Air and water are both fluids, so the hull drag goes up with respect to velocity squared as well. That's why planing and hydrofoiling craft are able to get up to these high speeds- they reduce surface area as their speed increases.
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You are right of course (take de Morrison's equation on forces on cylindricals for instance, the same is the case there for the "drag-part" of the equation). What I was thinking is that on these wessels, the wetted surface is relatively small compared to the sail area/area causing wind drag. Hence, the wind drag should be the biggest drag factor..?
