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