Laying Up: Inboard engines

Laying Up: Inboard engines by Cathy Brown

One of the most important jobs to do when laying up is to winterise the engine. It is one of the most expensive-to-replace items on the boat, and also the one most likely to suffer damage if there is a big freeze while the boat is out of the water.

The three potential enemies of the engine are freezing water, corrosion and condensation and the aim of winterising is to banish the first and minimise the other two.


The engine is largely protected from freezing while the boat is afloat, in British waters at least, because the sea temperature is extremely unlikely to get anywhere near freezing point, and so the engine remains effectively insulated.

Out of the water it is another matter: air temperature will often fall below freezing point during an average winter, so the engine must be protected from frost damage. One way to do that might be to provide some heating, and more and more people are leaving low power heaters on their boats all winter, where electrical power is available in boatyards.

But it would be dangerous to rely on this, because there is always a chance of power cuts, and they are most likely to occur during times of extreme weather. So even if you leave a heater on your boat, you should still frost-proof the engine.

If the engine has fresh water coolant, it must be checked, topped up or replaced with a 50/50 water/antifreeze mix as necessary.

Opinions are divided about whether raw water should simply be drained, or replaced with antifreeze mix. Draining will remove the risk of frost damage, but it can encourage condensation and therefore corrosion.

So it is better to replace it with antifreeze mixture. How this is achieved will vary from engine to engine. It may involve running the engine feeding the new liquid in from a bucket and hose. It may simply be a question of pouring some antifreeze in from the top. Read the manual and work out the best system for your engine.


Plug the air inlet and the exhaust outlet with oil-soaked rags to keep moisture out and prevent condensation.

Also fill the fuel tank to prevent condensation. Water in the fuel tank provides the breeding ground for the dreaded diesel “bug” so it is a good idea to add some “bug killer” (bactericide) when you fill up for winter. (Always add just before filling to make sure it gets mixed in properly).


Don't be tempted to leave the engine service until the spring. Now is the time to change filters etc to leave everything clean for the winter and so reduce the risk of corrosion. Change the primary fuel filter and check the secondary fuel filter for water or sediment.

Change the engine oil and filter, and run the engine to distribute the clean oil. Leaving dirty oil in the engine over the winter can cause damage. Change the gearbox oil too. If you have a sail drive, this can only be done out of the water. It's a good idea to make a note of all oil changes in the ship's log.

Some people advocate running the engine briefly once a month to keep it condensation free. But be aware of the need to replace/recirculate antifreeze. Others suggest turning it over without actually starting it each time you visit the boat. Don't forget to take the oily rags out of the inlets first.


You can either spray the engine with WD 40 or cover it with a cloth to help to guard against condensation. If you do leave a heater or dehumidifier on the boat, leave the engine compartment open so that the air can circulate freely, and the engine can share the benefit.

Don't forget to change the engine anode (if fitted) and get replacements for hull and prop shaft anodes, too.