Black smoke above 1800 rpms

Background. This is my first sailboat with Diesel engine. I purchased an 1984 IP31 in March and the engine ran good. It has a Yanmar 3gm30f. I live 700 miles from sailboat. The boat is storm damaged and needs repairs before moving a long distance.

For the first 3 months, I came to the boat monthly and ran the engine at idle for 1 hour to charge the batteries. Fourth month noticed that very little water was coming out. Began working at the thru hull to water pump first, changed impeller. It took me 3 months to determine mixing below was next. During these 3 months, I did not run the engine much.

This past weekend, 7 months since purchase, I replaced the mixing elbow. Water was finally coming out the back.

I took boat for 1 1/2 hour run on the diesel engine. Below 1800 rpms the engine ran fine. When I pushed the throttle above 1800 rpms, black smoke started coming out the back. Throttled down. Throttled back up 3 more times and each time black smoke came out.

I have google this. I now also know to not run engine at idle for long periods.

Has any one else experienced this? What was the solution?

First let me say I'm not an expert. I have owned a Hunter 34 with a Yanmar 3GM for 15 years and currently own an IP29 with a Yanmar 2GM20F for the past four years. I do all my own engine work. I've experienced your problem on both engines. Look for the simple things first. A few things to check:

What's the condition of the bottom of your boat. If you got growth, even fuzz, it will be a possible cause of black smoke. Had this myself.

Check your prop to be sure nothing is growing on it or it's not bound up with any line. Happened to me.

If you have a folding prop be sure it's settings are correct. Had this one too.

If you have a regular prop be sure the pitch is correct.

Each time I had black smoke it was a dirty bottom or a prop issue. Simple to fix each time.

Last thing I'd recommend is to Google something like "black smoke sailboat engine". You will find lots of information. But be careful not to go with the worst case scenarios before checking the easy stuff first. For me it was always the easy stuff.

Good luck,

Joe M