When talking about percentage of load in boat engines turbocharged and NA engines aren’t IMO apples and apples.
If you had an engine that was NA and was rated for continious duty at 2200rpm propped to rated hp and the guy in the next slip had the same engine w a turbo the turbo at 2200rpm would generate far far more heat and would be generating much more hp. But probably using the same con rods, crankshaft, bearings and most other parts most likely excluding exhaust valves and perhaps rings.
So the typical dock talk “3-400rpm down” may not apply. One needs to consult the manual for such information. However if you had a 6-354 Perkins and your friend had a 6-380 Ford you could probably assume they were very close in all respects including continous rpm rating. However one could still have sodium filled exhaust valves so checking w the manufacturer still would be a very good idea.
But most NA engines are probably quite similar as in approximately 300rpm down from rated rpm propped right. It would be interesting to me to see how turbo engines compare to NA engines. Especially those using the same base engine.
North Western Washington State USA