Propellers are usually up around 85% efficient at their design speed (with fixed pitch propellers falling off very rapidly away from this design speed, and vari

Propellers are usually up around 85% efficient at their design speed (with fixed pitch propellers falling off very rapidly away from this design speed, and variable pitch props having a flatter optimum). There's room for improvement here, but not a game-changing amount. A larger, slower prop captures most of the potential improvement with far less incremental complexity than something completely novel.