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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Marine Diesel Engine Hrs
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Thread: Marine Diesel Engine Hrs Reply to Thread
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Topic Review (Newest First)
07-17-2008 07:24 PM
chsjohn I still don't think that really good comparisons can be made between over the road (OTR) and marine engines, except maybe at idle. A diesel genset running in a low load condtion would replicate the battery charging conditions described quite closely. Conversations with the engineering staff at Cummins marine group here in Charleston would seem to support my conclusions.

Trying to compare OTR and marine engines under load isn't easy. The OTR diesel engine is usually operating at a much lower load for a given rpm than a marine diesel or generator engine. They also typically operate well below their maximum rpm. For an OTR engine to be placed under the same load conditions as a marine or generator engine it would need to be on a never ending uphill grade and operating at about 80% of it's maximim design rpm. These are the types of loads that a marine (crusing at or near hull speed) engine operates under. It is very simaler to a genset running under full load.

Carbon builup in marine engines is most often caused by high fuel schedues and low rpm, but can be reduced by ensuring that the engine is not running below it's design temp.

Moisture is a concern, but as long as the engine is brought to full operating temperature before shutdown it usaully is not a problem. Short run cycles that do not reach full operation are usually the primary cause of excessive condensation inside an engine.

The most common cause of engine failure is overheating. Overheating by itself doesn't usually do the damage. It causes a loss of lubrication by breaking down the oil flim between bearing surfaces. I would rather have an oil temp gauge than a watertemp gauge if choosing between the two.

The point that I was really trying to make is that the opreatiing temperature of an engine (hot or cold for coolant and oil) at a specific load and rpm has a tremendous impact on engine longevity.

Just my .02
07-16-2008 10:58 PM
sailboy21
Quote:
Originally Posted by camaraderie View Post
... If you run your engine at near full throttle for 10 hours, then let it idle for 6, then wring it out again for 10, the idle time will indeed have little if any impact. ...

I wonder how this discussion applies to commercial trollers... A good number of their boats are powered by diesels you might find in some larger cruising sailboats. They idle at troll speed for oodles of time (sometimes days), and then they haul butt back in. I find myself thinking they could save themselves in fuel costs (esp being so high now) if the throttled back a bit, but maybe this is their way of cleaning the pipes so to speak. I tried doing a little trolling for salmon from my boat.. but at fast idle I was still pushing over 2.5 knots on a calm day. Couldn't bear the thought of dropping her to a low idle just to catch a little fish.
07-16-2008 10:43 PM
camaraderie chsjohn...many cruisers will spend a lot of time on the hook just charging batteries if they don't have alternatives to their engine. We've had several discussions debating the damage this can do and comparing to truck diesels which idle for long hours without apparent harm. I wrote BoatUS and boat system guru Don Casey on this issue and here is what he told me:

"Diesel trucks idling in a Dakota truck stop have no implication for a 40-horse diesel powering nothing more than a 12-volt alternator. For one thing, the idle time in the truck is between long and continuous hours of the engine operating at full power. If you run your engine at near full throttle for 10 hours, then let it idle for 6, then wring it out again for 10, the idle time will indeed have little if any impact. That is not what we are talking about. In the case of a boat diesel used for charging the batteries, you fire it up each day just to let it tick over at a fast idle for an hour or two, then shut it off. And you do that again tomorrow and the next day and the day after that. The engine runs too cool for complete combustion, resulting in inevitable carbon build-up inside the engine and the exhaust system. Equally destructive, warming the engine attracts moisture into the crankcase but the idling engine never gets hot enough to drive this accumulating moisture out of the lubricating oil. The result is corrosion of the bearing surfaces. Every diesel manufacturer will tell you that unrelieved light loads are murder on their engines. Today you will find very few long-term cruisers depending on the main engine as the primary source of electrical power. The advantage of solar panels and wind generators is not just lower fuel costs. They reserve engine hours for propelling the boat, making a good-quality diesel engine in a sailboat almost immortal."
07-16-2008 10:15 AM
chsjohn Good point about the battery charging operation (didn't even think of that). I don't have any guess as to what number of hours are spent in that type of low load condition, but I suspect that most hours on an engine for the average cruiser would be spent used for propulsion ( I could be wrong I often am). If the engine has been proped correctly the load condtions would be fairly simalar to a genset (constant rpm fairly high in the power curve). I'm sure that the racing crowd isn't using their engines like cruisers, but they wouldn't be as concerned with the high number of engine hours because their engines aren't being used for the extensive amount of motoring that many cruisers do. Also, not many cruisers are going to motor their boat at speeds much lower than hull speed (unless manouvering).

As for operating conditions. The marine enviorenment is a harsh place to operate, but if the engine is freshwater cooled with a heat exchanger and is properly enclosed it shouldn't have as signifigant an impact.

Just my .02
07-16-2008 03:10 AM
sailingdog CHSjohn-

Comparing a commercial generator diesel to a marine diesel isn't really a good comparison IMHO. A commercial genset diesel will run at a fairly constant speed and load, and without the harsh marine environment. Saltwater and humid air does a number on engines... so does the highly variable loading on a boat's engine. Some days you're just idling it to recharge the batteries, others, you pushing it to make the last call at the bar...
07-16-2008 12:39 AM
chsjohn
Pre-oiler

Don't know if this applies to small engines (don't see why it wouldn't). I know of some diesel engines used in commercial generator applications that have run over 30,000 hours and are still running. These might be a better comparison than an over the road engine that is typicly only running at about 25-40% load. Marine engines and generator drives usually run at much higher loads. Preoilers supply oil under pressure before the engine starter is engaged. The system works by storing oil under pressure and a solenoid valve is opened just prior to starter engagement.. There are interlocks that engage the starter after oil pressure builds up. Start up is probably the hardest thing an engine endures on a regular basis.

Oil temp is also critical for engines running under a high load. Many marine engines have larger oil pans for increased quantity and this will often reduce temperatures to an acceptable level. It isn't uncommon for a marine engine to have a low coolant temp (because of the large quantaties of coolant available), but also havea high oil temp (due to higher bearing loads). Oil coolers are a must for engines running under very high loads, but they also must be properly designed or low oil temperatures will cause damage as well. Thermostatically controlled oil coolers are the best way to go. They function much like a radiator with a thermostat and will modulate the flow of oil or water to maintain proper oil temperature.

It is also criticly important to allow an engines temperature to stabilize after start up (ie: warm up).

Preoilers and oil-coolers don't come cheap, but they can more than pay for themselves in the right application by increasing the engines life.

Just my .02
07-12-2008 08:31 AM
cardiacpaul I just took a look at an '85 Endeavour with a hair over 100 hours, yea, 100 hours on it, and it was the cleanest thing I've seen years.
On the other hand, I know a fella with a perkins 4-108 in a '76 CSY, over 3000 hrs, its as cruddy as a 50 yr old crak who*e, but they both run like a charm.
Why?
Oil changes, clean fuel, and first aid before they need it.

Boat engines are like women...

Give them what they want, before they know they want it, and they won't leave you.
07-11-2008 06:29 PM
turfguy Thanks for all the great comments guys. I really appreciate all your knowledge.
turf
07-11-2008 06:00 PM
sailboy21 Puradyne? Haven't seen a oil filtration system on a pleasure boat but hey, why not? But 750 hrs... that is 5-7 years at what seems to be average use. Last summer I motored about 2000 miles and only put a tad over 400 hrs on the old beast. Even with a purification system I would be worried about keeping oil that long. One thing a filter can not do is replace detergents friction modifiers and other additives needed in modern engines... although I guess replacing oil lost due to normal consumption would replenish some of this. Interesting concept.
07-11-2008 04:52 PM
speciald Srvice is important. I did major surgery on mine at 2000 hour because it was 2000 hours - new injectors, pickeled heat exchangers, pulled and cleaned turbo and exhaust elbow. Still runs like new and burns no oil. I put a Puradyne oil filter system on 2 years ago and only change oil every 750 hours.
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