C12s: Any proven disadvantage of continuous low engine rpm?

[x2y3z4]

Our Alaskan 68 runs most efficient and comfortable at an engine rpm between 1050 and 1175rpm. This gives a cruising speed of 8.3 to 9kn. The engines are almost inaudible in this rpm range at the PilotHouse, dinette and galley. Everything and everyone feels relaxed. Usually we run 1085rpm at about 8.5kn. Of course, the engines reach their operating temperature.

Now, theory (and every Cat mechanic I've talked to about this) says that we should run 75% load for at least 30min every 8 hours of operation.

I have done this a few times, but it does not feel good to us. Neither the noise nor the speed fit our lifestyle, not to mention the diesel consumption.

How do you practice this?

Has anyone experienced any disadvantages ($$$) from this mode of operation?

 

[Portage_Bay]

I hear the same about CAT 3208s. Run 'em hard or you'll ruin them. I've heard the same about lots of engines. I doubt the truth. That's based on lots experience running a number of kinds of diesels at lower power. It really doesn't seem to hurt them at all.


That said I do run the 3208s hard for about 10 minutes a day. A day for us being 3 - 6 hrs, usually closer to 3.



Like you I don't like the racket and fuel burn.

 

[sunchaser]

The C12s have several HP and RPM ratings. If you’re at the lower full RPM of 1800 no problem running them for 15 minutes at around 1500. If at the 2300 RPM rating go for 1850. So yes, run them up to check for other issues such as vibration.

Also, when was last time the aftercoolers were serviced? In most Cat engines this is a key PM issue.

 

[Comodave]

Don’t know about Cats but we run our 6CTA Cummins up to about 2,000RPMs for the first 15 to 20 minutes each day to get the engines hot. Then we may slow down, or not, for the rest of the cruise. I like doing it at the beginning of the day to get the engines up to remo for the day rather than at the end of the day just before shutdown.

 

[DavidM]

Diesel engines don’t burn the fuel cleanly at low rpm’s because the injectors aren’t squirting much fuel so combustion temps are lower and the injection pattern isn’t as clean as higher rpm’s and loads.

Many of us have seen black smoke in the exhaust when the throttles are kicked open after a period of extended idling. That smoke usually clears up after a few minutes.

The smoke is due to unburned fuel being combusted under the higher temps of the heavy load. Under longer term low speed running, carbon is laid down in the combustion chamber, exhaust manifold, etc. That takes longer to burn off, maybe 20-30 minutes.

So how long and how hard to run to burn it all off depends on how low the rpm was and for how long.

1,100 rpm for eight hours sounds low and lengthy to me, so it may require 20-30 minutes of high rpm/power loading to burn all of the unburned diesel and carbon off.

The effect of 20-30 minutes of high power, at maybe half the kt/gal of the low speed fuel consumption in the context of 8 hours is pretty minimal.

I do understand the noise/serenity problem. We always timed our high speed running just before reaching the anchorage or marina to minimize that effect.

David

 

[Gdavid]

Honestly, I would put my trust in the Cat mechanic over strangers on the Internet, especially in light of the value of those engines and cost to repower. Might be worth paying for a second opinion. We all love sharing our opinions, but none of us are going to help you out if you run into problems with your powerplants, (of course the Cat tech isn't on the hook to bail you out either).

 

[Lepke]

Depends on the engine. Detroit Diesel generator engines run at 1200 rpm their entire life.
You will get some wet stacking depending on the engine and rpm. Unburned fuel will coat the exhaust and eventually burn off with higher rpm. With wet exhaust, you may show a sheen. All engines have some blowby. At low rpm, there's more moisture in the blowby so it gets in the oil. But regular oil changes solve that.What helps is a diesel treatment that increases the cetane and burns off carbon buildup.
But when I was a fisherman, I would run for days below 800 rpm. Drifting at night, not running in until the hold was full of fish or we were out of food. Sometimes for as long as 2 weeks. Jig fishing tuna, I might travel 1000 nm before raising the rpm to come in. I kept my oil clean and went longer between overhauls than others with the same engines.

 

[Simi 60]

https://www.sbmar.com/articles/low-speed-running-break-in-of-marine-diesels/

Long story short
Low speed diesels last near indefinitely compared to those run hard

We have run our nta855m at 1150 for days on end , our usual cruise RPM and see no evidence of black smoke when spooling her up to 1300 or even 1500 rpm after it

But C12 cats may be different

 

[ranger58sb]

Diesel engines don’t burn the fuel cleanly at low rpm’s because the injectors aren’t squirting much fuel so combustion temps are lower and the injection pattern isn’t as clean as higher rpm’s and loads.

I don't know, but I suspect that's not quite so true with more recent electronically-controlled common rail diesels. I think metering might be variable based on RPM, load, temps, etc.

???

-Chris

 

[DavidM]

Yes you are right, common rail injection burns cleaner at all rpm’s.

David

 

[Don L]

I have never understood why some think it is better to run hard. I don't think any piece of machinery likes to be run hard. Based on the sound etc. mine likes to be run around 60%.

But I do have to say I run mine up to around 80% every few hours and to burn off the extra fuel. It is noticeable when I do it and the engine always to run better after. But I only stay at the 80% for a minute or so.

 

[mickand]

Use some of your diesel savings to carry out regular oil analysis. That will tell you if any fuel is present.
Turbo'd engines benefit from running them up regularly to blow out and soot build up, but apart from that i would carry on using yours the way you like.
I have run my Lehmans at less than 1000rpm for many thousands of miles - they operate at a good temperature with no unburnt fuel in the oil. I'll keep doing it.

 

[Delta Riverat]

The manual for my Volvo TAMD 60B's advises not running prolonged under load under 1400.

1400 rpm is just about hull speed, 8 knots. 1600 will move the boat along at about 10.

The broker advised occasional full tilt blasts (which I did for 5 minutes yesterday) and the engines got to 2650 rpm and the speed was 13 knots. The froth behind the boat was amazing, a big foamy hump.

But yeah, a lotta fuel burnt for not much. I guess it's needed though for all the low speed time through the no wake zones.

But, I heed the manual and avoid prolonged operation under 1400. Which works well so far as the motors are smoother around 1600.

 

[fgarriso]

I did have a pair of CAT 3406 1800 rpm engines. They would run at 800-1800 RPM all day and all night, or days at a time without any problems.

 

[Wayne]

Continous Low RPM

Our Albin 43 had twin Cummings 6BT5.9M engines. I thought I ran it up often enough, but one time I found that both turbos had frozen up. Worked them loose, then ran the engine at higher RPMs longer and more often. No more turbo problems.

 

[ronobrien]

Diesels don't like to idle for long periods. Light loads aren't as beneficial for them as running hard for intermittent periods. That said, in marine applications they do experience fair loading as they are constantly "going uphill" pushing tons of water out of the way. There's probably not much, if any harm in what you are doing. A periodic hard run doesn't hurt at all (assuming the rest of the power train is in good shape) and could be beneficial. Running them at their limits often is going to shorten their life to some degree. A friend with a sailboat and Yanmar 60HP 4 cyl turbo was told by the Yanmar rep to run hard all the time. I'm not sure I agree but he is the rep.

 

[Simi 60]

. A friend with a sailboat and Yanmar 60HP 4 cyl turbo was told by the Yanmar rep to run hard all the time. I'm not sure I agree but he is the rep.

The faster they can blow them up the faster they get another sale
Butcher says it's a great day to buy meat.

 

[Delfin]

I did have a pair of CAT 3406 1800 rpm engines. They would run at 800-1800 RPM all day and all night, or days at a time without any problems.

I have the 3306. I was told by an elderly engineer at Cat who was on the design team for the 3406 and 3306 that they were built to accommodate the duty cycle on the North Slope project, which means the ability to idle for hours at a time in very cold weather without harm since they didn't shut them off. He thought this made them ideal for trawler life. Finally, he felt that with by the book maintenance and no abuse 25,000 hours was a piece of cake, and he referenced engines with 50,000 hours.

 

[ronobrien]

The faster they can blow them up the faster they get another sale
Butcher says it's a great day to buy meat.

I like to have more faith in human nature, but you are probably right.