Paul Spinrad says:
Here's a Youtube video of a wind-powered cart (with R/C steering) that goes downwind faster than the wind (DWFTTW — "No it is NOT impossible!").
Jack Goodman in Florida built this cart to end a long argument on the Amateur Yacht Research Society website (ayrs.org) about whether DWFTTW is even possible — but it just led to more discussions, with some believing the video is a hoax. Google "DWFTTW" for more, and there's a ton of discussion on the ayrs Yahoo! Group. Here's Jack's writeup (PDF).
It's taken me a while to put my brain around how this works– for me, the key point is that the propeller is a propeller, not a wind vane, and when the cart is rolling, the wheels are powering the propeller, not the other way around. With the right gearing, the propeller will always push backwards against the air, whether or not the air is moving forwards or backwards relative to the cart. The tailwind and the propeller action combine to make the wheels spin fast enough to keep the whole system rolling Faster Than The Wind. Definitely counter-intuitive (or a hoax I've fallen for).
please don't email me with your theories. Here's a link to discuss this. Link