You need to find out max continuous rpm
Diesel motors have a maximum continuous rated rpm and sometimes a 1 hr rating (rpm can be used for up to one hour, then you should lower rpm to cruising rpm) You can get this rpm from the owners manual or usually a data plate somewhere on the engine - usual wisdom is that the boat should cruise long-term at 70-80% of this continuous rating (for example, 3000 rpm max continuous=2400 cruising rpm). The boat should be propped to reach max continuous rpm or a little over with normal loads in the water- assuming your boat bottom is clean and everything is aligned correctly, it sounds like you are a little over propped, which loads the motor too much and causes the black diesel smoke as the injector system tries to dump fuel into the engine to make enough power for the load you are placing on the engine. But you should find out the actual working rpms for your particular motor, since it varies from one brand to another of diesel. long term overloading is not good for the engine, and may lead to overheating if run that way for a relatively long time. Does the engine reach max rpm when not connected to the prop(gearbox in neutral)? If so, then it sounds like your prop is pitched too much.
