It is pretty well understood, at least around here I believe, that operating a diesel at too low a power level will lead to increased maintenance and shorter life. Exhaust elbows coke up and low oil temperatures don't boil off water. Piston rings and valves are sealed by the combustion pressure and, if it is too low, gases will blow by causing premature erosion and wear.
What is probably not as well understood is how hard it is to operate a sailboat diesel at the desirable 80% power level that will minimize these problems. Marketing pressure to install oversize engines combined with cost savings in transmissions and props makes it almost impossible in many vessels. The basic problem though is in the nature of propellers.
Here is the power curve for my engine. The numbers will be different for others but the basic relationships will be the same.
The dotted line shows the maximum power the engine can produce for short periods. It's called a 1-hr rating but you could actually run it all day at those power outputs. You just wouldn't get the promised life. The solid line labeled "Continuous rating output" shows the power level you can run the engine at 24/7 with reasonable expectation of getting the full time between overhauls. The engine governor limits this engine to turning no faster than 2800 RPM regardless of how low the power draw is.
The Propeller power curve is for the prop that Yanmar expected to be put on the engine. This prop would draw 20 H.P. at 2600 RPM with the boat free running in smooth water. Open the throttle further and the RPM won't increase because the engine doesn't have enough power to turn the prop faster. The engine will just labor and pour out black smoke. A more carefully set up and sophisticated engine would have its governor set to limit RPM to 2600 for this prop.
80% of 20 H.P. is 16. If this were a controllable pitch prop that could be adjusted to draw full power at any RPM, the engine could be run as slow as 2100 without creating light load running problems. However, the nature of fixed pitch props is for the power draw to drop off much quicker as shown by the curve. The slowest the engine should be run for cruising and long periods with the prop envisioned by Yanmar is 2400. This is only 200 RPM less than maximum continuous. It's instinctive to think of 80% power being 80% RPM, or 1900, but that is actually only 40% power, way too low.
The problems become compounded in the typical sailboat installation. My engine turns right up to the governor limit and I see just a hint of darkening in the exhaust smoke at WOT so my prop curve is closer to the red line. It could even be lower. The target power level of 16 HP is therefore at 2500 RPM on my boat, only 100 less than the 2600 max you might infer from the graph.
My boat, like many sailboats, has plenty of power. It's nice when handling under power in line squalls or bucking head seas but it means she is positively jumping at 2500 RPM. From the standpoint of fuel consumption, comfort, and range, she runs very well at 2100. You can see from the red prop curve that this is only 9 HP. The engine feels great at that speed but it isn't a happy camper. I run actually run a lot at 2100 but I try to get up to 2600 for 10-15 minutes every time the engine is run and run harder whenever fuel consumption and comfort permit. From the standpoint of engine longevity and maintenance, a 12 HP engine would be optimum for my boat. I could get optimum fuel economy while running the engine at its best speed. I wouldn't have the reserves for severe conditions but those account for only a tiny percentage of my running time.
Many boats of my model were built with the 3 cylinder, 30 HP version of the engine as an option because people thought they were getting more safety in rough conditions. The props were the same size however. Even with a lot more pitch, there is no way the 3 cylinder boats are ever getting their engines up into the optimum power band. There are a lot of sailboats out there with similar power-displacement-prop relationships. It's no wonder sailboat diesels often have short lives.
Run your diesel as hard as you can, it will thank you for it.
What is probably not as well understood is how hard it is to operate a sailboat diesel at the desirable 80% power level that will minimize these problems. Marketing pressure to install oversize engines combined with cost savings in transmissions and props makes it almost impossible in many vessels. The basic problem though is in the nature of propellers.
Here is the power curve for my engine. The numbers will be different for others but the basic relationships will be the same.
The dotted line shows the maximum power the engine can produce for short periods. It's called a 1-hr rating but you could actually run it all day at those power outputs. You just wouldn't get the promised life. The solid line labeled "Continuous rating output" shows the power level you can run the engine at 24/7 with reasonable expectation of getting the full time between overhauls. The engine governor limits this engine to turning no faster than 2800 RPM regardless of how low the power draw is.
The Propeller power curve is for the prop that Yanmar expected to be put on the engine. This prop would draw 20 H.P. at 2600 RPM with the boat free running in smooth water. Open the throttle further and the RPM won't increase because the engine doesn't have enough power to turn the prop faster. The engine will just labor and pour out black smoke. A more carefully set up and sophisticated engine would have its governor set to limit RPM to 2600 for this prop.
80% of 20 H.P. is 16. If this were a controllable pitch prop that could be adjusted to draw full power at any RPM, the engine could be run as slow as 2100 without creating light load running problems. However, the nature of fixed pitch props is for the power draw to drop off much quicker as shown by the curve. The slowest the engine should be run for cruising and long periods with the prop envisioned by Yanmar is 2400. This is only 200 RPM less than maximum continuous. It's instinctive to think of 80% power being 80% RPM, or 1900, but that is actually only 40% power, way too low.
The problems become compounded in the typical sailboat installation. My engine turns right up to the governor limit and I see just a hint of darkening in the exhaust smoke at WOT so my prop curve is closer to the red line. It could even be lower. The target power level of 16 HP is therefore at 2500 RPM on my boat, only 100 less than the 2600 max you might infer from the graph.
My boat, like many sailboats, has plenty of power. It's nice when handling under power in line squalls or bucking head seas but it means she is positively jumping at 2500 RPM. From the standpoint of fuel consumption, comfort, and range, she runs very well at 2100. You can see from the red prop curve that this is only 9 HP. The engine feels great at that speed but it isn't a happy camper. I run actually run a lot at 2100 but I try to get up to 2600 for 10-15 minutes every time the engine is run and run harder whenever fuel consumption and comfort permit. From the standpoint of engine longevity and maintenance, a 12 HP engine would be optimum for my boat. I could get optimum fuel economy while running the engine at its best speed. I wouldn't have the reserves for severe conditions but those account for only a tiny percentage of my running time.
Many boats of my model were built with the 3 cylinder, 30 HP version of the engine as an option because people thought they were getting more safety in rough conditions. The props were the same size however. Even with a lot more pitch, there is no way the 3 cylinder boats are ever getting their engines up into the optimum power band. There are a lot of sailboats out there with similar power-displacement-prop relationships. It's no wonder sailboat diesels often have short lives.
Run your diesel as hard as you can, it will thank you for it.
