Foil section design

Showing how variables affect Foil lift and Drag

The simple variables - Boatspeed & Area - have the greatest effect on Angle of Attack. AoA has the largest effect on the speed along the surface, and this governs the boundary layer.

When the AoA is high, the speed peak at the nose is too high to slow down enough by the trailing edge and the flow separates and causes loss of lift and very high drag.

If the laminar b.l. extends too far aft, the b.l. flow doesn't have enough energy to stick to the surface and separates.

If the foil shape doesn't enough laminar b.l. the drag from friction will be higher

Boatspeed has a large effect because when sizing an appendage Lift varies with BS^2, so Angle of Attack varies with 1/ BS^2

AoA varies proportional to the Area - half the Area, twice the AoA

Span is usually a modest effect because very low span is rare in racing yachts.
Likewise with Surface finish - the difference between good and great is small

Needs revision to include effects of camber and cavitation!