Diesel Smoke Question

[Badsanta]

It is 25 years old and cant run as new. Cleaning the bottom will help alot. If it still smokes at full throttle, try it in neutral and see if you can get full RPM and check for smoke. If it looks clear, I would think that the motor is fine but just not as strong as when new and the prop was set for new and may now be to big for it. The bigger the prop and the larger the angle of the blade the more power is needed to turn it. Many boats are over propped. You may need the prop cut down or reset. It may be cheaper to buy a new one. You may just want to run it at a slower speed till another date. Chances are it has been like this awhile hence the lower speed of the previous owner. Check the exhaust elbow as previous suggested.

[Three888s]

First and cheapest, always check the air filter or for air restrictions.
Black smoke- overfueling. restricted air flow, injector nozzels not closing completely.
White smoke- water burning in cylinder. low compression (valve adj. or burnt valves)
( if low comp, smoke will burn your eyes from sulfer smell ) cold diesel engine will always blow white smoke til engine warms and pistons expand.
Blue smoke- burning oil.
after checking air filter, try accelerating slower from dead stop to cruising rpm. if still have problem, send injectors out and rebuilt and tune the engine.
Hope this will give direction, from cheapest to most expensive.
Best Rich