Planing or foiling Featured



A big issue in speedsailing is the discussion on planing vs foiling.  AC72 for instance now revolves around L-foils. Moths likewise. From this, it appears that foiling is faster than planing.Then again, kitesurfers,windsurfers and speed boats generally plane instead of foil. So what is faster, planing or foiling?

One strong argument in favour of foils is that foils are not affected by waves. Choppy water slows down a planing board or planing boat, but not a foiling boat, or at least to a lesser extent.

However, this is not the end of the story. An argument in favour of planing boards is that the wetted surface of a planing board decreases with increasing speed. This makes a planing board very efficient, because at high speeds, only a small part of the board is in contact with the water. Not so much with a foil. An AC72 can rise, but the horizontal part of the foil will stay under water and always has the same wetted surface. 

Another argument in favour of planing boards is that they limit the rising of the boat at higher speeds. L'Hydroptere and the AC72 are flying boats, but this flying also implies a huge gap underneath the wing, resulting in wind leakage and a complete lack of the endplating effect. The flying effect also increases the moments, because the centre of effort is raised, increasing the arm. Planing boards allow the boat to stick closer to the water surface, thereby maximizing the pressure in the sail. 

Further, a foil is essentially a wing having a high pressure side and a low pressure side. In contrast, a planing board only has a high pressure side. The low pressure side of a foil is the side where the turbulence and energy loss occurs. Without the low pressure side, you have less turbulence and are more efficient. Therefore, a planing board is more efficient by definition.

One more argument in favour of planing boards is that planing boards allow a functional division or rather, a spatial division between horizontal and vertical surfaces. With a L-foil like the one of the AC72, the horizontal surface is coupled to the vertical surface and located at the same position along the length of the boat. However, the horizontal surface would probably be better positioned more forward than the vertical surface. Due to the coupling with the vertical surface, this is not possible, which results in an imperfect design.

Moreover, the boat is in a continuous risk of making a forward flip. L'Hydroptere with its V-foils has the same problem. With planing boards, the position of the horizontal surface is uncoupled from the position of the vertical surface (the foil), allowing both surfaces to be positioned exactly where they need to be positioned. In particular with a stepped planing hull, this enhances pitch control and allows elimination of nose diving or forward flip problems.

Last, planing is simply more spectacular. When we add all the plusses and minuses, at Wiebel, we put our chips on planing. 


  • Emile Burnaby Lautier

    Planing is the propper choice but not for the reasons you mention here. Have yo considered the eficiency of foils at speed? Further have you considered the eficiency of the foils that you use for lateral lift at speeds over 45 knots?
    There is much to be learned from sailrocket and macquary innovation. or the swedish speedsailing challenge as well as the russian navy.

    kitesurfers are also planing and they create vertical as well as lateral lift from a palning surface. They achieve 50 + even when their airfoil has an L/D of just 6. sailrocket is a planing boat and its lateral foil is a wedge which is effectively a planing surface. and not a hydrofoil.

    anyway interesting project.

    Emile Burnaby Lautier Friday, 26 April 2013
  • Russell Miller

    Have you thought of adding suspension on the floats? Like a car has so the boat stays in contact with the water as much as possible rather than Jumping every wave. This would also have the effect of keeping the rig stable.

    Russell Miller Friday, 26 April 2013

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