Yes, you are overworking your engine which can cause damage. First, make sure the motor is running properly. If your engine is in good order, then a prop change is what you need. It does not sound right at all that a 17P prop is only turning 5100rpm, you may have a bad tach or a bad prop. Or simply the wrong prop. I'm not sure you should need to drop to a 13 or 15P prop on that rig, you might try a 17P of a different type. 17P is pretty popular size, someone you know probably has one you can borrow for a couple hours to test.
First thing is to make sure your engine is running correct, then check your tach and make sure it is correct, then check the prop and hub for damage, then try a new prop.
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