Cruising rpm's

[MJBrown]

Based on Yanmar's recommendations I always run apx 80% of max RPMs but will tweak it up or down a tad to find the sweet spot where there's the least vibration, noise ,etc. If you choose to run below the 80% range then make sure you run it wide open periodically to keep the carbon from building up on the injectors and cylinder walls. As for the difference in fuel consumption it's not even a consideration. Now if I burned 50 gals an hour I might pay attention, but not at 1.75 to 2 GPH.

[Maine Sail]

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..

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.

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..

[sailak]

Good info, thanks Maine Sail

[CaptainForce]

I have 5,760 hours on my sweet running 4JH3E Yanmar that moves me at 6.5 kts with a 37' waterline at 2500rpm with an 18x13 three blade prop using 0.9 gallons/hour. 'take care and joy, Aythya crew

[Liquorice]

My Yanmar Service Manual says my 2GM20F has a one hour rating of 18hp at 3600 rpm and a continuous rating of 16 hp at 3400 rpm.
My aim is to get it as close to the 3400 while still sounding "sweet".
The harder it works the better!
It uses so little diesel ($5 a season) that fuel economy doesn't come into it; it's engine longevity that I'm striving for!
sam :-)

[roline]

Take data.. hull speed vs RPM. This will show you where the engine likes to run for best performance with your boat and prop setup. I run at 2800, 2900 where the engine is the smoothest. Also with a 14Dx12P prop it is at 60 % efficiency/ 40% slip which is quite good for the narrow 2 blade Martec.

[captbillc]

Liquorice ----- at 3400 RPM you are operating the engine at the top continuous rating. i do not believe you will get the max amount of hours before overhaul that you would get running at a lower RPM because you are working the engine hard. i operate my 2GM20F most of the time at 2200 RPM and 5.5 kn. this is easy on the engine. i am a retired diesel tech and i want my engine to last a long time.

[klem]

I think that captbillc is getting at a common misconception that you should run a diesel as hard as possible. They do run well when run hard but you don't have to run them at WOT all the time. For this argument, I am going to make the assumption that you have proper gearing and prop pitch so lugging isn't a huge issue.

The real danger to running the engine at low rpm is glazing of your cylinder walls. This is caused by the engine temp being too low when running. If you are worried about glazing, make sure your temp gauge is reading in the normal range and try throttling up for a second and looking at the smoke color. If you see a big puff of blue smoke, your engine temp is too low and you need to throttle up but if you see black smoke, that is not an issue it is just unburt fuel because you are not moving enough air yet. Almost all engines now have a thermostat so they are more forgiving in terms of regulating their own temp.

The danger with running a diesel too fast is related to fatigue and heat. If you run an engine really hard, your exhaust gas temps will go up a lot and maybe even your block temps. Your engine can only take so much thermal cycling before it starts to have problems. Also, the increased rpm means that there are more fatigue cycles on the engine components. When you calculate fatigue, the variables are stress and the number of cycles and you are greatly increasing the number of cycles by running high rpms.

Most diesel engines will get their best efficiency at an rpm just over the torque peak. Some manufacturers will actually post this information, I know that cummins does on some of their engines. This does not take into account your specific boat which may have its own optimum speed. The last consideration is your time value and your wallet, you own an engine for a purpose and you might as well use it to suit your needs.

In truth, to properly answer this debate, you need to look at how everything is setup including hull form, expected conditions, gearing, and prop. For this discussion, I believe that we are assuming that these things are fixed but if you are repowering, you need to start with these variables.

[Liquorice]

I'm an engineer with 35 years in heavy equipment maintenance management.
It's hard to get these engines too hot with these cooling systems. I don't believe 3400 revs is a problem. Actually my "sweet spot is around 3100 revs.
However, I run my engine only from and too the mooring so it rarely gets a chance to really warm up. 2 or 3 times a season I give it a good run for a couple of hours so it gets a good workout. I also worry that my diesel will go off if I don't use it - A tank full usually lasts about 3 years!
sam :-)

[Liquorice]

It's me again!
On the subject of low load and bore glazing - It's not as much of a problem as you think - within reason!
I was involved with very large diesel earthmoving equipment in Northern Canada. We're talking huge Caterpillar and GM diesel engines - 1600 hp to 2400 hp. Because of the cold winters - minus 30 deg C to Minus 40 deg C (...same as -40 deg F) we couldn't shut them off from November to April.
We had a setting we called high idle (can't remember the exact revs - been retired for quite a while), but say half way between idle and full revs. We regularly would leave these units revving away in the field for many hours or days on high idle (...and very little load) with no adverse affect. Engine life was expected to be 20,000 hrs and we regularly achieved it. It was cold running that killed the engines, if we could keep them hot and not wash the cylinder walls with unburnt fuel, they would last forever!
Sorry about that - got carried away - I feel better now!
sam :-)