Foils (enterboards) are usually described as providing 'lift'. I see this in the case of the airplane wing, or the sail, where the sides have different shapes.
I do not see it in the case of a centerboard with a symmetrical foil. Clearly 'lift' is different from minimising leeway.
There's no difference. The leeway angle will adjust itself until the side force from the board equals the side force from the sail rig.
For each foil there will be an angle of attack at which the lift is zero. For a symmetrical board, that occurs when the flow is aligned with the chord of the board. For asymmetrical sections, it occurs when the flow is aligned at a negative angle of attack relative to the chord (typically on the order of 2 - 4 degrees). If you measure leeway from the zero lift orientation of the board, the two work the same. Their response to a change in angle of attack is the same.
On a tacking boat, why is an aerodynamic profile better than a thin flat blade? For a non tacking craft, ie shunting proa, can one capitalise on lift in foil design because the lee side of the foil is always the leeside? This rehearses the asymmetrical hull conversation, I know. And folks have moved away from the asymmetrical hull designs, I know. But is there an argument for a bidirectional lifting foil?
thx
Anatol
The difference between a symmetrical foil and an asymmetrical foil is the symmetrical foil has its minimum drag near zero lift, while the asymmetrical foil has its minimum drag at nonzero lift. This makes it easier to ensure that the operating lift occurs in the low-drag zone of the foil. Asymmetrical sections can also have a higher maximum lift before they stall than symmetrical sections.
If your operating range is biased in one direction, then you can tune an asymmetrical section so its low-drag zone encompasses the operating range.
For a proa, you need to decide if the board is going to have fore-aft symmetry or right-left symmetry. A board with fore-aft symmetry can be cambered to match the fact that the side force is always to the same side in a proa. But the ideal shape for a leading edge is quite different from the ideal shape for a trailing edge, so you have to make some compromises. It's probably better to give the foil sharp edges than rounded edges.
A board with right-left symmetry (conventional symmetric section) will need to be flipped around for each tack. The rounded leading edge will not be as susceptible to leading edge separation and ventilation as will be a sharp leading edge.
One way to have your cake and eat it too is to have two boards, and only use one at a time on any given tack. You could put both of them down while shunting, thus doubling your lateral area when at low speed, and then pull up the reversed board once you get going. Each board can then be optimized for its particular tack. This can also help solve the directional balance problem, because you can get the right lead for both tacks.
With regard to asymmetrical hulls, one misconception is an asymmetrical hull will provide "more" lift. But that's not true because the hydrodynamic side force always matches the aerodynamic side force. If you produce side force from the hull, then less side force will be produced by the board.
Another misconception is that an asymmetrical hull can provide the side force with less drag than a board. That's a false economy for two reasons. The first is an asymmetrical hull (ala Hobie 16) has more wetted area than a rounded symmetrical hull of the same length, and that eats into whatever wetted area reduction you make by eliminating the board.
The second reason is the lift-induced drag is inversely proportional to the depth. So when you create the lift using a shallow asymmetrical hull, there's a tremendous drag associated with that. It's well worth the extra wetted area to use a board to extend the span and get a huge reduction in the drag due to lift.
, 2' draft. With enough bouyancy. ...?
