William Mills recounts his experiences watching a sailing yacht skipper overhaul his marine diesel engine after it has been flooded with seawater in
Understanding Marine Diesel Engines
After a prolonged period of no use, we struggled to start our three cylinders Volvo 2003 marine diesel engine.
We held the starter button pressed for ages as it turned over and belched smoke from the exhaust.
Finally it started, and off to sea, we went on a sunny yet crisp winter’s day.
An hour later we decided to return to harbour. The engine won’t start even though it was still warm and should have started immediately.
Eventually, and after much nail biting anxiety, it did go, but as we were on a falling tide we failed to reach our mooring in time.
Our pontoon was tantalisingly close. Leaning over the bow, I could almost touch it.
“Put some revs on!” I implored my skipper as I was longing for a pint at the welcoming inn ashore.
He followed my advice and accelerated hard.
Sadly the boat was already firmly stuck aground and didn’t budge, and the water became very cloudy as it was stirred up by the propeller.
Fortunately, we had some drink on board and so spent a pleasant evening watching the sunset as the tide went out and then inched its way back in.
The next time we planned to set out, we got all dressed up and ready to depart an hour before high water, the engine simply won’t start even though we turned it over again and again until the batteries were nearly flat.
“Diesel engines run at their best when cold,” said the mechanic.
“They should also fire with the first turn of the starter motor,” he added.
If it doesn’t, we have lots of worrying diagnoses. Usually, diesel engines won’t start because water has got into the fuel.
The thing is if that is the cause it won’t start at all.
So how is it that some engines will start, but need to be turned over and over by the starter motor seemingly forever before finally chugging into life?
When the engine is working properly, the piston compresses the fuel gases inside the cylinder until it explodes, which usually is immediate.
However, if these gases are escaping through a leak it will be much more difficult to start.
The engine block is made from cast steel which when it gets hot it doesn’t expand but rather, cracks.
Understanding Marine Diesel Engines
All it takes is a small crack for pressure to escape resulting in underperformance and difficult starting.
The mechanic removed the oil dipstick and ran his fingers down it to reveal a grey, creamy paste.
Inside the engine are a number of pathways which enable engine oil to lubricate the moving parts and cooling water to circulate.
A tiny fissure, or crack and the two will emulsify into a sludge eventually blocking the whole engine up.
The mechanic next removed the oil filler cap which uncovered more of the telltale sludge.
“Turn it over,” he asked, and as I did so, he sniffed.
“The oil sump is leaking exhaust fumes,” he told me dolefully.
“New engine?” I asked and alas, he nodded.
Yet the age of the boat didn’t merit the cost of a brand new replacement, so the owner looked online and discovered he could buy a second hand one with a year’s warranty for around £1,500, which is far cheaper than a new one.
But first, we decided we would give the current engine one last try.
It took around ¾ of an hour to remove the injectors, fuel pipes and lift the head off. The cause of our predicament was quickly apparent.
Two of the three cylinder heads looked to be in good working order, yet the third showed signs of rust.
Although the cooling water which circulates inside the engine is freshwater, a secondary flow, this time of seawater comes up through a seacock and passes through a heat exchanger and is discharged via a tube attached to the exhaust pipe.
It’s all driven by a small rubber paddlewheel contraption called an impeller pump which is attached to the engine’s main flywheel.
If we rev up the engine in shallow water there is a risk that the seabed will be stirred up by the prop wash allowing sediment to be sucked up, and damage the impeller as it forces its way past.
If the engine is stopped at this crucial moment, this can end up dripping into the cylinder head, and wedging one of the valves open.
Any accompanying water will seep past the piston rings, and down into the oil slump where it emulsifies.
The cylinder head, having been removed was taken to an engineering workshop. There, it was machined flat and the valves reset before being returned to us for reassembly.
Once it is all back up and running it is there are a few golden rules to remember in order to prevent the problem reoccurring.
Firstly, the condition of the impeller in the seawater pump is crucial. When you next have your engine serviced, ask your mechanic to show you how to replace it.
It really is very easy – or just type ‘change impeller on marine diesel engine ‘ into U-tube search box and chose from the many short videos available.
When the engine is running there should be a continuous stream of water flowing out of the engine exhaust making reassuring splashing sounds. If this changes it needs to be dealt with now, and can’t be put off until next winter’s refit.
So always carry two spares, to be on the safe side, and change immediately if engine overheating is suspected.
When the annual service next comes around, ask your mechanic to check, and replace where necessary, all the thermostat wires to make sure the heat alarm works.
If it goes off, stop the engine straight away and treat the situation as a breakdown, unless of course, you are in a genuine emergency situation and have to keep moving.
Replace the seacocks and make regular inspections of the entrance area to the underwater intake pipes, these should be completely clean of any fouling.
When buying a second hand boat, the engine should start first turn, even if it is cold, just like your car at home.
However, it should be borne in mind that a seller can arrive early, start a difficult engine and warm it up, so as to ensure it starts easily when a potential buyer is present.
Given the cost of replacing an engine which has cracked due to overheating, it is well worth asking a mechanic to pressure test the cylinders.
It’s a simple procedure to carry out and reassures the buyer that there is no compression loss from inside the engine.
Always allow the engine time to warm up steadily when first starting, particularly in cold weather. There should be no initial throttle on the engine, as when starting the car.
Once an engine has overheated to the extent that it has cracked the engine block it will require major surgery, including replacement.
So long as the cooling water flows through in a continuous, plentiful current and the overheating alarm is working properly our engine should give us many years of problem free use.
Understanding Marine Diesel Engines
