What percent of a boats max horsepower is used after plaining to maintain a moderate cruising speed?

Question

I know that the max horsepower for a boat is needed to get the boat on plane i am curious however to see how much of that horse power is used once plained to keep at a moderate cruising level. The boat im asking for is a 21 ft. sea ray with a Mercury 228 (chevy 305) in it. Thanks!!

Answer 1

No.  The boat's maximum power is rarely needed to get it on plane.  The maximum power is required at the maximum speed.  Typically, a hard chine planing hull will begin to plane at a speed/length ratio of 2 (that's speed in knots divided by the square root of the waterline length).  Depending on design, the resistance may fall somewhat for a brief range of speed/length ratio but will climb again..  So even though there is a hump that the boat has to get over, the resistance does not necessarily fall off after that hump, and even if it does, it will begin to climb dramatically as speed increases.

With a 21ft Sea Ray with a Chevy 305, you should be able to run comfortably on plane at 80% power with no problem.