It is the boat that may be underpowered
The Yanmars of that generation are really long-lived engines. And, they put out the power they are supposed to. However, like many boats of that era, the Cherubini's tended to be equipped with smallish engines for the size of the boat. By modern standards, the boats ARE a bit underpowered, even without the loads of big alternators and other equipment Two horsepower per thousand pounds of displacement would be considered about the lowest acceptable power, nowadays. And that is a good bit more than you have. When does it matter? Well, your boat probably moves pretty well in flat water, but is slow in big waves or a heavy head wind. Of course, going fast is not too comfortable in those conditions, anyway. I don't think there are many Cherubini's with their original engines, that can truly get to their hull-speeds, under power, fully loaded. But then again, we used to call the engines in sailboats "auxiliaries", which may be to the point. Regarding your engine, most of the better Yanmar mechanics I have run into suggest that the engine should be able to reach its rated RPM's or very slightly less, when normally loaded and with the correct propeller size and pitch. Check that rating in your manual. And, if this is so, cruising speed should be between 70 and 85 percent of the rated RPM. They also suggest an hour at maximum RPM's every once in awhile, perhaps every thirty hours or so. This is particularly true of engines that are seldom used, not allowed to run long enough to truly warm up and engines that are run at anchor in neutral to charge batteries.
