# Cruising rpm's



## Gulfislander (Aug 10, 2006)

I have a 3 year old Yanmar 2GM20FC. I understand that the maximum rpm's are ~3500. How do you determine what is the best cruising rpm's.


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## celenoglu (Dec 13, 2008)

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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## Gulfislander (Aug 10, 2006)

My boat is a Niagara 31. I'm not sure of the hull speed but I think it's approx 5.5 knots. She seems to motor along nicely at 2800 rpms at 5.5 knots. But a friend told me I should only be cruising at 2000 rpm's.


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## St Anna (Mar 15, 2003)

Look at the performance curves. One will be torque and RPM - Another will be Fuel consumption, although it may be in grams of diesel & you will have to convert it. 

A max torque will end generating max HP to the Prop and you can see what is best cruising RPM for your yacht. Always will be a compromise. Revs or speed


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## dacap06 (Feb 2, 2008)

*around .75 * max RPM ...*

Over the years sailing OPBs, most folks ended up cruising at around 75% of max RPM or whatever got them to hull speed, whichever was lower.

My 30' Islander Bahama goes about 6.5 knots at 2700 RPM, which is 75% of my 2GMF's 3600 max RPM. My hull speed is 6.64 knots, so that's about right.

Those who do not know their hull speed should check out this Sailcalc page. It shows you all kinds of interesting statistics about your brand, size, and model boat (assuming it is on the list). I have no idea how accurate it is overall, but it was pretty close when I spot-checked a couple of its statistics for my boat using the original formulae.

Tom


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## sailak (Apr 15, 2007)

celenoglu said:


> The cruising RPM for your Yanmar is 2000 -2800.


What is that RPM range based on?

Thanks


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## bubb2 (Nov 9, 2002)

By Ear! The boat will tell you where she is the most "comfortable." I have the same engine in a Beneteau. 2100 rpm's gives me 5.4 knots and I burn 1/4 gallon a hour. 2800 rpm's I am moving along at 6.4 + knots, but I am burning 1/2 gallon a hour.

Depending on wind and waves she will settle in somewhere between those two, but you can "feel" where she (the boat) wants to be.


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## rikhall (Feb 7, 2008)

bubb2 said:


> By Ear! The boat will tell you where she is the most "comfortable."


Bubb2 is totally right! An old Yanmar rep from Down Under told me - "Run her where she sings to you"

Our old Tanzer 28 - 2GM20F - 2400 RPM
Our "new-to-us" Irwin Citation 34 - 2QM20H - 2500 RPM

And yes, she does sing when it is good - she "shakes, rattles and rolls" when it is not good.

Cheers

Rik


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## mitiempo (Sep 19, 2008)

I agree you should run where it seems comfortable. If you are not sure of your hull speed here's the formula: 1.34 x square root of waterline length. Example: 25' waterline, square root is 5 (5x5=25). 5 x 1.34 = hull speed which is approxmately 6.7.
Brian


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## captbillc (Jul 31, 2008)

my nimble 30 express has a yanmar 2GM 20F . LWL 25.8 beam 9.4 displacement 6,500 . 15"X11" pitch - perfect pitch prop. i cruise at 2200 RPM & 5.5kn at 3600 rpm (top) 6.9 kn.


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## MJBrown (Apr 1, 2009)

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.


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## Maine Sail (Jan 6, 2003)

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..



celenoglu said:


> 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..
​


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## sailak (Apr 15, 2007)

Good info, thanks Maine Sail


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## CaptainForce (Jan 1, 2006)

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


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## Liquorice (Nov 28, 2007)

*2gm20f*

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


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## roline (Apr 7, 2000)

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.


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## captbillc (Jul 31, 2008)

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.


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## klem (Oct 16, 2009)

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.


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## Liquorice (Nov 28, 2007)

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


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## Liquorice (Nov 28, 2007)

*Bore Glazing*

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


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## lancelot9898 (Dec 30, 2008)

I don't have the Yanmar but rather a Perkins 4-108 which is rated at 4000rpm intermitent and 3600 continuous. I also have a max prop which allows me to adjust the pitch to achieve various WOT readings. I finally settled on adjusting the pitch to 14 inches( 17 inch dia) and achieved 3000 WOT on my tach. However this year I discovered that the tach was out of calibration and when normally cruising at 2500 rpm, I was actually doing 1900. So with much thought I considered going back and adjusting that pitch smaller to achieve closer to the continuous rating, but now I'm not sure if that is necessary. One of the main factors on engine life is how much fuel flows through it and as long as operating temperatures are achieved then operating at lower rpms should be OK. I've read about over proping an engine resulting in lugging and while I can tell that I'm "lugging" my engine in my jeep when I don't down shift going at a lower speed, I have yet to tell that i'm doing the same with the boat. The only disadvantage that I see operaing at a lower rpm is that the horsepower available is less. My WOT reading is only 2300 rpm which is quite a reduction from the 3600 rating.


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## klem (Oct 16, 2009)

lancelot,

While I was one of the people arguing for lower rpm, the argument assumed proper gearing/pitch. Glazing will not be an issue for you but there will be other issues like lugging. I would recommend taking out some of the pitch when you get a chance so that your engine can turn over faster for a few reasons. As you mentioned, lugging is bad. Lugging is trying to get too much power out of each combustion cycle by simply forcing more fuel in. What this means is that rather than having the work done by the engine spread out over more combustion events, it is concentrated greatly increasing stress.

You most likely are not realizing the full power potential of the engine because of your gearing. This will be made even worse if you are trying to push into bad weather and you cannot get up into the engine's powerband.

Lugging will not improve fuel economy as many believe. You are trying to force more fuel into the engine than should be there and it runs rich. Diesel engines are fuel throttled not air throttled so improper gearing will result in excess fuel for each combustion event.

Does this make sense as to the difference between gearing and what you choose as a cruising rpm?


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## lancelot9898 (Dec 30, 2008)

klem said:


> As you mentioned, lugging is bad. Lugging is trying to get too much power out of each combustion cycle by simply forcing more fuel in. What this means is that rather than having the work done by the engine spread out over more combustion events, it is concentrated greatly increasing stress.


I'm not sure about the stress issue considering that I'm backing off about 20% from the WOT setting. Seems like if the WOT was 3600 rpm and I backed off 20% from that number, stresses induced on the engine should be similar? No calculations to back that up. Anyway that point about the jeep is that I can hear the engine "lugging" whereas with the Perkins it sounds "happy" at the lower rpm even pushing into a head wind. I'll probably reduce that pitch back to 12 inches approx. so to get more horsepower rating. At 2000 rpm the shaft hp is 8 whereas at 2500 it's 14. (3000 is 21)

One other thing that I noticed about rpm and WOT. When tied at dock my WOT rpm is about 150 rpm less that when out on the water. This I can understand since less force is needed to move the boat underway vs tied to the pilings. Not sure if I said that right. Anyway expect that I can achieve a higher WOT setting still if I'm going against a stiff current. (flow past the prop allows the prop to work less and I've seen around 2600 rpm on WOT) Hull spreed through the water would still be the limiting factor so there is a limit to how much rpm could be gained going against the current.


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## Maine Sail (Jan 6, 2003)

lancelot9898 said:


> My WOT reading is only 2300 rpm which is quite a reduction from the 3600 rating.


Most all engine manufacturers suggest the engine be able to hit max rated throttle with a clean prop and bottom. If you can not hit max RPM you are over propped. Being over propped/pitched is not good for the engine and is one more factor that can lead to bore glazing. You should adjust your prop so you can hit the max rated RPM.

You're not "backing off" 20% from max RPM you are forcing and over loading your engine and thus preventing your engine from hitting max RPM by over propping it. There is a big difference from being able to turn max RPM and then backing off by 20% for cruise RPM vs. not allowing your engine to hit max RPM by forcing too much pitch on it for a 20% reduction in available RPM.


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## lancelot9898 (Dec 30, 2008)

Maine Sail said:


> Most all engine manufacturers suggest the engine be able to hit max rated throttle with a clean prop and bottom.
> 
> You're not "backing off" 20% from max RPM you are forcing and over loading your engine and thus preventing your engine from hitting max RPM by over propping it.


From one of my Perkins manuals it states that the max rating of 4000 rpm should not be run for longer than 45 minutes of every hour with the rpm reduced by 20% during the 15 minutes before returning to max rating. However it goes on to state that if the wide open throttle setting is less than the max rating then it too should be backed off by 20% every 15 minutes on the hour. In other words the WOT setting should be treated as the max rating.

I've always heard the term "over proped" and understand that to get the hp rating of the engine that certain rpms should be obtained. However I do not understand if I'm operating the engine at a lesser rpm that I'm over loading it even when I'm not able to achieve the desired rpm rating. Is there some physical sign(temp rise, smoke from exhaust, strange sounds) to show that the engine is being overloaded??


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## Maine Sail (Jan 6, 2003)

lancelot9898 said:


> From one of my Perkins manuals it states that the max rating of 4000 rpm should not be run for longer than 45 minutes of every hour with the rpm reduced by 20% during the 15 minutes before returning to max rating. However it goes on to state that if the wide open throttle setting is less than the max rating then it too should be backed off by 20% every 15 minutes on the hour. In other words the WOT setting should be treated as the max rating.
> 
> I've always heard the term "over proped" and understand that to get the hp rating of the engine that certain rpms should be obtained. However I do not understand if I'm operating the engine at a lesser rpm that I'm over loading it even when I'm not able to achieve the desired rpm rating. Is there some physical sign(temp rise, smoke from exhaust, strange sounds) to show that the engine is being overloaded??


If you can not achieve the max rated RPM of the engine the engine is over propped. I don't know how to be any clearer.

I will use my own Westerbeke a an example as Westerbeke is very clear in their language about prop sizing. I was 350 RPM off this spring with a new prop and was told in no-uncertain terms by both the distributor, Hansen Marine, and Westerbeke them selves in Taunton, MA that I should most definitely have my prop re-pitched with no if's and's or but's about it.

Here's the basic gist. The MAX rated RPM on my four cylinder 44B Four is 3000. Here is exactly what Westerbeke says, in writing.

*"Westerbeke recommends a propellor that will allow the engine to turn 3000 RPM under way at full throttle."*

Pretty darn clear!!!

My MAX RPM is 3000 and my engine should be able to hit 3000 at WOT. This does not mean that this is where you run the engine but if it can turn max RPM then everything else falls in line behind it and you won't be loading the engine. I cruise at 75-80% of WOT and the 350 RPM difference made a large difference in the tone of the engine. Because I was 350 RPM off, Westerbeke recommended that I also temporarily back my normal cruise RPM off by 350 RPM to "try" and compensate for the over propping and to NOT overload my engine.

By backing my RPM off by 350 it put me at an RPM outside of the recommended cruise RPM range for the engine and one too low for long term use. This was according to the Westerbeke engineers in Taunton that I spoke with. Fortunately the prop maker had me a new prop in about two weeks time and I only had to put about three or four hours on the engine this way..

Many good surveyors also check to make sure an engine can hit MAX rated RPM during sea trials.


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## Northeaster (Jan 13, 2007)

When I bought my 30'er, which has an older (1982) Yanmar 2GM, 15hp max (3600rpm), 13hp continous (3200rpm), it was overpropped., with a 15x10 fixed 3 blade.
The engine "seemed" happy up to 2000rpm - in fact would push the boat to within a knot or so of hull speed, at these low rpms. But, if I tried to give it more throttle, I would get black smoke, and the engine wouldn't sound happy.

After much reading, and talking to a knowledgable prop guy, I had 1" of diameter and 1" of pitch taken off this winter, to make it a 14"x9". The pitch likely made more of a difference, as there was not alot of surface area in the tips. Regardless, I was told that I might see an increase of 200-300rpms, which would still leave me well short of 3600.

In reality, what happended is I can now rev 3200rpms, (a full 1000 rpms better than before), with no smoke, and hit full hull speed (6.4-6.8 kts) consistantly. I suspect that this is because I only had about 8 hp, to play with at the 2000roms range, whereas now, the prop is able to spin faster, and use more available engine hp as well (approx 13hp). Now, the engine sounds much happier (except for a squeal, that is another issue).

I am still a bit overpropped a bit, as I can hit about 200 higher in neutral, than in gear, with a clean hull / prop. I am not sure if I will have it changed again, as I am very close, and 90% better than it was. If I do, I will send it to the same prop guy, as he had 25 years experience working on props of all sizes (mostly commercial) and you could tell that he knew his stuff.


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## lancelot9898 (Dec 30, 2008)

Maine Sail said:


> . Because I was 350 RPM off, Westerbeke recommended that I also temporarily back my normal cruise RPM off by 350 RPM to "try" and compensate for the over propping and to NOT overload my engine.
> 
> By backing my RPM off by 350 it put me at an RPM outside of the recommended cruise RPM range for the engine and one too low for long term use. .


Appreciate your comments Maine. It seems to me that I am not overloading the engine by the statements above, however apparently my rpm is "outside of the recommended crusie rpm range." (1900 rpm does get me up on the torque curve so not sure if I'm really outside of "cruise range") FWIW...I bought the boat new in 1985 and have been operating the engine in this "out of spec" range ever since. I did not know until I put a photo tach on the engine that my tach readings were so much off however. Good discussion and look forward to any more comments.


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