I've seen an explanation [1] that in water it depends on having two aerofoils — one in the air and one in the water (they should be called fluidfoils. I guess t

I've seen an explanation [1] that in water it depends on having two aerofoils — one in the air and one in the water (they should be called fluidfoils. I guess the one in the water, the keel, is acting as a hydrofoil.) At any rate, in theory, with two fluidfoils and fluids with any different velocities you can go at any velocity (i.e. in any direction, at any speed.) [1] http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wind-or-faster-than-the-wind/ http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wi...