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Originally Posted by NightWing
When selecting an optimal RPM for sustained cruising, it is best to stay in a range where the boat is "on the step", with the least amount of wetted hull. Every boat has a planing speed that acomplishes the above. (Not including displacement hulls.) That speed is also when the powerplant is not working quite as hard. Once the boat is planed out with trim tabs and outdrive angle at efficient positions, the fuel efficiency is probably as good as you will get. Any more speed will just increase fuel consumption and engine wear.
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This is good advice. Some boats are so underpowered that this optimum hull efficiency speed can only be reached by really reving the heck out of the engine. Not a good thing for longevity.
Ideally, the optimum hull efficiency speed should fall close to the torque peak RPM of the engine, not the HP peak RPM. The torque peak of the engine is right where it has the best volumetric efficiency which means the intake system is operating at it's best at that engine speed and pumping losses are minimized.