Originally Posted by sony2000
Internal combustion engines are the most overall efficient at 75% of maximum RPM. The math is easy, 2700 rpm. It's a rule.
Err not so. An IC engine is at it's most efficient when it is run at it's maximun torque RPM.
We are talking fuel efficiency here. If you have some other definition of 'efficient' then what you say may be true.
So if you want your boat engine to be operating at it's maximun efficiency you need to 'over prop' it.
Just like driving down a flat road in your car. You want to be in 'overdrive' or a high gear.