Depends on flight speeds and prop rotation (tip speed specifically), but the propellers have for a long time had a 90+% efficiency already. Ideally, a long and

Depends on flight speeds and prop rotation (tip speed specifically), but the propellers have for a long time had a 90+% efficiency already. Ideally, a long and narrow one-bladed propeller would have best performance, somewhat similar to sailplane's wing. For practical reasons (ease of balancing and limited diameter) we use two or more shorter blades, even if it causes minor efficiency loss. A compounding concern is that of a propeller tip speed - when it approaches speed of sound, the physics of airflow become complex and it's very hard to maintain efficiency - and near impossible to maintain reasonable noise levels. This necessitates keeping blades of high speed propellers rather short. The Tu-114 was exceptionally loud for a plane, both due to propeller speed concerns, and also due to contra-rotating propellers inherently causing increased turbulence. There is a good four-part series on propeller theory and practice: https://www.youtube.com/watch?v=SgoPPg8oVt8&list=PLD2EcpzcvT-vGulRAAkOS6P73CfTo2dME https://www.youtube.com/watch?v=SgoPPg8oVt8&list=PLD2EcpzcvT...