Quoted:
Was talking to a buddy about boat propellors and was watching vids of some damn fast boats and was thinking about what specifically about the propellers that makes them move the boat so quickly. I assume there is some finite number of RPMs a prop can spin in the water and still "bite", so then what does more horsepower do if a lesser powerful motor is still capable of achieving the fastest usable propeller speed? Does a boat with a 7 marine 627hp outboard have a propeller that is 2x the size of a 300hp need outboard? I am interested in the relationship of size and pitch to power assuming there is a point of diminishing returns with RPMs.
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There is a lot more to think about than the size, pitch and power, like the size of the boat and the reasonable speed that a motor of a given hp will push it. If you put 300 hp on a barge that will go 20 mph, you need a big , low pitch prop that will push you 20 mph at peak output rpms, say 5k rpm. If you put that same motor on a lite go fast hull that can reach 80 mph then you need a prop pitched to hit 80 mph at the same 5k rpm. The faster you go the drag in the water increases dramatically, so this prop would probably not want to be as big because of drag and because it is pushing a lighter load. Also, once you are going 80 mph that large pitch doesn't feel so large because the water you are pushing on is going by at 80 mph.
To you bite comment, to be efficient a prop should only slip a certain percentage. If you have a high pitch prop targeted to go 80 mph, when the boat is barely moving, the slip is incredibly high, it will just cavitate with to much power applied. Once you get moving closer to target speed the prop will bite better as the slip comes down and more power is applied.