High RPM, Light Duty Diesel Engines

JLD says: "Given that I would bet that most folks run their Tugs in the 8 knot range, I wonder what would be the expected life of a well maintained 220 h.p. Cummins 6BT? I see many of these boats (1998-2002 ish) with 2000 - 2400 hours on them. Wonder if they would easily go another 2000 - 2400 hours? I would also wonder about the 270 h.p. version of the Cummins 6BT that came out in 2003?"

WESTERLY has a 1988 6BT5.9 210hp that just turned over 8000 hours while running 1650-1750rpm (7.4-7.8kts). I've had to replace injectors twice, but the engine is still good to go. 12000 hours before top end? I've heard of them going longer.

Also on my dock, there is a 2000 Nordic Tug 37 with a 6BTA 330hp that just turned over 8500 hours while running around 8 kts. Last year, they checked compression and blowby, reconfigured the exhaust system, and rebuilt the injectors; Cummins mechanic felt that the engine was about half-life.

Don't know anything about the 270hp model.

djmarchand said:

But I wouldn't buy a ten year old one that was run at 200 rpm off of top for a thousand hours or so.

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I suppose that would depend on the engine. Larger commercial vessels are often spec'd to run at 80% at the operational speed. But even for small recreational cruisers, a good engine operated at 70% for its entire life should not be an issue.

Regarding shorter engine life, if the OP takes a round trip Seattle to Anchorage every year then he may put 500 hrs on the engine. Even if 1000 hrs per year, that's a long time to add up to 10,000 hours or more, which is a reasonable life span.

JLD said:

Interestingly, this is a boat/engine combo that I am interested in finding out more information about!

Given that I would bet that most folks run their Tugs in the 8 knot range, I wonder what would be the expected life of a well maintained 220 h.p. Cummins 6BT? I see many of these boats (1998-2002 ish) with 2000 - 2400 hours on them. Wonder if they would easily go another 2000 - 2400 hours? I would also wonder about the 270 h.p. version of the Cummins 6BT that came out in 2003?

Jim

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Any of the Cummins Bs if run at trawler speeds pulling 50-75 hp from the engine will last a long as you will. Each in the series has their issues. The 210 hp version is the most basic with no after cooler to worry about. The 270 hp version has an after cooler but it is coolant cooled, so rarely gives any problems. The 330/370 hp versions have a sea water cooled after cooler which requires semi annual maintenance but if done will last at least ten years prior to replacement.

The guts of the engine: valves, rings, cylinder walls, bearings won't know if it is turbo charged, sea water after cooled or not at the 50-75 hp rate and will last almost forever.

As Jay N says above, 10-20,000 hours is entirely possible if propped, maintained and run light.

David

Arafura II said:

I love my Gardener 6LX 8 litres per hour at 8 knots 15000 hours TBO

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The Gardner is at the opposite ends of the marine recreational diesel spectrum from a 4,000 rpm high output engine. At 1,650 rated rpm it is less than half of these high revving engines and will truly last forever.

Let me talk a little about and rebut somewhat Eric's (I think) contention that high rpm is not bad. Well by itself maybe not, but all low rpm engines like the Gardner have massive displacement (10 liters for 150 hp), huge bearings etc and the BMEP (brake mean effective pressure- a measure of stress on the internals) is very low due to that high displacement.

The high revving engines have tiny displacement for their hp, 1/6 of the Gardner, and very high boost turbos to make that hp. That makes BMEP very high which stresses components, and rpms are 2-1/2 times higher than the Gardner and high piston speed does wear rings.

But to Eric's point, I would rather pull 100 hp from a mid revving engine at 2,000 rpm than 1,500 rpm. The lower BMEP more than compensates for the higher piston speed.

David

I would also wonder about the 270 h.p. version of the Cummins 6BT that came out in 2003?

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That engine version was available at least by 1999 because I bought one for a repower.
I was told by Tony A (who gave me lots of free advice) of boatdiesel fame that if I cruised it at 10 psi of boost I would get max longevity.
For me at the time that was 2050 rpm.

Nomad Willy said:

If you drove the boat the same speeds .. no problem.
As long as you operated the engines per the owners manual.

Re the qusetion about high rpm engines .. it’s misleading.
Should be about loading, not rpm. Running an engine at 4000 rpm should give a very long life if loads and temps were well into the safe zones.
Heat and pressure is the enemy .. not rpm.

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A case on point with regard to high rpms is demonstrated in gensets. Some gensets run at 3600rpm and these have proven to be a mess. Onan and Panda had some that used a 3cyl engines at 3600 and I have taken a few out for a final trip to the dumpster. All sorts of issues, many related to vibration. Just little things breaking. Vibrations breaking electrical things. Compression and blowby issues. Seemed to make it to around 1000-2000hrs when done.

Many examples locally of 1800rpm 3cyl Japanese engines with over 10k hrs still doing fine.

3000-4000rpm is spinning FAST. Vibration breaks things and the noise!! Ugggh.

Loading certainly is a factor in engine life, not too low and not too high gets engine into a happy spot. Any 6B Cummins whether turbo or not, aftercooled or not, making 40-80hp at 1500-1800rpm is going to have a long happy life. Provided maintained well and not killed by overheat, getting wet, etc. Presence of a turbo matters not. Turbos have proven to be very reliable.

The 270 is a nice engine, but not common. Nice Bosch P7100 pump that has awesome governor behavior, and a fresh water aftercooler. My fave 6B model.

I spend a lot of time around the sportfish world. Those engines are run HARD. Not uncommon for engine and to be needing internal work at 1500-2500hrs. Some die catastrophically. As in large chunks of metal scattered around the engine room.

Some sporties are powered where the engines run well below their rated hp and these get way better results, like no issues but maintenance and minor repairs and hour meters are over 10k hrs. With lots of that planed out!!

The 4000rpm engines are mostly auto based engines. Not a fan of those. Fortunately there are not many in the US trawler fleet.

I'd be looking for Deere, Cummins or if older, Perkins and Lehman, if you are shopping for 35-40ft trawler. Something where rated hp is in the 2500-3000rpm range and you can cruise at hull speed around 1500-1800.

Thanks Ski,
Well thought out post.

Something that occurs to me is the “what wears out” question. Don’t hear much about rods knocking. Or valves going south. Or crankshafts breaking. Or overheating.

One thing does come up very regularlly though and that’s smoking and hard starting. The reason for that is worn out cylinder walls, pistons and rings. I recall someone mentioned the problem w high output DD’s is liner wear. Pressure. Too much side force on the cylinder walls and pistons. That’s a load issue not rpm. And I’d guess the 3600rpm generators fail because of high rpm is coupled w high heat and pressures. The gen sets don’t limit themselves to 60% load. They take it right to the pin whenever the need arrises. And probably the full load continues for some time. Then we can look at the thermodynamics of the pistons in 2 stroke air cooled engines. Aluminum pistons get hot and bigger w heat. So hot w forces so great bits of aluminum from the piston get welded to the cylinder wall.

Sorry but I still think engine speed is not evil. And we can make more power w more power strokes per time as they are less forceful.

"And we can make more power w more power strokes per time as they are less forceful."

Spinning the engine faster with a "less forceful" power setting might result in less power.

The ancient 1920s method of looking at an engine ,operating piston speed in feet per second , or for longevity, piston miles traveled still works.

The metals , machining and assembly have improved in 100 years , but the concepts are valid.

FF,
That dosn’t take in the forces of combustion, pistons and cylinder walls. That I belive is the main source of engine wear.
I think you’d have a hard time wearing out an engine hooking it up to an electric motor and wracking up huge distances of piston travel. Not buy’in it.
Just an old mechanics tale like wifes tale.
Engine load must be the biggest factor of wear assuming the engine dosn’t run out of oil or overheat ect ect.

Thanks to all of the contributors! Your comments have been fascinating.

Right now I’m at the Seattle Boat Show. The trend in most of the new boats I’ve seen has been toward high rpm diesels, heavily weighted toward the Volvo 4D (?). I spoke to the owner of Kelmsman Yatchs and he’s thinking of moving toward a Hyundai motor. I wonder how much of this is pushed by EPA regs (Tier 3/4). I’m thinking that I want a good used boat, pre-2003 with a JD, Lugger, or a Cummins 6B Had one of the latter in a Nordic Tug (210, I think), run in the 1700-1800 range — slower and it wouldn’ truly warm up — and sold it with 4000 hrs. It still started on the first crank and never missed a beat. Needed nothing but fresh oil and clean intake seawater.

Again, thanks for all of your great, thoughtful comments.

Bob

Hi Bob,

I would give you the best advice I can give... Don't overthink it. Find a boat you like and take care of it. It will last you a long time. Trying to base your choice on a brand decision or a low/high rpm will have you still shopping 10 years from now. I am sure we can all state examples of every scenario of engine life, but the reality is that there are no rules that govern engine longevity. Please don't drive yourself crazy trying to prove the impossible because there is more to a boat than just the engine. And besides, by your description of future use, it will take you many, many decades to come anywhere near the point-of-failure you are trying to predict.

Go find yourself a well cared for boat that fits your lifestyle. Period. You will be a lot happier!

Good luck.

Light duty/recreational engines are not made to run at rated hp continuously. Usually 3-500 rpm less. If you look up the ratings, it's usually 2 hours in 12 can be run at full hp.
Any engine, properly maintained and used sparingly will last longer. Sometimes decades longer. The lifespan killer is high exhaust gas temperatures and dirty oil. Engines run wot have high EGTs. Those extreme temps eat away at the metal parts in the cylinders - cylinder walls, rings,pistons and valves. Slow down and let your engine live.