Almost mounting my mast so it would be underwater got me thinking (as
it should). Why not underwater sails? Force on a sail is
proportional to the density of the medium and that makes a 5 knot
water current give about 50X the force of a 5 kt air current on the
same size sail. This means you could sail across the Gulf Stream with
an underwater foil. However, I am not sure where your rudder and
centerboard would be, in the air? Could you make a planing boat with
an underwater foil taking advantage of the water current? That way
you would not be limited to the 1.2Xsqrt(waterline length) in speed
anf might get up to absurd speeds.
Then again, maybe this idea is the result of breathing epoxy fumes.