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Old 02-04-2019, 05:42 PM   #101
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And speaking of CAT:

"If maintained properly, diesel and gas generator sets can operate at light loads for long periods of time with no harmful effects. After operation at low load levels, each impacted generator set should operate under increased load to raise the cylinder temperature and pressure, which cleans the deposits from the combustion chamber."

- http://s7d2.scene7.com/is/content/Caterpillar/C10711038
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Old 02-04-2019, 11:27 PM   #102
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(snipped...)

Where it becomes really interesting is looking at jet RIB's, both gas and diesel. I might have to do that later, but back to a meeting for now.
I'm curious. Has your meeting finished yet?
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Old 02-05-2019, 12:03 AM   #103
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Diesels love to be run at their rated continuous operating speed with the correct load. Its all about matching the boat's hull shape and weight, the prop, the trans reduction ratio, and engine hp correctly to get the maximum performance and longevity out of any marine engine. The worst thing one can do is just assume that the former owners propped the vessel correctly. Overloading at low RPMs will kill an engine faster than anything
I think I'm a victim of this. My old boat's previous-previous owner took her from CA to Hawaii then on to Aussieland and back. The 1600 gal tank wasn't big enough so he filled the cockpit with fuel drums. Be he also slightly over-propped it with Kort nozzle props, to maximize his efficiency at planned operational speed.

It was a very strange setup, but the old log book was more fascinating to read than anything Melville or Verne. It definitely beat up those Jimmies, but they still ran strong.

I did switch out the props but kept the old ones as spares.
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Old 02-05-2019, 12:57 AM   #104
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I'm curious. Has your meeting finished yet?
Yes, and we've made it home. I haven't looked at those numbers though. Just been glad to be home.
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Old 02-05-2019, 01:49 AM   #105
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Normally cruise at 6.3 knots at 1800 RPM, representing a 43% load. RPM of 1600 results in 6.0 knots for a 38% load. Regardless, expect the JD 4045 to outlast me.
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Old 02-05-2019, 09:26 AM   #106
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Engine Longevity

I am glad to see that my article stimulated some conversation on this subject.

I would like to clarify a couple of points. First, the determination about how hard an engine should be pushed must be based on the rating of your particular engine. Cummins, for example, offers Continuous Duty and High Output ratings (along with others in between). Continuous Duty means they designed it to run at full power nearly all the time (commercial push boats for example).

The High Output rating is what almost all recreational boats have. Cummins specifies a power factor of 10-30% for these engines. Those who have a recreational rated engine and who push that engine at power factors of, say, 50-70% will experience shorter time between overhauls.

Second, I emphasized in the article that it is equally important for those who do run at light loads, to periodically run their engines up to WOT or to 80% load. The article included more detail about that protocol.


I hope these comments are helpful.

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Old 02-05-2019, 11:16 AM   #107
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the prop curve is a mathematical estimate of propeller load.

The equation is simple ((running RPM/max Rated RPM)^2.7)100= percent load

The 2.7 exponent is common though exponents from 2.5 to 3 are reported.

((2400/2600)^2.7)*100=80.5 %

Substitute your own numbers and paste it into your search box
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Old 02-05-2019, 07:43 PM   #108
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I am glad to see that my article stimulated some conversation on this subject.

I would like to clarify a couple of points. First, the determination about how hard an engine should be pushed must be based on the rating of your particular engine. Cummins, for example, offers Continuous Duty and High Output ratings (along with others in between). Continuous Duty means they designed it to run at full power nearly all the time (commercial push boats for example).

The High Output rating is what almost all recreational boats have. Cummins specifies a power factor of 10-30% for these engines. Those who have a recreational rated engine and who push that engine at power factors of, say, 50-70% will experience shorter time between overhauls.

Second, I emphasized in the article that it is equally important for those who do run at light loads, to periodically run their engines up to WOT or to 80% load. The article included more detail about that protocol.


I hope these comments are helpful.

Steve Zimmerman
Thanks Steve both for the article and your comments.

One question I have regarding rating. When I purchased Delfin, she had a new CAT 3306 C rated engine. The previous owner was a Naval Architect whose hobby was buying classic boats and refitting them. More money than brains, apparently, but in any case, his reasoning for using a C rated engine was that since it would be operated at around 25 - 40% load factor, the C rating was fine. I do take it up to EGTs of around 775 degrees every few hours to heat everything up. The C rating has the following parameters: load factor <70%, annual hours <3,000, max power per day < 1 hour.

My question is, how many actual physical differences are there between engines of the same manufacture and model, but different ratings, or are they primarily how the engine is governed? If the latter, then isn't rating somewhat unimportant in a full displacement vessel since whatever the rating, the engine will likely be run at less than 40% load?
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Old 02-05-2019, 09:03 PM   #109
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Jay on Westerly published a fuel consumption table that pretty much reflects what I see on a Cummins 6CTA 8.3 CPL1929. Using the approach of WOT fuel consumption versus measured fuel consumption over time, I come up with engine loading averaging around 9%. With 360 hours my average fuel consumption was 2.1 GPH (Fuel used divided by engine hours). I can achieve the WOT RPM with my current prop of 2650 RPM with a specific fuel consumption of 23.3 GPH at 2600 RPM on the Cummins Performance Curve. I would call that lightly loaded over time and tracks with our typical running RPM of 1250-1400 (7-8 knots).

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Old 02-06-2019, 02:55 AM   #110
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, but in any case, his reasoning for using a C rated engine was that since it would be operated at around 25 - 40% load factor, the C rating was fine. I do take it up to EGTs of around 775 degrees every few hours to heat everything up. The C rating has the following parameters: load factor <70%, annual hours <3,000, max power per day < 1 hour.
Surely that one hour per day, sucking like the proverbial, negates any fuel savings gained over the several hours previously by running slow?
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Old 02-06-2019, 07:00 AM   #111
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Surely that one hour per day, sucking like the proverbial, negates any fuel savings gained over the several hours previously by running slow?
Good point Simi. Some smart engine guys say that provided your oil temps are 180 to 190F that the one hour "high" load run up per day is an anecdotal reference to the 2 stroke DD era.

Dry stack complicates the issue too with lower exhaust gas temps causing stack coking, thus another reason for some to run periodically at "high" loads.

With all these rules of thumbs I need more hands. Or as many on this thread have said, run sensibly for your specific application.
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Old 02-06-2019, 07:52 AM   #112
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You are my new hero. That you keep the vehicles so long is amazing in this current throw-away society.

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Old 02-06-2019, 07:57 AM   #113
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"Overloading at low RPMs will kill an engine faster than anything"

While this is true , simply increasing the load at lower RPM can increase engine life , reduce fuel burn and make lofe onboard better with less noise and vibration.

The key is a PROPER load for the chosen RPM ,NOT an overload.

With a higher load at cruise , there should never be a need to run at WOT to "clean out the engine" .

An EGT gauge is a great idea , just to be sure, of no overloading.
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Old 02-06-2019, 07:57 AM   #114
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There have been a few comments here about the key to long engine life being keeping the oil clean with aftermarket bypass filters, Gulf Coasts, centrifugal polishers, etc.

However I've always thought that just running the engine properly, monitoring EGT and sticking to the manufacturer's recommended oil change intervals should be good enough, without having to resort to expensive aftermarket items or synthetic oils.

Is there really some magic to these aftermarket items?
Synthetic oils allow for extended drain intervals so, no, they are not more expensive. Plus, they minimize, comparatively, the start-up wear. By-pass filtration is quite effective at removing soot particles down to 1 micron as opposed to standard filters at 30 microns. Not magic, just science.
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Old 02-06-2019, 08:03 AM   #115
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Good point BK. Interestingly, Steve Z's PM article states 80% load is around 90% rated RPM. He must have been peeking at a JD sheet. On most marine engines, 35% load is 60 - 70% rated RPM.

Regarding pre-failure analysis (or better said repair it before it breaks) it is commonly done for tens of thousands of variable use engines under extended warranty programs. Especially during the last few decades as on engine diagnostics are downloaded during routine maintenance and oil sampling.
Getting back to Ford Lehmans, a 35% load (65% of rated RPM of 2,500 RPM) yields about 1,650 RPM. I think most Lehman owners run their engines between 1,600 and 1,750 RPM, perhaps 1,800 on occasion when necessary.
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Old 02-06-2019, 11:12 AM   #116
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Surely that one hour per day, sucking like the proverbial, negates any fuel savings gained over the several hours previously by running slow?
The 1 hr Max at WOT is the Cat spec for the C rated engine. My specific use pattern is 20-30 minutes with the EGT >= 750 degrees every 24 hours, or daily if we're making shorter day trips. GPH goes from 3.3 at 7.5 knots to 8 GPH at 8.9 knots, so yes, total fuel economy goes down but not enough to worry about. There is no point in my ever going to WOT at 2200 rpm since I hit hull speed at around 1850 rpm and any additional power provides no additional speed and only a slight increase in EGT.
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Old 02-06-2019, 11:46 AM   #117
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Synthetic oils allow for extended drain intervals so, no, they are not more expensive.
While that is what the oil manufacturer claims, the engine manufacturer's warrantee department may disagree. According to the manuals for the diesels I own (Ford, Yanmar, Cummins, Volvo) there is no allowance in maintenance interval for synthetics.
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Old 02-06-2019, 12:06 PM   #118
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Delfin wrote;
“My question is, how many actual physical differences are there between engines of the same manufacture and model, but different ratings, or are they primarily how the engine is governed? If the latter, then isn't rating somewhat unimportant in a full displacement vessel since whatever the rating, the engine will likely be run at less than 40% load?”

I see the perfectly powered (loaded) engine in a FD rec trawler as about 60 or so percent. Mine is 50% and I know I could do easily w less power. But it depends on who or what rates the engine. My Mitsubishi is a generic industrial engine. And Westerbeke and Vetus both claim more power .. 44 and 42hp.
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Old 02-06-2019, 12:17 PM   #119
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Delfin wrote;
“My question is, how many actual physical differences are there between engines of the same manufacture and model, but different ratings, or are they primarily how the engine is governed? If the latter, then isn't rating somewhat unimportant in a full displacement vessel since whatever the rating, the engine will likely be run at less than 40% load?”

I see the perfectly powered (loaded) engine in a FD rec trawler as about 60 or so percent. Mine is 50% and I know I could do easily w less power. But it depends on who or what rates the engine. My Mitsubishi is a generic industrial engine. And Westerbeke and Vetus both claim more power .. 44 and 42hp.
My question related more to what the mechanical differences are between engines of different ratings. Is it only the governed rpms, or are their others?
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Old 02-06-2019, 12:57 PM   #120
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Delfin,
I don’t know.
But exhaust valves may be of a better alloy. Pistons also. Lots of things could be different to handle higher loads and temps for longer periods of time.
Old Dodge flat head truck engines had different exhaust valves and there may have been many more differences. GMC made more than one 6 cyl engine for trucks. The V6 found in the GMC trucks (not found in cars) in the 60’s was designed specifically for trucks. Quite sizeable trucks at that. I remember I was amazed at thecooling system. Was a great engine but not very fuel efficient. Otherwise it would have been a good boat engine.
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