Here's a better explanation / discussion. The key insight is that the propeller is not a turbine. It's a propeller drawing energy from the motion of the vehicle. I was wrong about the variation - no variation is needed.That was the argument of the physics professor that bet $10,000 it wouldn't work. He claimed it was just temporary and the vehicle couldn't sustain a speed higher than the peak wind speed. He lost the bet.
DDWFTTW - Directly Downwind Faster Than The Wind
There has been suggestions to dedicate a thread to this interesting concept for boat or land vehicle propulsion. Jack Goodman created quite a stir...