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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