Hi Charles

It’s not easy to give a straight answer as so many factors can change the game:

A lot depends on the propellor. The optimal choice of the diameter and pitch would be influenced by the reduction ratio of the gearbox.

There is also a balance to be found between getting absolute maximum thrust for pure motoring in adverse conditions (when you could run the engine up to the speed of its maximum continuous power rating), and perhaps a slightly bigger pitch propellor to motorsail at maximum hull speed, without having the engine at full speed.

In pracice, if you find that if you gradually open the throttle, until the boat speed no longer increases, then there is not much point in trying to push the engine any harder. Any more will simply increase the fuel consumption and/or make a bigger bow wave for perhaps half a knot extra speed. Same thing if more throttle results in black smoke or soot on the exhaust. Weed or barnacles on the hull or propellor can give the same “overloaded” effect.