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.....The most efficient (thrust per hp used) prop is something like what you would find on a Cessna. They could have a 4 blade prop, but they don't need to because they have enough room for a large diameter, thin, two blade (efficient) prop.

Once you start getting into bigger planes, the wing size, clearance issues and the need to service more HP requires adding more blades to the props. This is a compromise, and if the planes had the room for it, they'd all have huge 2 blade props......
This generally correct, but there is much more to it. Drag is also a factor vs. the hp of the engine trying to turn more blades. The biggest reason, however, that more blades are used is noise. The prop tip travels through the air at a faster speed than the root. Faster is louder. In fact, there are some early aircraft where the blade tips reach the sound barrier. They make a very distinctive and extremely loud sound. Shortening the blades quiets down the ride, but you need to add blades to get the power back. Then you need to have enough hp to overcome the drag of more blades.

Boats and aircraft have this exactly in common...... everything is a compromise.
 
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