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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-wi...

Yes, check this out: http://en.wikipedia.org/wiki/Hydropt%C3%A8re

> Apparently sailboats do this all the time.

No, the article is about sailing dead downwind faster than wind. No regular sailboat can do that.

That would have been incredibly handy in the age of sail.

Yeah, they really do. I was looking into this a couple months ago when a friend at work, who has a sailboat, told me that they could do that, and that they could sail with a headwind!

Here's wikipedia's take on that: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind

That's not directly downwind.

http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Sa...