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Practices to help a marine diesel achieve a long and reliable service life

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What is the issue?
Engine development has largely been pushed forward by the automobile industry that has its focus on continuously varying RPMs and loads. These design principals are very different to typical boat engine usage which, by contrast, tend to driven at static RPMs with a steady load for long periods. Sailboats also tend to be operated for short periods to move in/out of berths and/or for long hours at light loads when charging batteries at anchor. This means a large proportion of the boat's engine lifecycle is therefore spent in the period immediately after a cold start and/or simply ticking over at a very low load. Although it may sound surprising, the typical boating lifestyle can be thoroughly destructive for modern diesel engines.

Why address this?
The practice of cold running, light or no loading will result in a very high engine wear rate and lack of efficiency and could reduce the service life of an engine to one-quarter of its capability. Several technical issues come together in the use of a marine diesel that causes degradation:

  • • Low-temperature running can lead to poor combustion which causes increased soot formation. It also causes moisture to condense in the engine. These condensates combine with any sulfur in the fuel to make sulfuric acid, which attacks sensitive engine surfaces.

  • • Overfueling creating an aggregation of unburned fuel in the cylinders. Diesel engines are inherently designed to be run loaded. As a result, they tend to slightly over-fuel when lightly loaded as only a small percentage of its output is being consumed at any given RPM. This over-fuelling cause's incomplete fuel combustion and a resultant build-up of carbon deposit or coating on the piston, rings and bore/liner of the engine.

  • • Low cylinder pressure is then brought about by soot, acid and unburned fuel deteriorate the piston ring sealing efficiency accelerating the process. This in time allows hot combustion gases, soot particles and unburned fuel to leak past the piston rings. This leads to the cylinder bore glazing and high lubrication oil consumption plus ring damage.

  • • Dilution of the lubricating oil. The pollutants and unburned fuel leaking past the piston rings starts to reduce the oil viscosity degrading its protective capabilities. This can cause premature wear of pistons, rings, liners and crankcase bearings.

All of these elements in combination can over time cause irreversibly damaged. The engine will smoke, power will decrease, and compression will reduce causing increased fuel consumption. As exhaust ports and valves foul up there will be very high of pollution/emission.

How to address this?
The good news is that modern marine diesel engines are highly reliable and durable pieces of machinery. They can no doubt withstand any or all of these bad practices for a long time. But eliminating them or adopting a few practices that vastly reduce the damage they do will help your engine to run more efficiently, be more reliable, to live longer, and run stronger.


The most difficult time for a diesel engine is when the engine is cold or running at less than the normal operating temperature which is typically 85°C (185°F), this is the time when it is completely subject to incomplete combustion. An easy way to make a big difference for your engine is to reduce this period to a minimum by bringing the engine temperature up as soon as possible.

Before highlighting the steps it is worth focusing on the biggest mistake people tend to make here. That is the practice of running the engines at idle for a half-hour before leaving a berth that is thought to be 'warming up the engine'. This is a fallacy as a diesel engine cannot come up to a proper operating temperature when it is not loaded and certainly not at idle. All the thirty minutes of idling at the dock archives is wasted fuel and carbonised injectors and pistons (which makes subsequent engine burn fuel even less efficiently), not to mention creating a lot of unnecessary white smoke and irritation for all the Mariners on the surrounding boats.

The below general engine 'Warm up' procedures will be much kinder to you, your engine, the environment and your neighbours:
  • • Prepare for departure first. When everything that is attended to get underway has been done, save for throwing off the dock lines, its time to think about starting the engine.

  • • Warm the Oil. In moderate temperatures, such as seasonal sailing, start the engine and run in neutral at a speed just above low idle for no longer than 5 minutes to warm the oil. As noted above, a diesel will not warm to operating temperature until it is under load but when the oil is cold as well it should not take on a load. You first need to let the engine idle just long enough to fully circulate the oil throughout the engine at a level temperature.

      It should be noted in passing at this point, always be certain:

    • The correct lubrication oil Experience is being used for the engine and operating environment.

    • ☐ That the oil light is off, the buzzer is silent and that the gauge is reading the recommended oil pressure.

    • ☐ That water flows from the exhaust outlet and there are tonal changes that would indicate there are problems.

  • • After about 2 minutes of warming up in neutral, you will hear the engine 'settle down' as the oil heats and reaches all of the engine spaces. At about five minutes of just above idling speed, the engine will have safely warmed the oil up enough for it to flow better and the engine may take on a light load to accelerate the process.

  • • Engage transmission and get underway using a light load whilst continuing to warm up the engine as a whole, but never exceed ½ maximum RPM during this period. If for some reason departure is delayed, or you may need to push the engine hard to get out of the berth, induce a warming load by dropping the engine into reverse at low revs and let the vessel tow against the dock lines.

  • • When temp gauge approaches operating temp, run at normal cruise rpm, usually about ¾ of max rpm or waterline length. If the engine temperature is higher than usual then review the supply to the engine. If you hear anything unusual in the sound of the engine then reduce speed, put the gearbox in neutral and take some time to investigate. A buildup of debris around the propeller will most likely be the problem.

  • • In generall use it is believed to be good to occasionally vary the rpm from about the ¾ engine speed to normal cruise rpm and not run at any certain rpm for long periods. This avoids the engine developing a fixed wear pattern.

  • • Generally when returning to a berth, after using a lot of power, it is advisable to let the diesel engines idle for about 5 mins so that they can cool down to normal levels before shutdown. Revving the engine just before shut down can be bad, especially for turbo-charged diesel engines. This is because the turbo-rotor will keep spinning even after shutdown without lubrication oil. However, some engine makers suggest to give a final rev and then shut down, to clear carbon from the cylinders. So it is advisable to check the manual or back with the manufacturer on this point.


Low load operations of diesel engines are defined as engine operations that are below 40% of maximum continuous rating. Reducing idling and /or adding load to the diesel engine is essential to help avoid incomplete fuel combustion.

The first area to focus upon here is excessive idling. Avoid running the engine at idling speed for any longer than necessary. That's running it at low idle with no load for more than 5 minutes. The best rule is, if you’re not underway, turn off the engines.

The biggest problem with leisure boating is charging the boat batteries. It's always best to charge batteries when the engine has some serious work to do, like moving the boat, and the extra charging load is not even a factor. But this is not always possible.

If you are running the engines to charge the batteries on anchor or alongside a quay place the boat in gear so that it is loaded by turning the propeller against the warps or ground tackle. This is particularly important when cold-starting, but be certain to provide the engine with the above mentioned five minutes of slightly above idling to circulate the oil before dropping it into reverse. Do be careful to disengage if a boat is preparing a berthing manoeuvre close by. I admittedly gave a neighbour an unexpected berthing manoeuvre when they found they were being pushed sideways by my unexpected wash.

If you plan to use the engine as a major contributor to the charge cycle you have to accept that nothing comes free. Additional fuel consumption, engine wear, and more frequent oil changes with top-quality oil and good filters will come as part of the parcel. Running engine under load over the long haul negates the very real concern of abusing a diesel.

However, you can make some specific arrangements to add load to the engine that will help the engine and utilise the load for productive purposes. Here are some ideas:

  • • Add the most powerful alternator you can possibly fit to the engine. This will place a heavier load on the engine and will create electrical energy in a shorter amount of time, so you can gain on both sides of the equation.

  • • Reduce the suboptimal regulator taper to address your exact battery requirements so that the alternator performance is optimised and the alternator is loading the engine for a larger percentage of the time. The typically integrated battery regulator, that decides the power output from the alternator when charging the batter, is very basic and more applicable to the charge profile required by an automobile. It applies a taper that causes the battery to 'bulk' charge rapidly at first and then drops down far too quickly to the lesser loading and output 'float' charge. The alternator's charging profile can be remedied Experience by a "smart" chip-controlled charger or a manual override. This will again place a heavier load and create more electrical energy in a shorter amount of time so you can once again gain on both sides of the equation.

  • • Use the engine for 'bulk' charging. The problem with batteries will always be the prolonged 'float' charge Experience. The first 80% can be 'bulk' charged within 2 or 3 hours, particularly so with AGM batteries that can handle a high bulk charging current. But then the charging current drops off dramatically as the 'float' phase commences and it typically requires another 9 to 10 hours. Smart chargers will maximise the 'bulk' charge phase and minimise the 'float', but it cannot eliminate it. Use solar, or wind for the 'float' charge.

  • • Consider adding engine-driven refrigeration. The refrigeration cooling process is achieved by a compressor, powered either by AC current from a generator or directly from the engine via a belt from the engine. If you select the belt driven approach you add another load whilst charging the batteries whilst also eliminating the AMPs you need to place into the batteries for refrigeration purposes. But You have to run the engine two hours every day, and maybe even twice a day, if you have food in the refrigerator.

If you have been operating the vessel for a period of low loading you can reduce the carbonisation by running the engine hard and at higher resolutions, but never outside the recommended band, so that it gets hot. As the engine temperature rises carbon traces can be burnt off.

This hard running technique of hammering vehicles up hills in high revolutions, half a dozen times, is the proven method to get older diesel automobiles through the UK's MOT emission tests. Some lorry drivers even go so far as to add petrol to the diesel every 1,000 hours. A ratio of petrol to diesel of 1:100 raises the operating temperature so that the engine decarbonises, and they pass emission regulations.

With thanks to:
Michael Harpur, Yacht Obsession.
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