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Ā· Closet Powerboater
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RE: prop efficiency, I once had it explained to me in a way that really made sense. I'll botch the details for sure, but here is the version I remember.

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.

On sailboats clearance is almost always the issue and drag when the prop is not being used (not an issue on an airplane) is also an issue. So, we tend to have 3 blade props, or if you have a heavy powerboat with huge engines, maybe you can't transfer all that thrust with only 3 blades, so you're forced to add a 4th blade.

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