I believe we may be approaching foiling all wrong...

Discussion in 'Hydrodynamics and Aerodynamics' started by Iridian, Jan 25, 2020.

  1. Iridian
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    Iridian New Member

    Most foil boats look a lot like normal sailboats with a couple of distinct changes.. Instead, lets go back to the drawing board and design something specifically for the hydrofoil.

    The Hobie Trifoiler is an example of something designed specifically for the foil, but I don't believe it went far enough.

    The top few concerns of foiling are cavitation, pitchpoling, and the drag necessitated by large foils to enable the boat to get up on a foil, which becomes a problem as the foil drag is too excessive. In addition, heeling force can cause issues.

    Kitefoiling solves these issues by using the explosive power of the kite loop to launch the foilboarder on to the foil, and human muscle memory to maintain stability. It's further improved by the complete lack of heeling force.

    Instead of relying on self-leveling foils for stability, lets change it up and rely on the ground effect. Additional horizontal wings can be utilized to maintain vertical stability, and counteract heeling force.

    See my remarkably shitty sketchup drawings below. Please note, this is concept art only, and I haven't put much thought into the dimensions necessary to balance the boat.





    The outside planing boards are currently at a 0 degree angle. In practice, they would be at a negative angle in order to catch the boat should it attempt to pitch pole. The shock absorbers are meant to stabilize it at 0 degrees.


    The horizontal wings are invertable, enabling asymmetric lift to cancel the heeling force. (I.e. inverting the camber and angle of attack of one of the wings.

    In addition, the usage of these horizontal wings enables us to pack on massively more sail for the weight/width, much like the Vestas Sail Rocket 2..
    Last edited: Jan 25, 2020
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  2. fallguy
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    fallguy Senior Member

    Foiling is not in my realm, but I am ever curious about it and why a forward foil could not mostly negate pitchpole while at the same time aiding in faster lift. But, I am just a curious fool on the matter.
  3. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Iridian - your proposal does look quite fascinating. However like fallguy foiling is not in my realm either.
    Re your 'planing boards', I think I would be worried about their form drag at slower speeds while trying to get up on the foils (?).
    And they might create a fair bit of wave making drag unless they are literally skipping along the surface.
    Would it be feasible to just have the lifting hydrofoils, and dispense with the planing boards?
  4. philSweet
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    philSweet Senior Member

    The trouble is that if all the foil does is lift, it has to do that better (higher L/D ratio) than buoyancy alone can lift. If we put buoyant hulls on equal footing and say that they also only have to lift, with no form stability restrictions, then an optimized displacement hull is the better performer over most sizes and speeds of interest (think rowing scull). But if we add in practical constraints such as static and dynamic stability, load factors, and form factors, a small displacement hull might go from an unconstrained drag of 1/30th of displacement to a constrained drag of 1/15th of displacement or worse. If the foil system can provide the performance of the constrained hull form, it can get by with a L/D of 15 or so. If it can't, it needs to be at least twice that good, and that isn't realistic. Foil stabilizers have been around for a long time. It's the lifting part that's hard to pull off. Lifting performance has a higher L/D floor than stability performance when compared to displacement hulls.

    So if you offload stability considerations to some other appliance, the displacement hull will probably benefit more than the foil system.
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  5. Iridian
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    Iridian New Member

    @philSweet I'm not sure if I'm representing my idea sufficiently. The idea is to use the horizontal wings for the vast majority of lift and to counter the heeling force.

    The foils would largely used as a traditional keel once the wings begin to carry the weight of the craft.

    @bajansailor I think it should work fine without the planing boards. I think the wings would likely be quite fragile though.
  6. CocoonCruisers
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    CocoonCruisers Junior Member

    With wings operating in air at the rather low speeds that can be achieved when sailing, i'd be a little wary of the effects of gusts.

    Perhaps you could also think of air cushions such as in Surface Effect Ships (SES) , or as used in thundercats or the hysucat approach. The air cushion below the hull builds with speed, so it will appear more or less when you need it. Beeing very close to the surface, it is less exposed to wind effects. And since the air cushion gets compressed when the boat comes close to crashing into the surface, it could probably provide more progressive damping than a Wing in Ground Effect.
  7. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    So, these are simply labels for similar outcomes.
    Wing in Ground Effect, SES, etc.
    Problem is, if your aim is to trap air, then your going to get wave interference that slows the boat down.
  8. ziper1221
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    ziper1221 Junior Member

    But sailboats only work by exploiting the energy boundary between the water and the air. Seems like offloading lift from the foils to the sails is somewhat misguided if you want to go to weather.
  9. Iridian
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    Iridian New Member

    @ziper1221 the energy boundary is resistance to the force of the wind parallel to the surface of the water, not perpendicular. I.e. zero mass would be optimal if a suitable resistance to sideways force was present.

    @CocoonCruisers I'm also concerned about gusts, but I believe the drag from the wings will be less than other surface effects given wave action. A similar concept was utilized by the soviets on the Ekranoplane.
  10. ziper1221
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    ziper1221 Junior Member

    Building ground effect wings won't reduce the mass.
  11. Iridian
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    Iridian New Member

    @ziper1221 It will decrease the drag caused by that mass across the surface of the water, while not reducing resistance to being pushed downwind.

  12. tspeer
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    tspeer Senior Member

    Ground effect isn't strong enough to provide the stabilizing force you want. However, aerodynamic lifting of the craft is exactly what Sailrocket was all about.
    The hdyrofoil pulled sideways and downward. The planing surface at the bow was more about regulating pitch and heave than supporting the boat. Both the wingsail and the beam produced aerodynamic support, and the wingsail had an extended horizontal panel for that purpose.
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