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Old 14-09-2014, 16:12   #1
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Minimum cruising RPM

Hello folks,

I am hoping you can help me answer a question about low RPM engine operation. I know the conventional wisdom is diesel engines need to be run under load i.e. high RPM, but I am selecting an engine for a new build that has a peak torque output at 1,800 RPM and would like to determine what my cruising RPM range might be.

I have the choice of either a Volvo D2-55 or D2-75 (same engine with turbocharger). The boat I am powering operates well at an RPM range of 2,000 to 2,400 with the D2-55 (Volvo recommended cruising RPM is 2,500). If I select the D2-75 I would get 35% more torque and the same power at 1,800 to 2,200 RPM with essentially the same fuel consumption, and having the extra revs and horsepower available when needed is a real advantage.

It seems that running the engine at lower RPM, but still at the top of the torque curve would reduce noise, vibration and wear, but would it cause any long term issues for the engine to run at these lower speeds?

Thanks in advance for any advice!
Doug
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Old 14-09-2014, 16:19   #2
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Re: Minimum cruising RPM

You will get many different opinions on this. My opinion, for what little it may be worth, is that modern diesels don't hate low revs as much as some people think. Operating any IC engine at its torque peak is certainly not going to be a problem. And lastly -- turbos and diesel engines are like cookies and cream.
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Old 14-09-2014, 16:58   #3
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Re: Minimum cruising RPM

The only issue with the turbo ones, is they MUST run at high enough load to have the turbo operating at temperature, otherwise the turbo will carbon up and fail.....
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Old 14-09-2014, 17:14   #4
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Re: Minimum cruising RPM

Mechanically there should be no issue running at lower RPM. In fact it could be easier on the hardware

Combustion - As long as the engine is burning clean there should be no additional issue with exhaust carbon and fouling.

Temperature could be an issue - lower temperatures may allow the turbo to carbon up and there is a risk of glazing on the cylinder walls that could cause an oil consumption problem.

Modern synthetic oils may be a solution for that.
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Old 14-09-2014, 17:48   #5
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Re: Minimum cruising RPM

You may wish to consider diesels that are specifically made to run at low RPM.
perkins and Cumins do these and also Lugger.Luggers commonly used in PNW fishing vessels.Often naturally aspirated (non turbo).My mind-these less potential probs though tradeoff is a few HP.

Synthetic oil with a turbo is good but you'll lose quite abit of the synthetics life.

In my turbo Cumins 6,I changed out to a synthetic and she hated it and told me by making alot more smoke ! Went back to Rotella T and she stopped fuming right off.I had big idea I'd do oil change with less frequency ! Best laid plans.
I do think that if my engine began its life with a synthetic the smoke never would have happened-unless I changed mid-stream.
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Old 14-09-2014, 17:53   #6
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Re: Minimum cruising RPM

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I do think that if my engine began its life with a synthetic the smoke never would have happened-unless I changed mid-stream.
You are surely right. Synthetic oil should be used in the engine from the beginning. Switching from mineral to synthetic oil can be real engine killer.
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Old 14-09-2014, 18:24   #7
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Re: Minimum cruising RPM

I left out my 2 cents on minimum cruising RPM.Generally the rule is %75 of peak.Though obviously there really is no hard and fast rule here.You learn your engine and what it prefers and go with this.Sometimes though you just gotta take it out to clean it out and get the perkiness back into it ! Lol.
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Old 14-09-2014, 18:33   #8
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Re: Minimum cruising RPM

I think if you can get an engine that pushes you at your desired cruising speed while turning at between 50% and 75% of its top continuous rating then you are fine.

b.
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Old 14-09-2014, 18:43   #9
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Re: Minimum cruising RPM

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Originally Posted by DoubleWhisky View Post
You are surely right. Synthetic oil should be used in the engine from the beginning. Switching from mineral to synthetic oil can be real engine killer.
Synthetic oil altogether will kill a boat engine. See:

Oil for yacht engines
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Old 14-09-2014, 18:49   #10
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Re: Minimum cruising RPM

Thanks very much for the input, this is very helpful!

-Doug
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Old 14-09-2014, 21:41   #11
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Re: Minimum cruising RPM

Thanks Dockhead. That was interesting and useful
information.
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Old 15-09-2014, 01:11   #12
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Re: Minimum cruising RPM

Thank's Dockhead, for telling folks that these Fancy new oil's are not really the thing to use in a lot of Marine diesels! More folks should know that marine diesels need high detergent diesel engine oil!! I've been useing Standard Oil Delo for MANY years and have had a couple of 7000 + hour engines! Changing the oil and filters are much more important then a fancy oil!! Im not saying the new oils are bad ! just that a good high detergent diesel oil is much cheaper and has worked well in many many commericial fishing boats for a lot more hours then on all our sailboats together!! LOL
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Old 15-09-2014, 02:14   #13
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Re: Minimum cruising RPM

+1 on the reply by Bob and Connie, the data on why we should never use synthetic oil in our engines is worth a ton! I had no idea, but will be sure to never make that mistake on our new engines!

I also read some of the other articles on the Cox website that Dockhead posted, and they were very interesting. Particularly the advice to have the props engaged when running the engines just for charging batteries (even at anchor) as having props engaged will provide enough load to avoid glazing.
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Old 15-09-2014, 02:38   #14
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Re: Minimum cruising RPM

I have a 210 6 cyl cummins running at 950 rpm , you don't know its running , swinging a 30 inch prop for 5ooo hrs with no problems
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Old 20-09-2014, 05:12   #15
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Re: Minimum cruising RPM

Quote:
Originally Posted by Poozer View Post
Hello folks,

I am hoping you can help me answer a question about low RPM engine operation. I know the conventional wisdom is diesel engines need to be run under load i.e. high RPM, but I am selecting an engine for a new build that has a peak torque output at 1,800 RPM and would like to determine what my cruising RPM range might be.

I have the choice of either a Volvo D2-55 or D2-75 (same engine with turbocharger). The boat I am powering operates well at an RPM range of 2,000 to 2,400 with the D2-55 (Volvo recommended cruising RPM is 2,500). If I select the D2-75 I would get 35% more torque and the same power at 1,800 to 2,200 RPM with essentially the same fuel consumption, and having the extra revs and horsepower available when needed is a real advantage.

It seems that running the engine at lower RPM, but still at the top of the torque curve would reduce noise, vibration and wear, but would it cause any long term issues for the engine to run at these lower speeds?

Thanks in advance for any advice!
Doug
I have not looked at the operational spec's on the those particular Volvo's, but if you contact them with your boat spec's and what you are trying to accomplish, they might offer up a few suggestions regarding RPM's and prop size. You don't want to put the engine into a lugging situation. The Volvo reps helped cure a few problems on a security boat that I was in charge of for maintenance. I knew that Boston Whaler company set the boat up for speed, and that was not what we needed. We did slow patrols around the sub base at 5-6 knots and the engine was always wet stacking. My chain of command would not listen to me at first, but after 3-4 turbo change outs........they came around. Give em a call.
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