Here is a bit of real sailing nerdism. Fast sailboats have no problem sailing faster than the true windspeed experienced by a stationary object, but they can't do it directly downwind. There's been lots and lots of debate, online and in yachty drinking places, about whether this is even physically possible. There was a consensus that if it could be done, it would involve a sail cart with a big-ass turbine to drive the wheels. Results have been ambiguous until now.
This video is long, boring, and heavy on the explanation of some pretty basic physics, but maybe worth skipping through for some. The basic concept to start with is that the turbine blades are not perpendicular to the wind, so the sailboat going directly downwind is not an analogy.