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Old 12-27-2010, 06:10 PM   #1
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Cummins....

I am a perpetual surfer of Yachtworld. *I am on there every day looking for THE boat. *And we are getting closer to being "back in the market". *I have nothing against Cummins engines. *Obvisously they enjoy a decent reputation. *My biggest issue is this....

"As of March 09, she had her starboard engine was hauled out rebuilt with limited warranty,Port engine with 900 lightly used hours..."


I see that VERY often with Cummins powered boats. *Almost always the other engine has less than 2000 hours on it and if we assume that they use both engines at the same time, are we to assume a Cummins 6BT is only good for about 2000 hours??? *I would never avoid a boat with Cummins in them but if I did buy one, this would always be lingering in the back of my head. *I also like the extra room they give you in the engine room versus the main competitor of that era....the Cat 3208. *But I see 3208s with 3 and 4000 hours on them all the time. *I realize there are many different versions of this engine(and the 3208) but it just gives me pause. *Anyway....discuss....


PS....I do find the cut and paste quote kinda humorous. *I guess they treated the other engine poorly and treated the other one with kid gloves.....hahaha
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Old 12-27-2010, 08:18 PM   #2
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RE: Cummins....

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Have also been a longago member and present lurker on BoatDiesel and can vouch for the forums' knowledge and experience.

When WESTERLY was purchased, the 6BT5.9 Cummins was not exactly my first choice of engine make due to past experience with other brands.* After 13 years, I have come to greatly appreciate the engine's design, ease of service, reliability and performance.*

There are a few well-known issues for preventative maintenance, otherwise this engine series is*very rugged and long-lasting for every application/installation EXCEPT over-propped and/or over-loaded planing boats.* In this latter situation, it does not matter*what kind of engine is installed as the result is*shortened-life, increased maintenance requirements and decreased reliability.

The engine in WESTERLY is now approaching 5000 hours service over 22 years, and should be good for another 22 years.* Service load at cruising is 20-30%, with an occasional run at 52%.

*
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Old 12-27-2010, 10:19 PM   #3
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RE: Cummins....

So are they less tolerant of running hard than other engines??? That is almost what it is sounding like.
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Old 12-27-2010, 10:33 PM   #4
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RE: Cummins....

I would like to know how tolerant they are to running slow and underloaded.
Also I've heard the 4cyl is not as smooth as others.
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Old 12-28-2010, 05:37 AM   #5
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RE: Cummins....

"I am a perpetual surfer of Yachtworld. I am on there every day looking for THE boat. "

TO be shown on Yachtworld the boat must be put up by a BROKER.

So you are paying 10% for nothing.

Other sites may not have this cost overide.
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Old 12-28-2010, 06:27 AM   #6
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RE: Cummins....

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DavidM wrote:Any marine engine producing 60-70 hp per liter is intolerant of running hard: overpropped or at WOT.*
Any marine engine producing 60 - 70 hp per liter is by definition running hard!

The most powerful diesel on the planet produces 109,000 hp by extracting just over 4 hp per liter of displacement at around 100 rpm and it will do it for tens of thousands of hours.
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Old 12-28-2010, 07:34 AM   #7
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RE: Cummins....

I think that it depends on the type of boat. It is common to see overhauls of all engine brands on sportfisherman because they typically run the s**t out of them. On motoryachts and trawlers the engines tend to run much longer.

Plus, any engine mfg can get a bad series. Early Cat 3116's and 3126's, DD's 92 and 8.2 series, Mangranades, etc.
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Old 12-28-2010, 09:02 AM   #8
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RE: Cummins....

Like it or not, Cummins is the world's largest supplier of "smaller" marine diesels. I view their choices up to 600 HP as the benchmark. As with all brands, prop right, especially if running at 70% plus rated fuel burn.

Boatdiesel is not necessarily a Cummins site, they have experts*who are Perkins, Lehman, Volvo, Yanmar and Cat also. Cummins gets the press because they likely*have more engines currently sold than*all the others combined in this size range.
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Old 12-28-2010, 10:53 AM   #9
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RE: Cummins....

I can tell you this...if I had the choice, I would take Yanmar over Cummins in a planing boat.....a boat that I am going to run loaded up and at around 70%.

So is an engine in a planing boat that might spend a good parts of it's life around 70% considered "overworked" or "running the piss" out of it???
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Old 12-28-2010, 12:19 PM   #10
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RE: Cummins....

Baker -*70% rated fuel burn for a well maintained and*a correctly propped diesel engine should be no problem. If you've not done so, read the boatdiesel archives, lots of good diesel info.
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Old 12-28-2010, 06:47 PM   #11
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Cummins....

John,

My two Cummins 6BTAs 330 HP circa 1997 have run fine for the little time Ive owned them. The boat was not run in the 2 years before I bought her and I had some issues before I took delivery, mostly due to it being unused. A clearing of the raw water system and new timing gear was all it took and with the exception of a mechanic over filling the oil and causing blow by, so far so good.


From what I have gathered over at boat diesel the make of the engine is a poor predictor of an engines longevity and reliability, but rather the care and how the owner ran them seems to be a better predictor of its longevity. Im certain many new boat owners have commented: "hey they're diesels, they'll run forever right?", thinking they can ignore and abuse them.


With that being said, each engine has its issues. The Catapiller 31xx series had a soft block issue and problems with swallowing valves, issues that are well documented on boat diesel and the Sea Ray owners forum. Cummins had and still has problems with poor reliability on their sea water pumps and idler pulley, both of which have caused catastrophic engine failures which is very rare. The Rolls Royce Trent 900/1000 series seems to be the king of catastrophic engine failures right now, but I doubt youll run across one in your boat search.


It seems most high performance diesel engine problems start in the raw water systems and if ignored grow until raw water makes its way into the core of the engine. When that happens of course the engine life is at risk. According to Tony Athens at boat diesel the #1 reason for early diesel engine failure is over propping causing the engines to be overloaded, apparently high performance diesels engines are very sensitive to this. I just took an inch out of my props pitch.


I generally cruise my engines at 1800 rpm and occasionally 2200 rpm, but never above this. Viewing boat diesel forum comments Ive come to the opinion that many problems are associated with running these high performance engines at the higher rpms. DavidM touched on this and RickB said Any marine engine producing 60 - 70 hp per liter is by definition running hard! I concur.


Concerning your concerns about running slow and unloaded my research confirms what DavidM stated above.







-- Edited by timjet on Tuesday 28th of December 2010 07:49:23 PM
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Old 12-29-2010, 06:19 AM   #12
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RE: Cummins....

Regarding Cummins 6BT:

The work boat marina I winter my boat at sold in their words " a jag ton of the 6BT 210 HP motors to crabbers. They install a hydraulic pump on them to run the pot hauler. The boats leave at 5am come back at 3pm. They run to a pot, stop, haul it, service, and move to the next. They don't go slow and they do this all day. It's nothing for them to put 10,000 to 20,000 hours on these motors. The part that breaks are the china copy morse controls. They are usually worn out in 2 to 3 years.

Had a 6BT 210 HP in my charter boat when I bought it. Ran it hard (80% capacity) for 2 years till I could afford to repower to the Cummins 6CT 300 HP (needed more HP to haul scubadivers). My 6BT ran great for me and is still running well for the guy who put it in his boat. Motor is now 18 years old (I think).

Regarding cummins 6BT at slow speed / low power:

I have a Dodge pickup with the 6BT 220 HP. Same motor, different turbo, injector pump, and probably different head. Truck runs down the road at 60 MPH (1,800 RPM) highway (my choice). Gets 22 MPG combined city and highway and 25 MPG hiway long trips. That's a fuel burn of 2.4 GPH (43 HP) max. In 8+ years I have 270,000 miles on the engine. I figure I have around 5,000+ hours on the engine. Doesn't use any oil between 5,000 mile oil changes with Rotella T. Only issue I have with them is the water pump. I am on my 4th one. The seals go bad in them. Easy to change though, 3 bolts and a big oring for a seal.

Cummins 6BT , yea there ok in my book

Ted
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Old 12-29-2010, 10:07 AM   #13
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RE: Cummins....

You name it...I've had them all. No brag...just fact! Cats, perkins, detroits, etc. I'm now on my 5th Cummins engine, in 3 different boats (2-twins and one single) and have had no serious trouble with any of them. (Had to change out the thermostat in my 330B that was a Cummins mod.)* I love them!
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Old 12-29-2010, 10:20 AM   #14
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RE: Cummins....

Dependent upon the selected motor output a "modern" diesel can have the same apparent block but yield very different HP. The Cummin's 6BT and its variants are designed for approximately 2500 to 2800 top RPM. If you go to the Cummins website it states that for leisure duty the 6BT engines are designed for a cruise RPM approximately 10 to 15% off the top rated RPM. Comparing say 2200 RPM to the fuel burn curve, it*shows about 70 - 75% of the rated fuel burn for that engine.

If your diesel cannot pull full RPM, it may not matter in your low RPM slow speed trawler. But in a commercial vessel or a*planing boat, not being able to achieve full rated RPM*without overheating can be a death knell.

Last year my company purchased many Cat haul trucks with 4000 HP engines. The trucks and engines are designed to haul uphill for about 25 minutes per cycle at full rated RPM day in and day out and last for about 15,000 hours between major rebuilds.

As Ted said in his above post,* diesels can do work and*lots of it. They need clean air, adequate cooling capacity,*by the book*PM*and properly filtered fuel to do this work.

As Eric, FF and others continually state, we don't need our big trawler engines. Too big an engine leads to under use and sloppy maintenance. When you then need the engine to deliver*near its rated RPM, it can't due to bad HX, wrong props or a dirty bottom. Their answer, get a small diesel and run it hard as intended. But unless a new build, our too big and under used engines are here to stay.
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Old 12-29-2010, 11:52 AM   #15
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Cummins....

All excellent info. Thanks so much for taking the time to post. Makes me feel much better about them as potential powerplants for our next boat. My post reference 70% was rated power...not rated RPM. I run my Yanmar at 2800RPM. I honestly don't know where that is in the power curve but my guess would be around 60% at most...realizing the power curve is nowhere near linear. I do know it's max continuous rating is 190hp@3100rpms and a time limited rating of 240hp@3300rpms(I can get 3600rpms out of it wide open so it is obviously not overloaded although you could argue I am leaving some power/efficiency on the table). So max continuous is 80% of WOT. So when we talk about percentage of power, are we talking percentage of max continuous or percentage of WOT???

Anyway, I am just rambling here because it is a good discussion. Sunchaser, your 4000hp Cats may run at 100% but I guarantee you Cat knew the application and there is plenty of margin above 100%...IOW, they are derated. And the longevity of a Dodge truck engine is not really relative unless we are talking about underloading. They just aren't loaded up like a boat is.

-- Edited by Baker on Wednesday 29th of December 2010 12:54:40 PM
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Old 12-29-2010, 01:34 PM   #16
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RE: Cummins....

"And the longevity of a Dodge truck engine is not really relative unless we are talking about underloading. They just aren't loaded up like a boat is."

That is a valid point. I had a 6BT in both a truck (250 *hp) and a boat 270 hp), and montored boost and egt in both.
Truck cruising on a flat at 70-75 mph,* 2000 rpm,*4 psi boost, 500 F egt (pre turbo).
Boat cruising at 1900-2000 rpm, 14 psi boost, 700 F egt (post turbo).

Truck was using about 1/4 the power that the boat was. Not close in duty cycle.*
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Old 12-29-2010, 02:24 PM   #17
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RE: Cummins....

Quote:
Baker wrote:
And the longevity of a Dodge truck engine is not really relative unless we are talking about underloading. They just aren't loaded up like a boat is.

-- Edited by Baker on Wednesday 29th of December 2010 12:54:40 PM
John, this was my point in posting the truck part. In a trawler application, if the propeller is pitched correctly to allow full RPM, the engine will likely run well at a very low HP for the cruising RPM. Visualize a 42' single screw trawler making 8 knotts at 1,800 rpm with a 6BT 220 HP. With and a fuel burn of 2.4 GPH (43 HP), it's running at 20% of rated capacity. My point was to show you can get good efficiency and longevity in the mid RPM range even though you are using a small percentage of the HP available at that RPM. IMO, this is far better than over pitching the prop and trying to generate that 43 HP at 1,400 RPM.

Ted
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Old 12-30-2010, 04:57 AM   #18
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RE: Cummins....

Quote:
O C Diver wrote:My point was to show you can get good efficiency and longevity in the mid RPM range even though you are using a small percentage of the HP available at that RPM.
Uh oh, better be careful, you will have the Underloading Police knocking on your door!

*
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Old 12-30-2010, 05:02 AM   #19
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RE: Cummins....

rpm with a 6BT 220 HP.

This is probably the truck rating.

If you find the 24/7 rating it may well be half of that , so 45 hp is not all that bad.

The key for a trawler is NOT to have a turbo, that may require some minimum RPM in order to survive.
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Old 12-30-2010, 05:25 AM   #20
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RE: Cummins....

Quote:
FF wrote:The key for a trawler is NOT to have a turbo, that may require some minimum RPM in order to survive.
A turbocharger responds to load, not rpm.

The rpm of a turbocharger depends on load and density of intake air.

*
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