Winterising your marine diesel engine

It’s a sad fact that most damage to small marine diesel engines comes from negligence rather than from hard work. A few months of sitting unloved in a freezing boat can cause tremendous damage so it is imperative that we try to mitigate this wear in as many ways as possible. Good winter hygiene can add tens of years to the life of your marine diesel.

Engine Oil

Engine oil is vitally important to our engines. It lubricates the entire engine and, if it’s the correct oil, it can help reduce wear down to virtually nothing but an equally important secondary job of the engine oil is to clean the by-products of combustion away. Over the sailing season, acidic by-products from the combustion process get past the piston rings and into the oil. This makes the oil slightly acidic which is corrosive to the engine block and sump that it is contained in. This must be changed for fresh oil at the end of the season to prevent corrosive damage over the long lay-up period.

Winter kills more diesel engines than hard work does


If your boat lives in a part of the world where bodies of water (ponds etc) may freeze over the winter, then any water held in your boat over the winter lay-up may also freeze. The water jacket surrounding the engine is full of water during operation so that heat can escape. During the season, this is either raw water (salt-water or lake-water) or it is coolant which goes through a heat exchanger. If it’s raw water, then it will be liable to freeze which could crack the engine block or cylinder head as it expands.

Winterising your diesel engine: jobs before haul-out

Fill up your fuel tanks. As your engine uses fuel during the season, the level in the tanks reduces. As the tank empties, air is drawn in through a “breather” to prevent a vacuum being created. This air is often damp because of the environment and it introduces a source of water to your diesel fuel. As the air in your tank cools, the water contained in it can condense against the inside walls of the tank and run down into the fuel. Water is more dense than diesel fuel, so it will settle out to the bottom of the tank but water condensation in diesel fuel storage tanks is a common problem. Moreover, diesel bug lives in the layer between the water and the fuel so to keep your tanks and fuel healthy, fill them up often and especially at the end of the season.

Diesel School: Click to learn more about Diesel Bug

Winterising your diesel engine: jobs after haul-out

Remove your batteries. Wet cell batteries do not like the cold and they do not like being left on their own.

Change the Engine Oil. As mentioned above, if you only change your marine diesel’s engine oil once a year, then you must do this after haul-out.

Flush all sea-water from the coolant system. Whether your engine is raw-water cooled or has a heat exchanger, you must flush all raw-water from the system and replace with a quality anti-freeze. You must also do this when the thermostat is open so either run the engine for a while to get it up to operating temperature or remove the thermostat (depending on engine type). Remember that you will need to provide fresh cooling water to the system as the boat is now out of the water – most folk disconnect the hose from the skin fitting and feed fresh water in there.

Clean the raw water filter.

These come in a variety of designs but all are designed to strain out the little bits of weed and grid that get sucked in through the raw-water skin fitting.

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