Foiler Design

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

  1. John Perry
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    John Perry Senior Member

    Presumably these formulae are for an infinite span?

    I have long been interested in the possibility of allowing hydrofoils to run on the surface in which case it could be assumed that they are fully ventilated on the upper side. Would the formulae then apply with Sigma = zero?

  2. tspeer
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    tspeer Senior Member

    My thanks to EPClement for pointing out this reference:
    Johnson, Virgil E., Theoretical and Experimental Investigation of Supercavitating Hydrofoils Operating Near the Free Water Surface, NASA Technical Report R-93, 1961.

    It shows good agreement between the theory and experimental data for rectangular planfoms with aspect ratios of 1 and 3.

    The lift of the flat plate is increased between infinite depth and operating at the surface. The cambered sections did not see as great an increase in lift at the surface. The drag also increased near the surface, with the result that the lift/drag ratio varied little with depth.

    When a flat plate with a rounded leading edge was tested, the form drag due to the leading edge was of the same order of magnitude as the skin friction and induced drag for low angles of attack. So it's important to keep the leading edge sharp and to operate at a high enough angle of attack to keep from wetting the upper surface. This makes it difficult to have a section shape that will perform well in both the subcavitating and supercavitating regimes.

    The aspect ratios considered were low because the sections had to be thin. So the chord had to be large to get enough physical thickness for strength at the high speeds. This, plus the need for a sharp leading edge, does not bode well for a sailing hydrofoil that has to be efficient at subcavitating speeds in order to get to supercavitating speeds. It may point to the need for a ladder foil that carries the load on a subcavitating element at takeoff and and transitions to flying on the supercavitating foil at high speeds. With the attendent drag of hauling the supercavitating foil through the water, wetted on both sides, before reaching supercavitating speed.

    The maximum practical L/D for an aspect ratio 3 supercavitating hydrofoil was around 12, at a lift coefficient of 0.2, operating one chord-length below the surface. As the lift coefficient increased, the L/D dropped very rapidly. A foil designed to operate at a lift coefficient of 0.2 would only have a L/D around 3 at a lift coefficient of 0.4, or about 70% of the speed for max L/D.

    So it's important to operate in a narrow range of lift coefficients. This can only be done over a reasonable speed range by using a surface piercing foil that changes its area, instead of a fully submerged foil.

    When one considers the additional drag due to side loads on a sailing hydrofoil, the minimum L/D required for flight is probably on the order of 6 - 8. This can be achieved over a wider range of lift coefficients by lowering the design lift coefficient to 0.1. The maximum L/D is somewhat lower at around 9, but the lift coefficient can be doubled before it drops below 6.

    If the cambered sections are operated at too low an angle of attack, they cavitate on the lower surface, with a loss of lift and increase in drag. so it looks like a supercavitating hydrofoil will probably be operating on the backside of the drag curve. Once it's flying, it will accelerate out to very high speed. But getting it flying is going to be very difficult because of the thrust required.

    A surface piercing foil is likely to be a better candidate than a fully submerged foil because it can be operated in a narrower range of lift coefficients and because ventilation is actually beneficial in the high speed range.
  3. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Report of first U.S. foiler Moth from the new Modern Moth newsletter "Mothballs":

    International 10 sq. Meter Canoe champ and US Mothist Bill Beaver is reportedly working on a foiling Moth for the Brigantine nationals.​

    For your copy of the newsletter contact Scott Sandell (e-mail on opening page at
  4. John ilett
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    John ilett Senior Member

    There is also a foiling Prowler Moth in San Francisco which Nigel Oswald has had for around six months.
  5. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Cool, John. You're the pioneer, man. I modify my statement. Any chance Nigel might come east to race in New Jersey in June? (Brigantine is adjacent to Atlantic City)

    Please be sure Nigel gets a copy of the new newsletter. It has a great interview with Rohan Veal.

    On another subject: interesting human powered foiler:
    Last edited: Nov 11, 2005
  6. Doug Lord

    Doug Lord Guest

    Main foil/ rudder foil interconnect

    Of the many excellent contributions that Tom Speer has made I think this is one of the most brilliant and most promising. In my own application I believe that it can lead to better control in "normal" foiling and that it will help make jumping feasible on a "peoples foiler"...
    Has anyone tried it yet on a Moth?

    SEE SKETCH ON P. 23, POST #335
  7. Kiteship
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    Kiteship Senior Member

    I think this may be obsolete, Tom. Supercavs I've seen all had rather thick "wedge" shaped sections (more like fig 57 in R-93 than like fig 32). In fact, I often see supercav rudders with dead straight faces and a really thick TE--a regular log splitter wedge shape. (I've seen these even on powerboats not expected to need super cavs--they force the rudder into super cavitation--or ventilation--to avoid a sudden loss of lift if the foil transitions.)

    I wonder if it might be possible to build a "cap" for a super cav foil, which would form the upper surface of an efficient section at sub-cavitating speeds. At some transitional speed, the "cap" would be quickly moved above the water line (or perhaps abandoned, like a sabot??), leaving the super cav working alone...

  8. tspeer
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    tspeer Senior Member

    Supercavitating sections sometimes also have an "annex" - a rectangular shaped addition behind the lifting surface - too. This is solely for structural support and is entirely "dry", being inside the cavity.

    The wedge shape of supercavitating sections is largely determined by the design minimum angle of attack. You can fill in the cavity with foil and not affect the performance as long as the upper surface is not wetted. Since the upper cavity streamline diverges from the leading edge, that dictates the wedge-shaped section. The lower surface shape is what determines the performance, so the NACA report is still relevant.

    I suspect the wedge-shaped rudders are actually base-ventilated sections for much of their operation - wetted on both surfaces with a cavity behind the blunt base. At some angle of attack, they'd become supercavitating, although I suspect there would be a flattening of the lift curve slope at that point.

    I suppose, but once you shed it, you could be toast in the first lull you encounter. I'd prefer a design that could decelerate and accelerate repeatedly.
  9. alans
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    alans Alan

    I agree Tom's is correct in postulating improved contol using both a flap on the centreboard foil and an elevator on the rudder foil. The two control surfaces can be linked or driven inderpendently by separate wands. A bow mounted wand measures a combination of pitch attitude and height while the flap acts forward of the centre of gravity and the elevator acts aft of the CofG. Consequently if the control surface motion is linked then they must have opposing motions and the moment generated by the elevator must not over ride that of the flap otherwise height control is lost. On the moth a elevator gearing set at 20% of that of the flap is a good starting place.
    If the two control surfaces are driven by seperate wands, there is no gearing reversal but the elevator wand must be aft of the CofG so that the pitch attitude term is in the correct phase.
    The boat motion of the two options is quite different, the former tending to hobby horse over waves and the later lifting over waves. Both system can still jump out of the water when faced with adverse conditions. As far as I can determine form math modelling both adjustment of "flight" height and wand to flap gearing are required to gain best control response to the wave conditions of the day.
    In regard to the actual mechanics, if the spring or shock chord that pulls the wand into the water is connected directly to the flap lever instead of the wand then a chord can provide the wand to flap linkage with close to zero friction.

  10. dimitarp
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    dimitarp Junior Member

    What do you mean for that foils?Coment please!!

    Attached Files:

  11. Doug Lord

    Doug Lord Guest


    Not sure I understand the question but those look like they could be a foilset for a windsurfer; I just completed a set using straight foils(7/1 aspect ratio; 63412 mainfoil with flap) for a client for experiments in the low to moderate windspeed region on a board. My mainfoil and vertical fin were made by Fastacraft and I designed and assembled the dual hydrofoil/vertical fin system. This system by Neil Pryde/Rush Randle seems similar as well:
    NeilPryde Maui --Rush Foilboard Windsurfer
  12. Buildboats
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    Buildboats Senior Member

    Those boards are pretty cool Doug thanks for sharing
  13. RHough
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    RHough Retro Dude

  14. Doug Lord

    Doug Lord Guest


    I don't want to say I told you so just 'cause I told you so! The next stage in the evoulution is the aeroSKIFF™-- a Peoples foiler that literally flies-in or out of the water...Sorry, I couldn't help myself.

  15. John Perry
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    John Perry Senior Member

    Just happens that yesterday I went along to an interesting lecture given by a representative of Austal, a company which is at the forefront of the development of fast commercial vessels, eg. catamaran car ferries and more recently trimaran car ferries. One thing that was mentioned was the 'interceptor', this being a vertical plate which slides vertically down into the water flow immediately behind the transom. This causes the pressure in front of the plate to rise to be close to stagnation pressure resulting in lift on the aft end of the hull. A servo system is used to move the vertical plate rapidly up and down to control pitching in heavy seas. Apparently, and perhaps surprisingly, this device has a better lift to drag ratio than a variable angle transom flap and it only needs about one tenth the actuation power than does a transom flap since the actuator is moving the device in a direction perpendicular to the hydrodyanamic force.

    Just wondered if you could attach such a device to the back of a supercavitating foil to control lift with minimal control force? - or has this been done already.

    Latest trimaran vehicle ferry built by Austal seems an impressive craft. 1000 tons cargo, 1250 passengers, 40knots cruise speed. Sea keeping ability is apparently best determined by counting the number of sick bags which need to be issued and this is approximately 80% less than a catamaran ferry operating on the same route. The main reason for this is that the trimaran with its very small waterplane amas has much lower roll stiffness than a catamaran and this gives a better ride in beam seas. The ride is also improved by both the servo controlled interceptors on the transom and servo controled adjustable incidence Tee foil mounted under the bow of the main hull.

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