Proper Loading of Diesels

I have twin John Deere 6068AFM75 M3 300 h.p. engines with 113 hours on them. It appears the most comfortable cruise speed and RPM is 7.5 to 8 knots at 1450 RPM. At this RPM the gauges show about a 37% load.

Several people have suggested I run them up to 60 to 70% for about 10 minutes of every 4 hours of operation. I have always heard the danger of running a diesel under loaded. Others say it is not necessary to do this and they will run forever at my cruise RPM and load.

What are your opinions?

Thank you,

I'm in pretty much the same boat as you.

I have a 47' boat with 330 HP cummins engines that are new <10 hrs.

After breaking them in, the boat runs great at 7-8 knots 12-1400 rpm.

I'm not planning on running this boat at a 15 knot fast cruise except to break them in for 20 or 30 hours.

I read an article in passagemaker this last month warning about underloading.

I read an article by Tony athens on boatdiesel.com, regarding my exact engine indicating that he's never seen a premature engine failure due to under loading.

This is sure to generate some controversy, but in my case I'm going to run the engines easy and not worry much about it.

There is also on interesting piece on the blog of MV Dirona, a 52 Nordhavn esentially saying the same thing you are.

I think temperature has a lot to do with it and I think I need to pay more attention to different temperatures. We are leaving Oriental on May 4th to come back to the Chesapeake for the summer, I'll have a week to log a lot of information that I have not had an opportunity to record to date.

I did run them much harder during the break-in at John Deere's suggestion.

Temperature is the key. These diesels have to reach "proper" temperature for efficiency and longevity. That can be independent of rpm. I would join and check with the folks at www.boatdiesel.com

Some say minimum oil temp of 175 so get your IR gun out and shoot the pan. My oil temps are 180 to 185 at about 30% load (of maximum, rated fuel burn).

Just Bob said:

I did run them much harder during the break-in at John Deere's suggestion.

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Exactly!

We're leaving on our "maiden voyage" with out recently re-power'd boat next Saturday.

The plan is to run the engines hard for a couple of long cruising days to get the rings seated, then take it easy and enjoy the ride after that.

If the weather's too rough for 15 knots, I'll just push the bow wave for a couple of days and accomplish basically the same thing.

Tom,
I think the temp thing is related to engine parts like valves, pistons, rings ect.
I think these engine loading discussions happen because everyone knows it's best to load about 50 to 70%. Most are not even near 50% and want to hear from others that it's not bad. Kind of a group therapy session. If I had most of the boats on this forum I'd underload big time ...like 25% on most boats and not worry much about it. I still think it's better to run them about 60 - 70%. I picked a small engine for Willy but I did'nt pick small enough. I'm at 50% load or a tad less. Should have 30 to 35 hp. And the performance curves show an engine is most efficient at about 75% load. But what do you want to do..........run efficiently at 5 GPH or less efficiently at 2 gph? Obviously it would be best to get a much smaller engine and run it at a "healthy" load right in the most efficient zone. But repowering costs lots so most everyone is going to continue to underload and most likely everyone will survive in a happy state and will live happily ever after.
Tom,
If your oil temps are 180 at 30% load what are they at 80% after an hour of running?

The group therapy analogy is not that far off. Yes we are looking for validation of our behavior.

So I take it no one thinks it necessary to run it up for period of time while underway?

I specified these engines and in hind sight would have spec'd smaller ones. We're running pretty efficiently at about 4.5 gph at 8 knots. That drops pretty significantly to about 3.2 gph at 1225 RPM doing 7.2 knots, but then then loading is only about 25%.

I do carry an IR gun and will check oil temperature. I've also heard monitoring exhaust temperature is equally important, I'm investigating the installation of hardware that will give me that at the helm.

Thank you for your responses.

Electronic injected diesels suffer less from under loading.

Some can be retuned to a lower rated power .

The worst is a turbo engine that is run so lightly there is no turbo boost at cruise.

("The worst is a turbo engine that is run so lightly there is no turbo boost at cruise. ")

FF, why is that a problem with the turbo?

I think I can monitor boost pressure at the helm, is there a recommended minimum?

Thank you,

FF said:

Electronic injected diesels suffer less from under loading.

Some can be retuned to a lower rated power .

The worst is a turbo engine that is run so lightly there is no turbo boost at cruise.

Click to expand...

I doubt that's such a huge issue. My car is turbocharged, and the reason is to give it V8 type power when quick acceleration is needed, but as soon as required speed is achieved and one backs off to cruising speed, turbo boost drops to zero anyway, and thank goodness for that, because then I get 4 cylinder economy when I don't need 8 cylinder power. Why would a marine diesel be in principle any different. Diesels already work on such a high compression the charge self-ignites - adding turbo boost would seem unnecessary to achieve desired run temps. My understanding is the turbo is to give that extra power to lift a heavy boat onto the plane, and then hold it there, but at a huge cost in fuel consumption. I suspect maintaining hull speed, just off boost, as long as the water and oil temps are in the ideal zone would not really be called underloading, and wear and longevity probably enhanced if anything, as long as properly serviced. After all, what would de-tuning to lower output really entail that would be that different? I have always been a bit suspicious of the need to "run her at wide open throttle (some magic period) every 4 hrs", to prevent gunking up. I wonder how scientifically based that story really is...?

At least 5 times per year on TF the infamous beware of "turbo boost" issue arises. But never yet have I see a question or comment that says "my diesel engine died because I ran it too slow."

In reading from and talking with engine experts (celectric is one, Cat and Cummins designers others) and as per Tony Athens (who is better on the internet and in real practice?) my understanding of low power/RPM and turbo boost is:

If your engine was designed for Tier II it is a non issue. If your engine has a wastegate (most if not all do since about year 2000) your turbo will kick in at pretty low (for me 1250 - 1300 RPM) RPMs and proportion or waste the boost at various throttle settings/fuel demand. So long as water and oil temps are "up" you are in safe territory. Some run their engines at 80% power for 15 minutes or so per day of cruisng to "blow things out", this is OK but geared more towards sport fishing boats that have been idling all day to fish or lock waiting.

If you have a vintage 2 stroke DD best follow FF's advice.

Turbo'd engines have lower compression and at low (how low I do'nt know) loads less heat will be generated and the underloading issue could be more of an issue. However the most economical (fuel burn wise) trawler should be a boat w a small (relatively) engine so that normal cruise load would be about 60 to 75%. But I know of no such boat.
Peter,
Your turbo car engine during it's travels comes under heavy load VERY often unless you live in an extremely flat place w no stop lights or stop signs. Your boat is going along at 35 mph on a perfectly flat road w no stops ....maybe occasional slow downs (to mimic no wake zones) so to compare is to compare Florida to Point Barrow Alaska.
The big issue is that diesels run so cold at low loads. You can't even fry an egg on the exhaust manifold of a diesel at idle. That's not how I make my eggs ....I've been told that.

Detroit Diesel

My 4-53's have been service since 1976, my boat started it's life with a delivery trip to Seattle from NC. My Starboard engine is original my port engine rebuilt after the blower came loose on a return trip from Guatemala off the coast of Baha. They bolted it back on and finished the trip. North Harbor in Seattle rebuilt it 10 years ago. I really have no clue how many hours are on the port engine as one of the owners disconnected the hour meters because he trolled so much he didn't the want hours shown as he felt they didn't represent hard hours. I cruise at 1800 RPM, the governor is set a 2300 and the engine is rated at 120 hp 2800 RPM. The previous owner of 25 years cruised at 1800 RPM. These engines have been running for 36 years underloaded. They start instantly with no smoke. I've had mechanical survey's done on both engines as well as oil sampling and both engines are still in very good condition. I've had Detroit's in backhoes , forklifts , and dump trucks, all of which sat and idled for long periods of time. I've never had one break or not start. I may have permanent hearing loss from running these engines as well as kept Standard Oil in business with the amount they leak. But based on my results I've not had a problem with under loading a Detroit two stroke diesel.

But based on my results I've not had a problem with under loading a Detroit two stroke diesel.

The biggest problem with a lightly loaded DD is fuel burn.And depending on the injectors sometimes stack smoke.

Until you get to about 60% of the build rating the economy sucks.

Ideling at 1 gph vs ideling at 1.5 gph is a 50% difference , but not much bother.

Same with most marinizations , changing engines or gearing and prop is hardly a big deal , getting 25% better economy at 3 gph , for 200 hours a year.

An engine thatshould go 7000 hours of constant use is no problem if it runs 4000 hours at 200 hours a year in a pleasure palace..

I'm not sure I understand your last statement

It looks like my 1800 cruise speed is about 64% of rated rpm. There are three jet sizes available for the 453 natural and remember having the largest which again from memory is 124hp at 2800rpm. Why they call these blown engines naturals is beyond me but they did turbo some of these. I looked at two of these boats when I bought this one and both boats were governed at around 2400rpm. Both owners had the same story that they were set that way to prolong the life of these engines. With this hull design the 2300rpm is over hull speed and I suspect that more horsepower in the would just dig a hole in the water. I don't know how much you know about the 53 series Detroits? They are an interesting engine as they have four valve heads and a counter balance shaft to smooth vibration. They are also modular and can be built asymmetrical and in several configurations, twins, triples, 4's , v-6's and v 8's. Starters are available in several different voltages as well a air and manual hydraulic. They can be rebuilt in frame for around 6k. The weak point if there is one, is the valve size. the valves and stems are quite small and heads are usually good for about 8000hrs when run hard. Old technology maybe ahead of it's time. As time has shown a very popular engine. The injectors are mechanical plunger pumps easily rebuilt in the field and operated by rocker arms referred as a rack, thus rack adjustment, a term loosely used to set injection on other engines. But in this case it really is a rack adjustment. This maybe the most difficult adjustment to get the feel for. They are noisy and known for leaking oil. However they are very durable smooth running engine's. The old industry saying is if it starts you'll get another days work out of one. When I bought this boat we kicked the idea around to repower with a pair of JD's to too improve economy and reduce engine noise. The cost of replacement is a lifetime of cruising kitty. I carry 1500 gallons of fuel so range is not a problem. I think the availability of parts is good as no matter where you go there's a 53 series engine powering something whether it be a log skid, ag pump or fish boat. MTU still manufactures some new parts for these engines. I had 330b's on my last boat and they shook so bad at idle the doors would rattle. Not these Detroits, smooth as glass.

"Why they call these blown engines naturals is beyond me but they did turbo some of these."

The Rootes blower in most DD is simply to push exhaust gasses out of the engine, not to run the engine above atmospheric (supercharged or "blown").

Since the fuel is not injected till later the scavenging can be to excess with only minor penalty in power consumed.

Which is probably repaid by having a fresh charge of air , rather than sort of as most engines do.

Just BOB said:
"...Several people have suggested I run them up to 60 to 70% for about 10 minutes of every 4 hours of operation. I have always heard the danger of running a diesel under loaded. Others say it is not necessary to do this and they will run forever at my cruise RPM and load.

What are your opinions?..."

When I first went shopping for the engine for Rainha Jannota, I went to visit the MWM factory and, coincidentally, a major Engineering Manager from International was present. As we were visiting the same person, we both got invited for lunch.
One of the issues brought up to the table, while eating a glamorous Brazilian BBQ, was exactly this so discussed issue of under loading marine engines by running them at very low RPMs. I will reproduce now his final statement about the issue:

Quote”…There two different fields in this discussion. One is the new “Electronic” diesel engines which will suffer if/when running at very low RPMs for long periods of time.”…Now comes the surprise, “…very low RPM on a max 2500 RPM boat engine means less than a 1000 RPM. Yes we have seen some premature charcoal build up in Electronic engines that runs 4000 hrs out of 4500 hrs at idle rpm. The other field refers to conventional non-electronic engines. For these, as far as I know, there aren’t reliable technical/scientific studies/tests that prove the damage on a engine that runs indefinitely at under load regimens, in boats, trucks or even industrial engines…” end of Quote

After listening carefully, I considered this a definitive statement about the issue and bought a mechanic engine.

Regards

Fernando
Note: MWM is a Brazilain company created by Deutz in the 40's, nationalized by the military dictatorship in the 70's, who recentely bought US INTERNATIONAL.