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The propeller in the water is acting as a torque converter and the final drive in one. Unlike a car where road speed is directly proportional to the speed at the final drive unit a propeller is constantly variable almost like a CVT. If you want the engine RPM down at max speed you would change propellers like you would change your final drive ratio in a car. I'm sure your boats manufacturer matched the prop size and pitch relative to your engine and boat size/ hull style. Its probably a good compromise for whatever your boat was meant to do.
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