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Old 07-22-2019, 04:13 PM   #1
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Diesel life

Howdy, wondering about a 6.7L Cummins 250 HP versus a much higher output engine, say 380 HP on the same block having different engine life. Is it correct, ( all else equal, e.g not over propped) the 250 HP will have a longer life?

For a trawler Iím looking at the top speed seems to be about 1 Knott between the 250 HP and 355HP.

Thanks
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Old 07-22-2019, 04:29 PM   #2
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Howdy, wondering about a 6.7L Cummins 250 HP versus a much higher output engine, say 380 HP on the same block having different engine life. Is it correct, ( all else equal, e.g not over propped) the 250 HP will have a longer life?

For a trawler Iím looking at the top speed seems to be about 1 Knott between the 250 HP and 355HP.

Thanks


What type of boat running at what speeds?
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Old 07-22-2019, 04:44 PM   #3
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Any boat, say NP 43 running at 8 knots normally.
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Old 07-22-2019, 05:13 PM   #4
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What condition is it in?
Dependable and easy to maintain is something worth considering.
How long is it going to last?
Almost any engine at all will last longer than you can buy fuel for it and run it in your boat. And unless you’re a very unusual boater you’ll sell the boat in 1-5 or so years wondering how long the next boats engine will last.
Just get an engine that surveys well.
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Old 07-22-2019, 05:32 PM   #5
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Wellllll!


The 250 hp Cummins and the 380 hp Cummins can both be run at the same hp continuously, something near 250 hp for continuous use while cruising. At that load they will each probably get the same life. Same block, same hp, roughly the same rpm all equals the same life.


If you run the Cummins 380 at 200 rpm off of top or about 300 hp it won't last as long. If you run the 250 hp engine at 200 rpm off of top it should do much better.


It is all a matter of rpm, load and engine block.


And a 1 kt difference for 40% more hp going from 250 hp to 355 is pretty piss poor.


As others have said, the type of boat and how fast do you want to go will determine if the 380 hp engine is better for you.


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Old 07-22-2019, 06:12 PM   #6
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Wellllll!


The 250 hp Cummins and the 380 hp Cummins can both be run at the same hp continuously, something near 250 hp for continuous use while cruising. At that load they will each probably get the same life. Same block, same hp, roughly the same rpm all equals the same life.


If you run the Cummins 380 at 200 rpm off of top or about 300 hp it won't last as long. If you run the 250 hp engine at 200 rpm off of top it should do much better.


It is all a matter of rpm, load and engine block.


And a 1 kt difference for 40% more hp going from 250 hp to 355 is pretty piss poor.


As others have said, the type of boat and how fast do you want to go will determine if the 380 hp engine is better for you.


David
Iím not disagreeing with David. Longevity is more about how much fuel you put to the engine than rpms. They do have a tendency to go hand in hand. So if you feed both engines the same amount of fuel, you will get the same amount of HP and the same amount of longevity.
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Old 07-22-2019, 06:20 PM   #7
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One day I asked the chief diesel mechanic in our locomotive diesel shop what is the life expectancy for one of the diesel engine, at first he did not understand the question, then he replied to me that there is no limit. This made me think, what are we calling life expectancy? Is it time to failure? Time to next major maintenance? I see some engine that are 50+ years old and still running so is there any real limit?

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Old 07-22-2019, 06:35 PM   #8
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locomotive diesels have replaceable cylinders, so there is practically no life limit, unless something really bad happens to the main block.
Cummins 5.9 or 6.7 do not have replaceable cylinders, but honestly dont need them.
Machine shop could rebore and install sleeves. But unless they break their rings, cylinders dont wear much. These engines self destruct in other ways, from bad injectors flooding the cylinders. Common rail Cummins are great engines if they use the Bosch CP3 injection pumps, but if they use the CP4 pump that is one that causes big problems with ULSD in the USA.
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Old 07-22-2019, 06:36 PM   #9
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I believe you are referring to a new build based on your past posts?

Most people go with the higher HP engine on a FD/SD trawler to run at the lower end of the curve for new builds.

We all know that operating within the lower portion of the power curve is fine with common rail, so there is no reason to go down that path again.

The extra horses allows for greater speeds periodically when in a tight situation with other boats around, passing, currents, etc.

There is another factor to consider - Resale. Many people who are shopping for newer used boats want the higher HP engine. I am not saying the technical argument for the higher HP is necessarily superior to a lower rated engine, but is what it is. I discovered that when I was researching what size engine to use for a new build.
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Old 07-22-2019, 06:37 PM   #10
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Well, a locomotive diesel can be rebuilt indefinitely unless there is a rod thrown through the crankcase or something equally catastrophic.


When I talk about recreational diesel life, I am speaking about wear- rings and valves primarily and much less about things like bearings. When a diesel engine is worn, it takes a lot of cranking to start and blows smoke even when warm. This is due to high loads or lots of hours at moderate loads.


I am not speaking about single component failures like after coolers, injection pumps and injection elbows. These can usually be fixed if the engine core remains sound.


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Old 07-22-2019, 07:44 PM   #11
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I’m guessing a 380 QSB run conservative would go 20,000 to 30,000 hours.
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Old 07-22-2019, 07:54 PM   #12
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Quote:
And a 1 kt difference for 40% more hp going from 250 hp to 355 is pretty piss poor.
That's on the hull form and maybe the prop match, not the engine. My guess is the hull is pushing up its displacement speed limit at whatever speed the comparison is being made.
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Old 07-22-2019, 08:10 PM   #13
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Originally Posted by Lou_tribal View Post
One day I asked the chief diesel mechanic in our locomotive diesel shop what is the life expectancy for one of the diesel engine, at first he did not understand the question, then he replied to me that there is no limit. This made me think, what are we calling life expectancy? Is it time to failure? Time to next major maintenance? I see some engine that are 50+ years old and still running so is there any real limit?

L
I think most people refer to life expectancy as time between rebuilds. If one has the tools and knowledge to rebuild themselves, then you're right life expectancy is indefinite. If one doesn't, that rebuild is tens of thousands of dollars and is a very important number.

I can buy the parts to rebuild my Detroit naturals for less than $5k each. "Life expectancy" isn't a huge concern. To a guy with the hot turbo aftercooled version of the same engine who is paying a yard to rebuild, life expectancy is very important.
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Old 07-22-2019, 10:12 PM   #14
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Any boat, say NP 43 running at 8 knots normally.

Well, I have a NP 43 with a 380hp QSB 5.9L engine. I had the same questions and concerns you did before I purchased this boat used. What I learned from folks that know these things is that if your running the boat at the loads that would get a NP 43 to cruise at 8 knots (about 1800 rpm for my boat) you wonít notice any difference in engine life or fuel burn.

FWIW, I generally run at 1450 rpm, burn about 2.0 gph and it gives me about 7 knots. At that speed we have very little wake, and the boat is very quiet.
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Old 07-22-2019, 10:15 PM   #15
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I had 28,000 hours on one of my Isuzu Diesel engines (10t truck). It smoked a lot on start and after any prolonged idling. However, the Engine is not why got rid of the truck, everything else about the truck was worn out.
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Old 07-23-2019, 05:51 AM   #16
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Many, many more diesels in boats are killed by not following Da Book than ever worn out.

Few folks have" Da Book", the engine repair manual, in addition to the Operators Manual.

How many have read and followed the "Storage Procedure" for out of service for over 30 days ( your engine specific) and all the rest for winter storage?

The engines with the longest service life are usually run long hours or 24/7 like a prime generator or farm water pump.
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Old 07-23-2019, 06:11 AM   #17
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Whether the QSB is rated at 250 or 380 does not matter. In a trawler neither will wear out unless run ungodly hours or run at the extremes of their operating envelop.

These motors rarely "wear out". The get "killed" by corrosion problems from sea water, sitting long times (especially in humidity), poor maintenance, etc.

They are both nearly identical engines aside from the fuel map data stored in the computer. In a 43' trawler a reasonable operator will run either at just under hull speed which is probably around 1500rpm, 60hp, 3-4gph. Right about where they cruise in the pickup trucks, where they are known to be able to go 1,000,000miles. Which at an average of 50mph is 20,000hrs. And yep, at that many miles and hours, the engine might still run, but the truck (or boat!) is usually ragged out.

If you ran each wide open all the time, the 250 will most likely live longer. But it would make zero sense to run either at that mode.
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Old 07-23-2019, 09:46 AM   #18
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I noticed on Cummins web site that the 250/305 have a lower max RPM, (2600) versus the 355/380 and other higher HP engines at 3000 rpm.
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Old 07-23-2019, 09:57 AM   #19
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Yep, the higher hp ratings often have higher rpm. You can use that higher rpm (and hp) if you need it, but it matters not if it you don't use it.
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Old 07-23-2019, 11:08 AM   #20
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Yep, the higher hp ratings often have higher rpm. You can use that higher rpm (and hp) if you need it, but it matters not if it you don't use it.



And if you look at the two prop curves for those engines, they will both be producing the same hp from 2,600 rpm and lower.


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