500RPM down from WOT is very safe ,I would consider 1800 to be safe , if you can stand the noise and deck angle.
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"as M1 through M5 with M1 being most heavy duty full time WOT use and at the other end M5 usually at a higher WOT and HP setting light pleasure use with very little WOT use expected. "
There are RATINGS the engine would be run at , only the speed boat M5 would be at WOT , for a very short time.
The engine at M1 rating will still be capable of more HP and RPM , and many will use the short term ratings to go faster ,make a bridge or lock, , by observing the short time limits , no harm happens.
The propulsion engines are usually identical , the owner chooses the M rating he prefers . to operate at .
Pumps and generators may be set up differently at the factory .