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[hopeitfloats]

just an enquiring mind here. how can a yacht travel 2x the speed of the wind - as per americas cup trimarans.  sounds like it should be physically impossible.

[malcolmfrary]

Running directly downwind, it is impossible.  Running across the wind, including tracking up to about 45 degrees off-wind both up and downwind, however, different forces come into play.  It all comes down to the energy that is gathered by the sails working against the resistance to sideways motion of the underwater part being turned into forward motion.
Have a look at
http://en.wikipedia.org/wiki/Sailing
and
http://www.sailingahead.com/information/science_sailing.htm
They both explain it better than me, and I think I understood it better after reading there.
There are some interesting subtleties in windy boats.