There are a lot of discussions on other forums (like in this one: http://www.boatdesign.net/forums/boat-design/wind-powered-sail-less-boat-24669.html, but there are a whole lot more) about sailing direct downwind faster than the wind (direct downwind, not sailing at an angle, of course). There's people claiming they've got it. See: http://www.youtube.com/watch?v=MfZt19F-OA4&feature=related This seems to me to contradict the principles of energy and momentum conservation. Nobody in such forums debate with a proper phisics-mathematical approach. That's why I bring the subject to this Forum. Any help? Thanks in advance.