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Old 16-03-2014, 23:06   #1
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Diesel Maintenance

I have an '89 Carver 4207 with twin Cat 3208's (375 HP). We plan to take the boat every weekend once we get some repairs taken care of. How often should we be starting up the engines, how long should we run them for and at what RPM?
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Old 16-03-2014, 23:27   #2
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Re: Diesel Maintenance

Diesels running at light load need to be taken up to the top of their rev range (in neutral, if necessary) every couple of hours, a handful of times, or they will carbon up.

Generally "light load" means anything under about 60% of maximum revs.

Even with the above proviso, it's not a great health option for diesel engines to regularly run at light loads.

As long as they're put away properly (including 'winterising' if necessary), there's no need to run them when the boat is not in use. Batteries will need charging by other means.

Ideally diesels should not be run for periods insufficient for them to reach a steady-state temperature.

And NO diesel should be run at a higher setting on the speed control lever than the point at which the injection pump is supplying as much fuel as the engine can burn cleanly at the revs it's running at.

You can tell this setting (which will vary depending on conditions, and some underpropped/overpowered boats will NEVER reach it) because advancing the speed lever past this point will not produce any increase in revs.
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Old 17-03-2014, 01:51   #3
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Old 17-03-2014, 04:57   #4
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Re: Diesel Maintenance

How about the OP's question about how often to start them?
I think much wear occurs when an engine is started (relative to steady state running.) IMO the less you start them the better. What say you all?

Regarding maintenance, "experts" agree that providing good fuel to the engine is probably job #1. Make sure the lubrication is correct (we change O & F every 100 hours) and that the coolant is no more than 2 years old.

What is the group's opinion on analysing the oil on a regular basis?
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Old 17-03-2014, 05:13   #5
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Re: Diesel Maintenance

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Originally Posted by Andrew Troup View Post

Generally "light load" means anything under about 60% of maximum revs.
With respect, load and RPM are not the same. Problems of bore polishing and carbon-fouling are worse at light loads and higher RPM, as far as I know, which can be observed in constant-speed gensets run without a load overnight when aircon cycles off.

I'm not sure I believe that less than 60% RPM is harmful for a boat's prime mover, if you are in gear and making way. There is some controversy about this, but this guy: Bore glazing suggests that as long as a prime mover is in gear and driving a prop, and has been properly broken in, it cannot come down with bore glazing or polishing.
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Old 17-03-2014, 05:24   #6
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Re: Diesel Maintenance

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Originally Posted by mikekomm View Post
I have an '89 Carver 4207 with twin Cat 3208's (375 HP). We plan to take the boat every weekend once we get some repairs taken care of. How often should we be starting up the engines, how long should we run them for and at what RPM?
Diesel fuel will varnish parts just like gasoline if left to sit. The local diesel injector shop advises running the engine one a week for 20 minutes. When stored, I try once every other week.
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Old 17-03-2014, 18:28   #7
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Re: Diesel Maintenance

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Originally Posted by Dockhead View Post
With respect, load and RPM are not the same.
That's true, I was oversimplifying ...

So I will qualify what I said.

In the absence of any or all of the following:

Variable pitch prop
Changes due to towing or carrying heavy cargo
Punching into steep seas
Coping with (or benefiting from) major windage or sails
Out of gear running
(and other similar considerations)

Load and RPM for a propellor-driven boat are directly related.

I was hoping people needing a more nuanced view would be able to apply their own corrections for variables like those I've listed.
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Old 17-03-2014, 18:48   #8
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Re: Diesel Maintenance

@DotDun:

I'm a bit skeptical about varnishing being a problem if an engine is not regularly started - at least, a comparable problem with the inevitable problems which come from starting an engine from cold, like the inevitable condensation in the bores.

Admittedly sulphuric acid in that condensate is less problematic in this low-sulphur diesel age, at least in some parts of the world.

More of an issue (I think) is the irregular and delayed detonation (often incorrectly believed to be premature detonation) caused by the atomised fuel recondensing in a cold engine. Hence the "clatter" which all diesels exhibit until the bores and combustion chambers warm up. The oil is (as yet) not well distributed through the engine to cope with the severe shock loadings from the irregular detonation.

In a 'gold standard' installation, the coolant and the lube oil would both be able to be pre-heated to running temperature, and the respective circulation pumps both be run for a few minutes before starting the engine, and if this was done (and the engine spun a few times with the decompressors open and no fuel being injected), I don't see any impediment to running the engines once a week or fortnight: I think it would be beneficial.



This is just anecdotal, but I've just been present at the inspection of the injector and associated parts of a single-cylinder marine diesel engine which has had multiple long periods of inactivity, often not started even once in a year.

The injector was the original, never removed in 20 years. There is no evidence of varnish accumulation.

Maybe some sources of diesel are (or in former years, were) more prone; I can't comment on this, but I've not personally found varnishing to cause identifiable problems, or had it visibly affect parts I've stripped and checked.

There may admittedly have been injector pump problems I have been unaware of arising from this cause, though ...
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Old 17-03-2014, 18:55   #9
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Re: Diesel Maintenance

I think Dockhead is narrowing the scope of potential light-load problems unrealistically when he talks solely about bore glazing and polishing.

The problems of a diesel running at light load importantly include the formation of carbon particles. Partly this is because the dynamics of swirl at low piston speeds are suboptimal, partly it's because an engine at light load will be running too cool (particularly a typical marine installation), resulting in inefficient and incomplete combustion.

These carbon particles cause multiple problems. As well as accelerating wear, they build up (notably in the exhaust elbow, at the injection zone) causing excess back pressure, accelerating the rate of carbon production. Furthermore carbon in this graphitic form acts as though it were a highly "noble" metal, causing that elbow to rot from the inside out, and causing corrosion elsewhere in the engine.

Thinking more about his contention that load and RPM are not inextricably linked:

I struggle to imagine a real-life situation where a marine diesel engine with a fixed pitch prop, running at low revs, would not also be lightly loaded, unless the sterngland was binding, or the prop was fouled.

If there were a headwind causing the engine to struggle, it's not plausible that the speed control would not be used to increase engine revs, to take one example...

I think his objection is a misapplication of a general dictum (Load is not necessarily tied to Revs) to a particular situation where there is effectively a tie.
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