wind powered, power generating ships
Someone: 'useful' is subjective. That mast would not have to be 'very large' to power one's GPS system, and having that certainly could be useful. At even lower power, a tiny impeller that measures boat speed could be very useful.If you are aiming for large amounts of power, I would expect it to be more efficient to replace part of the sailing by a wind turbine, and to get rid of that water turbine.