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Old 02-07-2006, 03:32 PM   #1
Auzzee's Avatar
Join Date: Jan 2005
Home Port: Darwin
Vessel Name: Sandettie
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Default Diesel engine

Periodically, I fire up my diesel whilst I am at the dock. My owners manual (GM Detroit 3-53) says I should idle the engine for 5 minutes before putting it under load then, before shutting it down, idle the engine again for five minutes. I have been in the habit of loading it by turning on refrigeration and other electrics and running it in gear. I am not a mechanic. This afternoon I was told that running the engine at idle will glaze the cylinder bores, and that running it under load against mooring ropes is of no use. I am self employed and currently working all the hours I can in an effort to power-boost the cruising kitty. In Darwin, I keep the boat in a marina managed by lock gates, so I rarely am able to run the diesel unfettered. Is my routine correct, or am I contributing to possible engine damage?


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Old 02-07-2006, 07:03 PM   #2
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Hi David,

Your informant was right. The worst thing you can do with a diesel is to run it at idling speed. Even running it with partial load, such as against mooring lines, will not help unless you get the revs up.

A diesel should be run at about 80% of its maximum load for prolongued life.

Idling before running under load and doing the same in reverse afterwards is a good policy only because it warms up the engine and cools it down slowly. Also, when warming up, you are spreading the engine oil around. Starting a cold engine and going direct to operational speeds and loads would result in added wear due to the initial lack of lubrication.

I spent a couple of years as a very junior marine engineer and, on merchant ships, we always ran one of the lube oil pumps (electically driven)for about 5 minutes before starting the main engine, just te ensure adequate lubrication. Obviously this cannot be done on a smaller engine without a seperate pump so that initial few minutes operating without load is designed to do the same job.

My advice is not to run the engine, even if this means leaving it for a month or two, unless you can get out and put the thing under load. Make sure also that the engine runs long enough to be really warmed up.

I would advise, if you have a mechanical stop or decompression lever, using the starter motor to turn the engine over once a week or so whilst operating the stop / decompression mechanism. This will ensure that some oil is transported through the engine, lubricating and protecting from corrosion, as well as changing the position of the pistons and valves so that they don't get 'stuck' in one spot.

Good luck with it.



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Old 02-08-2006, 12:04 AM   #3
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Hi Nausikaa,

Appreciate your input as I'd perhaps wrongly assumed it may help to periodically run the engine when moored - as opposed to not running it at all.

Re your recommendation about 80% 'loading'? If you mean 80% of the revs Q1 is surely the loading will be the same whether underway or moored?

Also - our Yanmar 75hp maxed will do 3,500 revs. Our 'normal/ fuel efficient' motor cruising revs are usually 2,000 or possibly 2,500 in a blow.

So Q2, is it actually better for the engine if we were to adopt something closer to 2,750 (80%) at all times?


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Old 02-08-2006, 01:49 PM   #4
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Hi John,

The load issue is a complicated one. It depends not just on the engine but on the propeller. If you visulise an engine coupled to a variable pitch prop, you can with feathered blades run the engine at 80% revs but the load is marginal. On the other hand, if the engine develops say 100 Kw (or HP) you can, be increasing the pitch reach a stage when the engine is developing 80% of its power output (i.e. 80 KW or HP) without reaching 80% of mamximum revs.

When you reason that the effect should be the same, under way or berthed, then I would agree, if you can load the engine sufficiently. However, that is not normally possible because without moving through the water you will not get the nowmal flow past the prop, which will mean the prop operates inefficiently and may even cavitate. In your case, running at 2,700 revs (ot there abouts)would increase the losd but it would not reach 80%. Also, the problem arises, in most berths, that the prop wash you develop at those speeds would be distyurbing for boats berthed around you. If it is not, great! Go ahead. otherwise, don't just run it and let it idle or run at low power output.

The answer to your second quetsion is yes! But, again it depends on the load. When you are running against a blow, the load is greater than running with the wind. Not a lot greater I will admit. The theory is, if you think of a diesel driven vehicle on a hill, going up the hill (added load) the revs go down but the load is high, going downhill the revs go up but the load is marginal.

As the load, at sea, does not vary so much unless you are towing or breaking ice, then running at 2,750 revs your engine will develop about 80% of its power output (assuming you have the correct prop fitted). This will, all things being equal, give your engine a longer term of life.

Why is this, you may ask? Well, almost all diesels are designed to be in 'balance' at 80% load. It matters not if the diesel is normally aspirated or fitted with turbo blowers. MTU however produce an engine with a stageless turbo blower which makes their engines effective over a much larger range of loading. This type of engine is used in such vessels as coast guard cutters which at times are to run at high speeds and other times are running for long periods at low speeds.

The glazing of the cylinder (you mentioned this in an earlier input) occurs at lower loads because the engine is not generating sufficient heat; the piston rings have thus not expanded to their most efficient operating size alowing blow back when the fuel ignites. This raises the pressure slightly in the crank case but also causes a slightly less efficient combustion which, in turn, results in deposits on the cylinder liner.

Incidentally, the 80% loading, as you rightly pointed out, is not the same thing as 'most economical speed'. What is good for the engine is not necessarily good for fuel efficiency.

I hope my explination has been understandable and that your engine performs well for very many years.

All the best,


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