Turbo vs conventional
Thank you all so far for you input. I am on a time line, and trying to make the right decisions, so your input really is valuable.
To clarify some questions raised, the engine sounded like the 3 cylinder Universal on my 1986 Catalina... thump thump thump... like an old fishing boat (or the diesel from the Humphrey Bogart movie African Queen)
The cylinders are scored, and well outside Yanmar specs max specs. The oil was new, and the likelihood of getting an accurate indication of the engine condition from a lube analysis was quite low.
Please comment on my rational for a 75HP turbo:
I am comparing a Yanmar 4JH4-TE 75 VS 4JH4E 55 HP
Based on the sea trial shaft speed of 2750 RPM, I was close to hull speed. From the factory performance curves, this requires a propeller power of 40 HP and generates a torque of 97 Ft-lb and 2.5gal/hr fuel consumption.
Going to the 75HP engine, the same 40HP prop power requires 2500 RPM crankshaft speed, thus a different gear ratio is needed to get the correct prop rpm. So the engine rpm is lower with the turbo. My crankshaft torque goes to 140 ft-lb, and fuel consumption drops to 1.8 gph, almost a 30% reduction! With this improvement in efficiency, I would expect that there would be less heat generated. The gearing on the new engine is also over sized for the typical load.
The Yanmar 3JH4 is a 75HP engine that runs at a higher speed, and is not one I am considering.
The advantage for extra HP is fuel economy, more low-end torque, lower rpm for normal operation, and the extra HP if needed to fight a heavy headwind, or pull off a soft grounding. I have the power curves and can send them out if anyone wants them, or you can find them on Yanmarmarine.com
Please comment on my logic, Thanks for the help