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Old 09-09-2016, 09:29   #31
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Re: Why are high RPMs bad for diesels?

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Originally Posted by HighTemp View Post
I have a Nissan GT-R that, at one point, I apparently thought wasn't fast enough, so I spent entirely too much money making it go faster, and I still sit in traffic on the freeway...... And no, I don't run around at 8000 RPM very often.

Are the governors on marine diesels specifically set to limit the rev to say 80% of the theoretical maximum? I am trying to equate it to my car in that the engine is built to turn as high as 8600, but we electronically limit fuel and ignition timing to 8200 for "safety" purposes.

Diesels governors are set by the manufacturer for whatever number they desire.
Let me explain that a little, I had until I moved the biggest lawnmower John Deere made, it had a three cyl Yanmar Diesel very similar to the common sailboat motor, a Lawnmower though you run wide open to get the highest blade speeds you can and it cuts grass faster and cleaner, the governor was set to allow full, continuous RPM as you were supposed to operate the machine that way.

But there is a little more to it than than, as Dockhead says you can tell when a motor is happy, really any gearhead will know from the sound, I know that sounds like BS, but it is true, non gearheads will look at me cause they have no idea and don't believe me.
But there is of course a drive train too, and it will have harmonic resonance frequencies that you should avoid, or you are a very lucky person, but every drive shaft boat I have ever owned had certain RPM's that it vibrated a little, above or below that spot and everything was smooth, so pick a spot where everything is smooth.
I don't know about saildrives, never had one
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Old 09-09-2016, 09:49   #32
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Re: Why are high RPMs bad for diesels?

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Originally Posted by Cheechako View Post
However, it's obvious that some engine manufacturers believe higher rpms = shorter life. Many commercial diesels are rated at different rpm for different use. A big one might be rated at 3000 rpm for recreational intermittent use. but the same engine might be rated at 2400 rpm for continuous use.

This probably is a better answer
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Old 09-09-2016, 10:37   #33
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Re: Why are high RPMs bad for diesels?

Because this can happen:



Or this:

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Old 09-09-2016, 10:58   #34
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Re: Why are high RPMs bad for diesels?

...a final thought for the day..the original of the species mentioned hire boats being run at high(bad) rpms..well here in the sunny south of France the 2000+ hire boats on the waterways are fitted with speed governers.. that spelling looks weird.... I believe it might be to save the engines wear & also to stop the german boaters from racing everybody else, night all, Norman
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Old 09-09-2016, 11:09   #35
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Re: Why are high RPMs bad for diesels?

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Any engine if constantly ran at excessive rpms is not going to last as long as if ran at recommended rev range. Charter boat people i guess hop in and think its not their engine so why not push it to the limit for the 5 days we have the opportunity to.

But, this is why most charter companies tell their guests not to run the engines above, typically, around 2400rpms.....when maximum may be 3500, and in fact (as with our Yanmar 3GM30's) manufacturer's recommendations is for optimum running speed of approximately 80% of max, or maybe 2800. More wear / damage may be done to charter boat engines by running them too low, especially boats without generators where main engines may run with no/low loads for endless hours at 12-1800 rpms to charge batteries!

Diesels, like human bodies, need exercise!!
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Old 09-09-2016, 11:36   #36
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Re: Why are high RPMs bad for diesels?

My boat has a Yanmar 4JH. The sweet spot where prop. pitch, fuel consumption, and speed seem to be the best is 1,500 to 1,800 RPM.
Engine temperature is a factor. Diesel seems to be best at 180-190 degrees. I hear many engines turned on and raced before reaching proper temp. I am also in the camp of idling the engine for a while before turning off.
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Old 09-09-2016, 11:47   #37
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Re: Why are high RPMs bad for diesels?

It's the weight of all those heavy duty parts flailing around inside. You could jack up the quality of all those parts but you would still be limited by the burn speed of diesel fuel, about 5000 rpm.
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Old 09-09-2016, 12:50   #38
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Re: Why are high RPMs bad for diesels?

Maximum RPM vary widely depending on the model of the diesel engine.

Various engine models have ratings as to the maximum time at maximum continuous power.

Diesel engine speed is controlled by a governor controlling the RPM, not a throttle controlling air/fuel volume.

My diesel engine runs "happy" around three-quarters of maximum RPM. This represents less than a 50% load (actual fuel consumption rate versus maximum fuel consumption rate) on the engine. My engine's maximum speed is 2400 RPM. I try to limit idle/no-load running to five minutes.
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Old 09-09-2016, 17:03   #39
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Re: Why are high RPMs bad for diesels?

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I've ever heard that before. My understanding is that diesels like to be run at high load and high rpm. Keeps carbon buildup to minimum and even burns it off.
Much much worse is running the diesel at idle or low rpms with little or no load for any longer periods.

edit: poster #2 beat me in replying. my reply is to op's post #1.
cheers.
In the previous post, "Nauticalnomad" explained the diesel very well that even I cannot do this any better. The only thing that I can add is that you have high speed revolution diesels (4800 - 5000 rpm) and low speed revolution diesels (2200 - 2800 rpm max) all depends of the design of particular diesels. I run twin Isuzu and their max is 2800rpm.
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Old 09-09-2016, 18:00   #40
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Re: Why are high RPMs bad for diesels?

The advice I had received many years ago was from my master mariner father inlaw, my chief engineer brother inlaw and 1st class engineers with a combined 80 or more years is that if you idle you'll kill your engine faster. Early in my sailing career I was told that I need to feel like I was abusing my engine for it to be happiest. Diesels need to be under load and running hot.
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Old 09-09-2016, 18:41   #41
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Re: Why are high RPMs bad for diesels?

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Originally Posted by lindabarzini View Post

What exactly is the damage caused by running the diesels engines above 2500 or 2800 rpms?
Fuel consumption chart is logrithmic. ie if you go slow your engine doesnt use much diesel. Go a bit faster and you are broke.

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Old 09-09-2016, 19:26   #42
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Re: Why are high RPMs bad for diesels?

You also need to see how many RPMs gets the boat moving at hull speed in calm conditions and then why wast fuel???? You won't go faster!

In rougher conditions more RPMs may get your speed higher... but again you may reach a point of diminishing returns.

With a clean bottom in calm conditions I don't need to exceed 2000 rpm to get hull speed...

And fuel consumption is about .6 gallon / hr.
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Old 09-09-2016, 22:12   #43
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Re: Why are high RPMs bad for diesels?

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governed rpm.
Thats really all you need to know right there.
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Old 09-09-2016, 22:54   #44
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Re: Why are high RPMs bad for diesels?

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Originally Posted by nortonscove View Post
The advice I had received many years ago was from my master mariner father inlaw, my chief engineer brother inlaw and 1st class engineers with a combined 80 or more years is that if you idle you'll kill your engine faster. Early in my sailing career I was told that I need to feel like I was abusing my engine for it to be happiest. Diesels need to be under load and running hot.
There are many myths about diesels. I'm not sure what you mean by "running hot". Warmed up to its design temperature is better than cold, but excessive heat is just as bad, if not worse than excessive RPMs.

It's good to know what the design parameters are for the engine that you have and stay within those limits.

I have over one million miles on my Cummins 475 HP ISX engine in my semi truck and I keep a close watch on my water temperature, oil temperature, exhaust temperature, RPMs, oil pressure, etc. Also no unnecessary idling.
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Old 09-09-2016, 23:47   #45
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Re: Why are high RPMs bad for diesels?

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Originally Posted by nortonscove View Post
The advice I had received many years ago was from my master mariner father inlaw, my chief engineer brother inlaw and 1st class engineers with a combined 80 or more years is that if you idle you'll kill your engine faster. Early in my sailing career I was told that I need to feel like I was abusing my engine for it to be happiest. Diesels need to be under load and running hot.

Finally someone speaks the matter of the truth to the point. The title of this thread couldn't be further from the truth and is pure BS.

sub removed, cheers.
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