Cummins 6BT 5.9M hours opinions

1991 Monk 36 with original Cummins 6BT 5.9M (220 hp) with 8655 hrs on it. Assuming it was well maintained, is that a lot of hours? For a boat that is 29 yrs old, that does not seem out of line to me. Roughly 298 hours per year. The boat was definitely used though; it did not sit at a dock.



I know diesels, in general, can go many tens of thousands without issue, but I know next to nothing about Cummins engines. I know it's an inline 6 with a good rep and seems the BT means turbo charged.



I am awaiting a response from the broker on the state of the fuel tanks, whether it's a one owner boat etc...


Thanks for any input.

great motor. Mine is a 1994 210 6bt 210hp. Bought the boat with 200hrs & now I have 3500 hrs.

Just regular maint. the manual specs when and what. Very trouble free so far. No injectors or HPP yet.
I have replaced the M71 raw water pump several times - usually good for around 1000 hrs before it starts to drip.
Replaced the wet exhaust elbow at 2000 hrs - didn't really need to, but seemed like smart preventative.

I would imagine the owner has a maint. log - that will indicate overall care and feeding.


The motors short of overheating or bad fuel should last a long time

At 29 years an 8900 hours I would budget for a rebuild. They could run trouble free for a long time but then again they may not. As long as the price is right and you can afford a potential rebuild then just use it and hope for the best. Low time engines can also fail.

That engine in a generator application will often go well into the 20,000 hour time frame before needing a rebuild.

Read the article at sbmar.com on marine age.

https://www.sbmar.com/featured-article/marine-age-the-real-age-of-a-marine-diesel-engine/

"Low time engines can also fail."

Low time engines seldom fail, they are KILLED.

A boat that for decades has run 300+ hours is more likely to have been maintained , than one someone just starts and idles dockside on weekends.

The higher time engine is more likely to have a collection of oil samples and been properly put to bed for the off season.

Localboy.
Check out sbmar.com (Tony Athen's website). He has lots of free info both in his Tony's tips and in the forum.
I have one of these engines, but mine is a 2002 version and it is aftercooled (330 hp).
If the engine has been well maintained, it should be fine for many more hours. A friend of mine has the same engine as mine (6BTA), he has maintained it properly, and he has almost 10,000 hours on it, still going strong!
FF made some very good points.
Check out Tony's info, especially on "marine age" and marine exhausts, and carefully inspect the engine along those lines. The aftercooler (and some exhausts) on these engines are items that require careful checking (with the aftercooler needing regular maintenance). The engine you are looking at does not have an aftercooler, so that issue is eliminated. With the engine producing lower horsepower (than some it's cousins like mine and the friend I mentioned), all things being equal, it should in theory last longer than the higher HP, harder working blocks.

Change out the poor raw water pump (Sherwood) to the better designed one that Tony sells (a direct plug and play).
Otherwise, this engine has been used in Dodge pickup trucks for years, parts are readily available, there are lots of them out there in various uses so most mechanics are familiar with them, so servicing should be easy.
Good luck with your decision.

One other factor to consider. Under what level of load was the engine likely to have been historically operated at. With it being a Monk, I am guessing that the engine was most often operated at 1/2 throttle or below (relatively low load)? Also, check to ensure that it has not been overpropped. Will it reach approx. 100 RPM above rated WOT?

I would think that diesel could burn 60,000 gallons before rebuild. You do the math.

localboy said:

whether it's a one owner boat etc...

Thanks for any input.

Click to expand...

I feel like I hear people talk about one owner boats as if it’s beneficial to have one owner. Usually when a boat sells, it gets a full survey. That means a fresh set of professional eyes looking at everything. Then a fresh owner who comes in and spends money on correcting anything that slipped away from the last owner. They add some new equipment, zero time the maintenance to ensure trouble free cruising, and sometimes their spouse adds ‘feathers in the nest’ with upholstery, carpet, and new mattresses.

So seeing a boat with multiple owners can be a good thing. Not saying there aren’t outliers in any data sample, but don’t rule it out.

As for those engine hours, maintenance records and a good engine survey will tell you a lot. My guess is that it didn’t average 289 hours, but had years of high use and years of nearly no use. If the last few were low use, I’m more concerned. Either way, budget for a rebuild.

Good comments all. I have these engines at 210 hp and consider them to be the most solid system on my boat. Mine have 6 years, 1300 hours after a repower. As recommended, Tony Athens at sbmar.com knows more about these engines than about anyone and you can learn a lot from him. Boatdiesel.com also has a lot of information. In the case of this particular boat, get a mechanical survey, sea trial it, and run it to the pins, wide open throttle for 10+minutes looking for abnormalities. If it won't come up to rated rpm, it may be overpropped, which may have shortened its life expectancy . Best case scenario at 8,000 hours is you're still going to be replacing hang-on items....starter, alternator, manifolds, heat exchanger, oil cooler...but the base engine may be ok. As a buyer, I'd still be looking for a price reduction based on high hours and marine age.

"If it won't come up to rated rpm, it may be overpropped, which may have shortened its life expectancy ."

Or if the boat cruises at half throttle a cruising prop has been installed.

This allows power consumed at lower RPM to more closely match engine output.

Lower cruise RPM, quieter on board , better fuel consumption and a longer wearing engine with fewer piston miles.

A new red line on the tach ,or an EGT gauge is the usual giveaway the owner understood what he was doing, to not overloaded the engine.

I’d rather have an engine that has been well utilized and maintained. Engines hate not being operated often falling victim to things like internal corrosion.

If you look at the 6BT in the automotive world they have been known to go 500,000 to 600,000 miles. Average is probably 300,000 miles as the body rusts away or it is damaged in a wreck. If the typical auto averages 30 MPH this translates to 10,000 to 20,000 hours.

In a fixed application, like generating electricity you would expect the engine to last longer. The unknown here is how the engines were run and how they were maintained. If the owner has kept good maintenance records that will tell you a lot. Oil consumption is an indicator of engine wear. Looking at the exhaust may tell you a few things as well. If the engine has some hours on it since it’s last service an oil analysis may tell you some things as well.

At best this engine is at mid time. At worst it can be near end life. A good engine survey would be indicated for this engine.

Good engines in the lower HP range (210 to 220). The 6BT in my Dodge has 488,000 miles (9,760 hours) on it. Normal replacement items like water pump, alternator, starter, and lift pump. Still has original turbo. No oil consumption and perfect oil analysis.

Ted

I think that when considering an engine with 5,000 to 10,000 hours on it you should probably consider the boat itself more than the engine.

Is it a sport fisher, is it heavy and slow, a commercial fishing boat, a tour boat, privately owned or part of a fleet. Things like that are probably more important than hours.

A privately owned Monk 36 with that Cummins sounds like the perfect boat/engine combination for long engine life.

It may scare other buyers and drive the price down. If it is priced right, Go for it.

pete