Diesel soot appearing on the rear of my boat?

It happens a lot with post# 1 when people come here just for a quick answer instead of general knowlege. They come. They go. From his request for where to buy "yearly maintenance parts" I'm inclined to think he's looking to do his own yearly maintenance. From Rcraps answer he may have started realizing that there's a reason that diesel mechanics are certified by the manufacturer. Once they started thinking about pollution control and using brains, diagnostics became complicated. Hopefully he'll continue the conversation or at least come back to update us on what his soot problem turns out to be, so others can learn (even if what's learned is that it needs to be hooked up to a computer diagnostic program).

They come. They go
They come. They go
Nah...
Now I shooold feel comfortable.
I am wathin' Sienfeld ...
Already!

A sooted transom after a 900 mile journey isn't unusual. Get some Roll Off (not On-Off). Cleans it up easy. I believe we had those motors on a Carver I used to run, and they sooted us up pretty good. That's when I found Roll Off. You won't find it at WM. Get it on line or at an independent marine store.

Years ago I read an article in a small boat magazine about a guy who rented a boat for a trip from Lindenhurst to Block Island (just east of Montauk Point). To get there from Long Island's south shore you basically come out the inlet and head east, keeping the coast about a mile off your left shoulder. The guy had made the trip about a year earlier. This time though he got lost because the baloons weren't there to mark his course. People like to have weddings and such on the beach, and often release balloons. They generally drop into the ocean about a mile off shore. He thought they were nav aids. When I picked myself up off the floor my mind drifted back to my early days, and how little I knew. I think that article made me a better instructor. After all these years I'm still more impressed by how little I know than by how much. There always being more to learn is one of the best things about boating IMHO.

Good advice Zud!
We can all get a bit abrupt, no harm meant.
Peripheral info, just the words needed for that post with HTM09 and the generators, when you were posting for motors.

I had a soot problem on the transom when running @ 1800 rpm as recommended by previous owner. Read somewhere that my Perkins should run at 70% of Max continous RPM.
Changed to 2100 RPM and no more soot.

You could be running those engines at an rpm just below where your turbos spool up. Most engines don't like that rpm range and often will send out some black smoke just before the turbos kick in. If you are seeing black exhaust all the time, this could be a simple cause. I would run your boat at some different rpm ranges and see if all produce the same results or not.