I saw this article in Wired which asks "Can a wind-powered craft move faster than the wind which pushes it?", and have been pondering ever since.  Thin Air Designs claim to have created such a craft; it consists of a land yacht with a propeller linked to the drive shaft of the wheels.  There's even a movie showing the craft in action.

You could imagine such a craft going into the wind quite fast, as it goes faster into the wind the apparent wind is faster, and the craft goes even faster, etc.  Sailboats do the same thing.  But can you imagine such a craft going downwind?  It seems like the faster it goes, the slower the apparent wind, and when it reaches wind speed the apparent wind will be zero.  At that point there is nothing to turn the propeller, and no power to the wheels.  Right?

Or ... maybe the wheels turn the propeller?  Maybe the faster the craft goes downwind, the faster the wheels turn the propeller, which bites the air, moving the craft faster?  In which case once it is going the same speed as the wind, the propeller is grabbing at air which isn't moving at all.  But that would imply a sort of perpetual motion machine.

Huh.  I'm not sure about the physics here, what do you think?