Thanks Annie, that was an interesting read.
My rig seems to have more power on the starboard tack, but I already know that I didn't build it with both sides equal so that's more likely the problem.
I wonder if rather than simple turbulence the extra distance air must travel to go around the mast doesn't partially defeat the whole purpose of a foil, that air travel faster on the side that develops lift, the lee side on a sail, because it has further to go.
So when the mast is on the lee side of the sail the wind has to go the extra distance to get around the mast and the camber, the extra air speed necessary offsets the turbulence from the mast.
With the mast on the windward side of the sail the air goes around just the camber on the lee side but has to go a bit faster to get around the mast on the windward, effectively neutralizing part of the advantage gained by putting camber in the sail.
Hmm, well, I'll have something to think about next time I go out :)
Bill