There's blade-water friction as well as water-water friction. The link you provided does not quantify their relative contributions. [edit: Actually, I'm not eve

There's blade-water friction as well as water-water friction. The link you provided does not quantify their relative contributions. [edit: Actually, I'm not even sure how much blade-water friction there is, due to the boundary layer effect.] There is also heating from cavitation bubbles collapsing in the water. If you look up cavitation heaters you will find a wacky rabbit hole with unsubstantiated claims about achieving heating efficiencies over unity. However, apparently there are some industrial processes that benefit from cavitation heating by getting even liquid heating and avoiding heating elements that can develop scaling. More info: http://hydrodynamics.com/cavitation-technology/scale-free-heating/ http://hydrodynamics.com/cavitation-technology/scale-free-he...