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Old 07-16-2008
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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
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Old 07-16-2008
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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."
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Old 07-16-2008
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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.
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Old 07-17-2008
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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
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