Spinning the rotor allows you to generate more power from the wind than what you would have gotten by putting the same energy into spinning a submerged propelle

Spinning the rotor allows you to generate more power from the wind than what you would have gotten by putting the same energy into spinning a submerged propeller. From what I understand it's not hugely more efficient, but something like 10-20% fuel savings. Not really equivalent to traditional sails, but also much easier to retrofit and doesn't need additional crew