The excerpt below is quote from Yanmarhelp.com:
"For prolonged running, a pleasure boat engine a good operating speed is around 85% of maximum continuous rated RPM (see the engine data plate)
assuming the propeller size is correct and the engine can reach maximum rpm).
See this list for approximate rpm:
GM - 4JH - 3/4JH2 series | 2900rpm |
YM series Depending on WOT rpm | 2600-2900rpm |
4LH and 6LY series | 2650rpm |
| 6LP and 3/4JH3 series | 3000rpm |
3/4JH4 series | 2400rpm |
| 6CX series | 2200rpm |
Operating for extended periods below 80% rpm may increase longevity but the cylinder bores could glaze causing an increase in oil consumption and blue smoke especially when new.
If you're operating these engines COMMERCIALLY, different rules apply. For engines without a tacho, run it where it sings best. If the ship is shaking and growling, alter the RPM. If the engine VIBRATES unpleasantly at a particular rpm, don't operate it in that rpm band or it will eventually break! Yanmar TACHOMETERS usually read about 100rpm too high at maximum rpm so don't be pedantic about lining up the needle exactly. The latest Yanmar panels combined with combined tacho & LCD digital hour meter as fitted to some GM series and all YM and JH4 engines that have a VDO electronic taho that operates from the flywheel sensor on GM series and from aletrnator pulse on other models.
You may note that the MAXIMUM RPM of your engine under load is higher than the figure I have given above
USE THE MAXIMUM RPM IF YOU ARE IN TROUBLE, or to get the boat on the plane, don't be frightened, the engine will handle this for an hour provided it has been installed correctly. Once out of trouble or on the plane, reduce rpm to 85% for better fuel consumption and longer engine life."
You can actually run an engines at to low an RPM and glaze cylinder walls by not putting enough load on it. Diesels prefer to be run hard. Most of the manufacturers out there recommend 75-85% of WOT with a properly sized prop. If you also idle for charging on top of low cruise RPM you will have a good recipe for glazing.
For example:
My Westerbeke manual states that max RPM is 3000 and the max cruise RPM is 2500. 2500 is 85% of max RPM just as Yanmar's is.
Westerbeke also states a
MINIMUM cruise RPM of 2000 which is 68% of max RPM. So the range for my Westerbeke is 68% to 85% of max rated..
Quote:
Originally Posted by celenoglu
The cruising RPM for your Yanmar is 2000 -2800. This depends mainly on your needs and the boat's hull speed. Normally it is best to attain speeds near to hull speed without too much load on the engine. My cruising rpm's are 2200 -2500 for my Yanmar which means 5,5 -6 knots.
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The 2GM has a max rated RPM of 3600. A 2000 RPM cruise on that engine is a mere 55% of the maximum rated 3600. Even a 2500 RPM cruise is only 69% of max rated. Loading a diesel, but not over loading it, is really what you want to do. Running it with to light a load, like 55% of max, could lead to premature glazing, increased oil consumption and smoking..