Sailboats engines are many times “over-sized” as boat owners wants to have reserves when motoring in high winds and when maneuvering.
The side effect of over-powering, is that we only use 50 – 60 % of the available power to make cruising speed in normal conditions. This means we will be running the engine on light load most of the time.
Diesel engines are not made for running on light load, and will suffer from carbon build, due to low combusting temperature. Carbon deposits is a result of incomplete combusting of diesel fuel.
If you have a modern “marine engine” with computer controlled fuel injection, carbon deposits will be less of a problem, as the amount of fuel injected will be ideal compared to the real load. At least one advantage for the new technology!
All engines has a recommended max RPM. It is important that the propeller is dimensioned to reach the recommended RPM. The proper size of the propeller is a factor of engine HP, reduction ratio on gearbox, water line, propeller diameter, number of blades and pitch. There are softwares that can calculate the size of propeller and get you close, but as always personal experience is important to get it right.
If you are considering a new propeller, make sure the supplier accept to change it, in case the calculations and experience did not take you close to the expected RPM. (Check the technical specification what the recommended RPM is.)
It is important that you know what the maximum RPM is with WOT (wide open throttle). If you do not know, go out and check with a clean bottom, clean propeller in smooth water. Write up the numbers in your boat manual, and do this test on a regularly basis. It is a good indicator how your engine is doing, growth on bottom or propeller.
If you would have any engine problems in the future this is one of the test you can do yourself, before contacting the workshop. Your mechanical shop will ask for the number.
As long distance cruising means a lot of motoring, we tend to use a much lower RPM than what engine manufacturer recommends. On Bella Luna we normally cruise at 2000-2200 RPM (Maximum RPM is 3200), in order to get a better fuel economy and better comfort in general. If we run the engine on 2000 RPM for extended periods we get carbon building up in the combusting chamber, on valve seats, injectors, turbo and exhaust system.
To burn the carbon deposits it is important to exercise your engine regularly. If not your engine will get carbon deposits that will damage the engine. I got following advice from the technical manager at Yanmar, Sweden.
Run the engine at 2900-3000 RPM for minimum 20 minutes after every 4-5 hour on light load. (Maximum RPM for our engine is 3200) He explained it takes about 10-15 minutes for the combusting temperature to reach 560 °C. Carbon deposits will not burn at lower temperatures.
It is important to understand that you are trying to burn deposits, and not trying to blow the sot out by running on high RPM. The sot that you see coming out when rewing up, is just the tip of the iceberg.
This high RPM running is sometimes called “Italian tuning” and perfumed also by respected diesel mechanics. If you already have carbon deposits, I recommend you try this before visiting a repair shop. It might take several runs before you see an improvement. There is a big chance, that the engine performance will improve and you do not have to visit the workshop.
Turbo engines suffer more, as the turbo is driven by the exhaust and is a part of the exhaust system.
Non-turbo engines are less sensitive but not immune.
I think it is unrealistic to think that cruisers will change behavior, and start to use the engine on the high load as recommended by the engine manufacturer. Fuel consumption and engine noise will not be acceptable.
An acceptable compromise is to run on light load, and give the engine a workout every 4-5 hour.
Just to give you an idea:
On Bella Luna we burn about 0.6 l/Nm at light load. (2000 RPM)
If we would run at 2700 to make the engine perfectly happy, we would burn twice the amount and gain only one knot boat speed.