Would it make sense, and perhaps be better for the engine, to increase the prop pitch, 1 to 3 inches higher than normally recommended by the engine manufacturer, if one rarely, if ever, runs near max rpms?
I own a 40-foot J/120, a lightweight cruiser/racer, with a Yanmar 3JH3E that is rated at 40 hp at 3,800 max rpm. Our boat is easily driven and we cruise at more than 7 knots at 2,400 to 2,700 rpm and never run for any extended period at more than 3,000 rpm. The Yanmar reps stick to their normal recommendation that the prop pitch should be selected so that the engine attains its maximum rpm, while also cautioning against extended light loading.
It seems to me that overall efficiency and longevity of the engine could be improved, very simply, by increasing the pitch of the prop to bring the engine power and propeller power curves closer together at lower rpms — say max rpm of about 3,200 instead of 3,800 for my engine. Our boat would then cruise at an even lower rpm, say, around 2100, decreasing the gap between the engine power and prop power, decreasing noise and maybe improving fuel efficiency. Of course, we would have not run the engine flat-out for an extended time, but we never do anyway.
Nigel Calder responds: It seems to me that the idea of increasing the pitch and/or diameter to get the propeller and engine power curves to cross closer to normal cruising rpms has merit from an efficiency perspective. However, by the time the engine gets to normal cruising speeds in the conventional installation it will be sufficiently loaded to be operating reasonably efficiently, so you will not see that big of a reduction in fuel consumption. You should see bigger percentage gains at lower speeds.
There are several caveats. The first is, if this is a new engine and problems develop, the warranty may be voided. The second is, if you overdo it, because diesel engines have very little torque at low engine speeds, the engine may not be able to swing what is, in effect, a grossly oversized propeller, and you may not get to your cruising speed, especially if the boat has a foul bottom or there is other other resistance. The third is, as you noted, if you do open the engine up, it will be overloaded.
Interestingly enough, the latest generation of 50-100-hp Volvo Penta engines are designed such that the torque curve comes up very sharply and then flattens out and starts to decline. This gets the engine to full-rated power at lower rpms than is typical, and then the power curve is just about flat all the way to top speed because the increasing engine speed is counteracted by a falling torque curve (power = rpm x torque). With such an engine in a sailboat, it is a good idea to size the propeller such that the propeller curve crosses the power curve several hundred rpm below maximum speed (more-or-less where the engine first reaches full power).