A lot of the conventional wisdom thrown about is from seat of the pants I go passed down from person to person and not born out by hard data that the engine manufacturers diligently work
out and after seeing the results of all the things owners try out on their rigs.
Engine life is all about load. Manufacturers make up fuel burn curves and show what you should be burning at any given rpm. If you prop your boat do that it does not achieve its top rpm it will be overloaded at all rpms. May not hurt much at trolling speeds but at cruise
and over you are wearing it out regardless if the RPM and burning excessive fuel.
But because we really never know and if the boat seems “right” a d it doesn’t seem like a real hog then everything is OK. No matter the hull
type the engine doesn’t care it just wants to meets it’s designed performance curves. Different boats may run at different mph but the engine doesn’t care. It only cares about load.
Propping the engine do its top rpm is lower than the rated maximum “seems” intuitive that you are working the engine less but the opposite is true. The bigger prop means a bigger load at any given speed. It’s like pulling an anchor
out behind the boat at all times.
Go to Tony’s tips at SBMAR.com and look up the stuff about propping boats. Also check out the stuff about marine
age vs actual age. Great starts to understand this stuff.