Almost all sailboat engines are diesels because these avoid the danger posed by gasoline and its fumes. A marinized diesel engine is basically an automotive engine or electrical generator engine fitted with a marine cooling system and perhaps other accessories, such as the alternator, that were modified for a marine environment.
Optimal
sizeModern marine diesels run at lower RPM and produce more torque than their earlier counterparts. An auxiliary engine made recently should therefore be of a size that offers about 3 HP for every ton of displacement. More is not necessarily better, as fuel economy will suffer. Less is bad because the boat might not move to windward in a gale.
A marine diesel that is started only occasionally and run briefly tends to carbon up and wear out early.
Life
expectancyDiesel engines like to run long, hard, and often (but not too often). A expectancy marine diesel that is started only once daily and run at 90% of full throttle all day might last for many thousands of hours. Running the engine less often, or at low greatly reduce this life expectancy.
The main enemy of marine diesels is not wear, but rather, corrosion, particularly in the cooling system where it leads to overheating. Running the engine long and hard will counteract the effects of corrosion. A 10-year old engine with 5,000 hours on it is therefore often in better condition than the same 10-year old engine with only 500 hours.
Exhaust
smokeExhaust can diagnose some diesel engine problems. Black smoke contains smoke carbon soot from partial combustion caused by lack of air (check filter, air intake or exhaust restriction, engine room ventilation), too heavy a load (fouled prop or bottom, engine slipped out of alignment), or too much fuel from a bad injector. Blue-tinged smoke signals that oil is being burned as a consequence of worn parts in the engine or overfilling with oil. White smoke that rises and quickly dissipates is harmless water vapor (ie, steam). White smoke that hangs low near the water and fails to dissipate is unburned fuel (bad injector or else worn value seals or guides, rings, or cylinders). White smoke is normal, for some engines, until they warm up and also on cold, humid days when you can see your own breath. An engine that has idled for a long time will be smoky under load until the cold stacking fuel deposits burn off. Water in the fuel makes steam or white smoke. Low quality fuel burns inefficiently and makes black smoke.
Running
at IdleMost diesel engines prefer to run at or near their rated RPM except when cold. After starting, the engine should run at fast idle
at Idle of 1,000 to 1,500 RPM for a few minutes until the coolant temperature rises into the operating range. Some engines also benefit from a few minutes of fast idle before being shut down to prevent vapor lock. Between these times, it is best to avoid running at idle, when possible, particularly if the engine is not under load. Cold stacking Running a diesel engine at low RPM without load causes a phenomenon called cold stacking where the vaporized fuel condenses on the cylinder walls and piston faces. The condensed fuel causes soot, which can damage the valves and pushrods, and it also washes fuel down the cylinder walls into the crankcase, diluting the oil. Overload How heavily an engine is loaded at its rated RPM depends on the size and pitch of the prop. Unlike gas engines, where a throttle controls speed by changing the amount of air supplied, in diesel engines the fuel pump controls speed – by changing the amount of fuel – while the air supply stays relatively constant (except in a turbo-diesel). At low speed, when there is too much air, the excess goes out as harmless exhaust. At higher speed, adding more fuel can use up all the air and the excess fuel then goes out as black smoke. The speed control keeps on adding more fuel until the governor cuts in at max RPM, which might never occur if the overloaded engine runs out of air. An overloaded engine will wear out sooner and be quick to overheat.