This is the age old question! Should I run at a higher RPM for an extra knot but half the fuel efficiency? The diesel engine manufacturer says yes, my wallet says no.
Our personal preference under engine only is about 1800 RPM (Yanmar 4jh4). This is where we burn about a gallon of fuel per hour and can get about 6 knots pretty easily. When motor sailing – we will run at 14-1500 RPM. The fuel burn is about ¾ gal per hour.
If you decide to run the engine at lower RPM to save fuel (as we do), then it is very important to run the engine hard for about 5 minutes every hour. For us this is 26-2800 RPM. This will eliminate carbon build up in the exhaust system and prolong the life of the engine. Diesel engines prefer to be run hard about 80% of max RPM. Running a lower RPMs without load will shorten the life of the engine unless it is pushed up at least once per hour for about 5 minutes. This is outlinede in the Yanmar operation manual. I would assume the same is true for Volvo.
Hope this helps
With best regards,
Sailing Vessel - Cream Puff - SM2K - #275
Currently cruising - Curacao
I've dug through the forum history on the topic of the "correct" RPM range for the Super Maramu, and I haven't quite found what I want. What I have learned is there probably is no single, correct answer -- it depends on the engine and the prop. But what I really want to know is not what RPM I should be seeing at wide open throttle, but what RPM I should be cruising at? By this I mean the RPM you would use if you needed to motor for a long period of time. If the max RPM is different for everyone, it stands to reason that cruising RPM should also be different for everyone. Is there some way of calculating this RPM from the max? I've heard a rule of thumb of 80% of max RPM, but given that some of our engines are "over-propped" by design, I don't know if it applies, or even if it's a good rule in the first place. Should we rev to whatever is necessary to go a certain speed? Is there any harm in operating the engine close to/at wide open throttle for extended periods of time?
For the record, I have a Volvo TMD22 (B?).. I typically cruise at around 2400rpm, which yields a boat speed of about 6.5kts. I have a fixed three-blade prop (probably the original spare). I think the boat went faster at this RPM last year when I first bought her, but that could be me misremembering the RPM, or the tachometer going out of calibration, or the fact that I badly need a bottom job (though I had it scraped only a couple weeks ago). I do not know my max RPM; my engine needs a new timing belt and I don't think it's prudent to stress it until that is replaced. The fastest I've gone is about 2600 and I'm sure it can go higher. I saw around 7.2kts STW at that speed.
Ryan and Kelly
SM 233 Iteration
Boston, MA, USA