Re: Ink black exhaust water - soot?
I have a 1980 Yanmar 3gmd. I believe same as the OP's engine but 3 cylinder. I think my boat is also over proped as if I run at anything over 2500 rpm the engine just goes faster but boat is already at hull speed so goes no faster. I do not blow black smoke, by the exhaust does become more gray and the engine seems to start running hotter and seems stressed.
I have also heard the yanmars like to run at 80% max rpm. But to me, that is really straining the engine. I know what Mack Boring and even the Yanmar manual says, but it just seems to be straining the engine, and higher rpm mean more wear on moving parts. I do however run the engine in neutral for 30 seconds or so at full rpm prior to shutting down- to blow out the carbon as the Yanmar manual says you should do.
I believe my engine has been run this way for the past 34 years and does not burn oil or have low compression. Exhaust is clear.
I would just run your engine at the lower rpm if that is what your boat is happy with.
The I also agree with the OP's PO- it is difficult to get the perfect prop specs considering the effects of wind and waves will have on the load the engine will see.
As far as the engine oil dip stick, sounds to me the your range is too wide, I would go to a yanmar supply and compare what they would have for your engine