A boat with a windmill rotating a water propeller can sail directly upwind. It has been done many times, so take that as a given. (The maximum speed so far is about 0.5 times the speed of the wind. For a land yacht, the maximum speed so far is about 1.0 times the speed of the wind.) My question has to do with the correct physics terminology to describe the following situation. A windmill boat on a rapidly flowing river, under windless conditions, sails directly downriver faster than the river by using its relative wind. From the frame of reference of the river, the boat is sailing normally directly upwind. However, how does one describe what is happening from the conventional frame of reference of the ground? What is the source of energy? Obviously, the river is the only source of energy. But then how does one describe the windmill's interaction with the relative wind? It appears to be extracting energy from the stationary air, which is not possible. So is the interaction with the air essentially just the equivalent of a fluid transmission to transfer energy from the river to the boat? Or what?