I mean was it actually a windmill generator? I have windmill-looking things on my boat for measuring wind speed and such (they are small). It's just that using the actual sails to pull your boat along, and then using the speed to turn a water turbine on the hull is way more efficient. Especially since there's no meaningful difference between a motor and a generator, apart from which way the power flows.

Edit: also solar. Been waiting for reliable flexible solar panels for the sole purpose of replacing my sails with solar panels. That would be dope as hell.