[Dick Powers]

One primary design parameter of propellers is “Activity Factor”. The larger the Activity Factor, the more power a propeller can absorb, and turn into thrust. A wide blade has a higher activity factor than a narrow blade. Hence, three wide blades may be able to absorb as much power as four narrow blades if the activity factors are similar. The wider blade, however, may be heavier requiring a beefier hub but fewer blades reduce the number of mechanical parts required – movable pitch mechanisms, etc. So prop design, like aircraft design is balancing the compromises.

There are many other parameters that effect the total propeller efficiency, such as blade airfoil thickness, and twist along the blade length.