in the last 31 days, progress has been made on the prototype - or perhaps even on the actual boat and that it is now ready for order through Microsail?

I surely don’t want to miss the opportunity to let people interested in the F-48 Class know that it is now available for purchase from your company.


Also thought you would enjoy this photo regarding foiler stability.

Just a note on foiler stability…
If memory serves, that is a picture of a foiler that “tripped” over that mark by hitting it at high speed. Me thinks a “normal” boat of a similar size would do the same thing if it were going at the same speed…

Oh yeah, Lorsail, are there any of your gofast machines in the St. Louis (MO) area. I’d like to see one for myself…


The X3 foiler is not yet available but a basic outline of the retract system is available to anyone wanting to experiment.
As to the picture of the Rave fullsize foiler shown pitchpoled it is very important to understand the differences between full size Raves and the F3 type: the full size Rave and most full size foilers are susceptabe to what is called ventillation. That is air being sucked down the vertical fin and “ventillating” the hydrofoil. According to Dr. Bradfield it is rare on the Rave. And it is 100% UNHEARD OF on the F3 foiler- presumamably something to do with Reynolds numbers. In fact, the F3 has been sailed in rough water numerous times where the foils actually came out of the water with no ill effects whatsover. On a full size foiler that could cause instant pitchpole or capsize depending on the boat. The International Moth frequently breaks the surface with the foil and only crashes a fraction of the time that happens; It’s probably best on a full size foiler to keep water around the foil unless the foil is specifically designed to resist/prevent ventillation.

Mr. Walker-none in St .Louis-only four F3 sold so far-in Minnisota, Illinois,Washington State and Hawaii-video and stills on ; when the X3 is introduced it will be much less expensive…

edt:ad info
Doug Lord
–High Technology Sailing/Racing

To me, this post seems more an ad/promotion for a specific product than a general information post. Its also info that I think has appeared here more than a few times in the past. What do others (particularly moderator’s)think? Is this the right direction for this forum?

It would help to read posts that are to be discussed; the posts presented here ,particularly the first post, was a detailed description of the considerations involved in designing a Bradfield type foiler with auto stability.
This information was posted as a direct response to Ian’s saying that he might consider foils on his multi; I wanted to give him and others the facts they need to know so that they would not have to reinvent the wheel.
Not only do I publish the exact details I say that I’m willing to help anyone design one of these. While it will certainly take an expert builder to put one together it is cetrainly possible given the information I have presented and my willingness to answer questions along the way.
Objections to the help I have provided and to the detailed information provided for anyone interested are way out of line.

Doug Lord
–High Technology Sailing/Racing

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