Re: What RPM Should I Run At?
i guess roughly by the position of your lever .
In aircraft engines the manufacturer determines 75% "power" from a "family" of engine operating curves.
These curves are provided at several (different) RPM, manifold pressures and altitudes.
With a variable pitch (governed) prop you can vary the throttle at a fixed RPM and of course, you can select many different RPMs and throttle settings.....
The power curves are always a percentage of maximum rated power and you can determine actual HP from them at various throttle settings (manifold pressure) , ENG RPM's for many different altitudes.... etc.
Since a boat has 2 of those parameters "fixed" I.E propeller pitch and (pressure) altitude, the operator can only vary the RPM directly with the throttle.
If you have a marine engine with a prop pitch selected to allow MAX (rated HP RPM) at full throttle, then you can only estimate the 75% point by assuming 75% of max RPM approximately equals 75% HP.
That may not be an accurate assumption, but it's all we have.
The only way to accurately determine the exact 75% HP point (assuming you're actually getting FULL rated HP at WOT) would be to find that point using a dynomometer, run the engine to max rated HP at full throttle and then pulling it back to 75% of that number. (and note the RPM and throttle [manifold pressure] setting) AND, you would have to try to approximate the same load that would be present while operating the boat. you couldn't vary the load (total boat weight) while doing it either...............
This would probably not be worth doing.
I suspect that Volvo and Mercruiser are just recommending using 75% of max rated RPM as a suggested approximate continuous operating RPM instead of just an arbitrary throttle position.
Seems arbitrary, but agree this is the only practical way. Probably over-thinking, but what else is a forum for?
And I may be "over-thinking" this!