The answer is it depends on the engine and the reduction gear
ratio. Each engine is different, from old large tugboat engines that redline at 300rpm, and fast gas engines that are meant to run up to 6500 rpm
engines in boats are installed, and then tested, to see if they over-rev or under-rev the engine with the hull
. It is important to the longevity of the engine to have it max out at the proper rpm's.
The other factor is how much fuel
do you want to burn. While the engine may be capable of turning over at 3500 rpm maximum, with the boat at 9 knots; quite often running 2700 rpm will push the boat at 8 knots using half the fuel. The key is to experiment
. Usually around 70-75% power is the sweet spot, but it depends on the boat.
If you give us the engine make/model and the reduction gear
ratio, we should be able to give you some idea.