Why aren't side thruster fins on kite surfboard symmetrical? The fin that is in the water going upwind seems like it is around the wrong way to be efficient when digging the windward rail in. I'll have to try some symmetrical ones and see if they are crap when in surfing mode.
At a given fin size and angle of attack (=toe in), a single symmetrical fin has less lift compared to an asymmetrical one. If you look at the polars of symmetrical profiles vs. asymmetrical profiles (=the lift coefficient as a function of the angle of attack alpha), you find that the symmetrical profile produces the same amount of lift with positive and negative AoA, whereas the asymmetrical profile produces more lift at positive AoA and less lift at the respective negative AoA.
This means the overall lift of both side fins together, as a function of AoA of the thruster set (=by turning the board upwind or downwind), is higher with asymmetrical fins. When you turn your board upwind, the upwind fin produces a lot of lift (= down force on the upwind rail), whereas the downwind fin goes near zero lift AoA and produces a minimum of drag.
tl;dr: asymmetrical fins in a thruster setup are more efficient.
