Sailing Downwind Faster Than The Wind

[Leopolis]

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.

 

[russ_watters]

It is possible and it is not a violation of conservation of energy any more than a fast-moving sailboat is (at any angle) or a wind turbing with blades moving much faster than the wind. The best way to prove it to yourself is by drawing a vector diagram of what is happening. You may also consider a vehicle like those shown moving directly up wind and consider (again, with a vector diagram) how they are different.

We've had discussions of this before and they tend to run away and get very heated/hostile. I've led tightly moderated versions as recently as a month ago that I may start again (I have a few days off work...). Do a search of the forums for several threads on the subject. You can PM me with questions if you still have any after reading up on them and I'll answer or incorporate them into the new discussion.

I'm locking this pending the reopening of a tightly moderated discussion.