Did you know that many of us yachties are gradually, and sometimes not so gradually, wrecking our engines, as well as spending more on fuel and generating more carbon than we need to?
It's true, so let's look at the problem, and then the solution.
Let's define the problem: the single best thing we can do to keep our diesel engines happy—after regular oil changes and clean fuel—is to run them hard. What, I can hear you say?! Surely, if an engine is underloaded it will last forever (you are not alone, I used to believe this). Well no, the exact opposite is true.
In fact, what happens if you run your diesel engine at low power setting a lot is that the cylinder walls will glaze-up and then the rings won't seat properly, power will drop, and your engine will start to suffer from the dreaded blow-by syndrome (as well as all kinds of other bad stuff), in which combustion gasses and unburnt fuel bypass the rings because they are no longer seating properly.
If you are seeing a slick on the water around your engine's exhaust, and particularly if that slick remains after the engine is fully warmed up, you may have a glazed engine. Another tell-tale symptom is that the engine will no longer reach full throttle RPM, even though the boat hull is clean and the prop has not been changed.
If you catch this situation early enough, you may be able to correct it by running the engine hard for long periods—I have fixed a glazed engine myself this way—but if not, the only answer will be an expensive rebuild or a new engine.