Because we tend to stop using boating in the winter in the UK we need to winterise the boat engine to help preserve it through the long, cold, dark days of winter.

We need to do something to protect the cooling system of our engine because it can freeze here in the UK.

I’ve seen the harbour seawater in Pwllheli, where we used to keep our boat, start to freeze over, and if that’s the case then seawater left in the system can freeze too.

Now is a good time to change the oil and filters, as used oil can contain acidic products which can harm our engine if left unused over the winter. It’s also a good time to do basic engine checks.

How to winterise a boat engine: Which type of engine?

Most of our engines are raw water-cooled directly or indirectly.

Directly means that the raw sea water is pumped around the engine cooling passageways and exhaust system by a raw water pump before leaving the engine by being injected into the exhaust system to both cool it and silence it.

Indirectly means that the engine cooling passageway system is filled with a mixture of fresh water and antifreeze, a generic term that describes a liquid that stops the coolant from freezing; it also contains anti-corrosion chemicals, which prevents the engine from suffering from internal corrosion.

This system is then cooled by the raw water pump pumping seawater through an intercooler/heat exchanger connected to the fresh water system.

The seawater is then injected into the exhaust system to both cool and silence it.

Protection is key

In Portugal, where we now keep our boat, it is just as important to use antifreeze because of the anti-corrosion element of the antifreeze fluid.

I’ve seen cases where people have thought: “Oh it’s hot here, we don’t need antifreeze, just use fresh water,” and then wondered why some of the components of the engine cooling system, such as core plugs, rust away from the inside.

Fresh water contains oxygen – that’s what fish breathe! – and that mixture of fresh water and oxygen does corrode steel very quickly!

So to winterising, or preserving in my case, if we leave the boat for a while.

Whether the engine is indirectly or directly cooled, there is a simple way of protecting the passageways of the cooling system.

If you intend to change the oil as part of the regime then start the engine and let it warm up first – warm oil flows easier than cold when pumping it out – and change the oil and filters first.

Using my boat as an example, I turn off the through-hull raw water seacock and open the raw water strainer.

I then prepare a bucket of fresh water mixed with some cheap glycol-based antifreeze, mixed to a ratio of about 75% water and 25% antifreeze.

I start the engine and slowly tip the bucket of antifreeze mix into the raw water strainer which feeds the raw water pump.

The mix is pumped throughout the raw water system and out through the exhaust.

If you are in the water, pumping antifreeze into the sea nowadays is a big no-no.

A simple bucket system can be set up under the exhaust pipe outlet to catch the excess product so it can be disposed of responsibly.

This leaves antifreeze/anticorrosion liquid in all the nooks and crannies of the raw water and exhaust system.

I then remove the raw water pump impeller and tie it to the ignition key as a warning that there isn’t one in the engine, so no mistakes about starting the engine without a water pump impeller can be made next season!

The impeller can be checked at this time for cracks or wear and the seals in the pump can be cleaned and lubed, a trick I learned many years ago which prolongs their life.

Don’t forget that some engines use zinc pencil anodes in the engine – check whether yours does and change if needed.

Not just the cooling system

Winterising of course is not only just about the cooling system but includes the fuel system, gearbox, checking belt tensions, and lubricating other parts as well as the batteries, including the batteries in your flashlights or headtorches!

Our water tanks and house system also need some thought.

I had two Jabsco plastic strainer housings crack over the winter before I twigged what was happening.

The fresh water inside them was freezing and cracking them.

One of the jobs of engine oil is to neutralise acidic by-products of the combustion process.

Leaving this in the engine for six months over the UK winter is not such a good idea so it is best to change the oil before mothballing, and carry out a full engine service before winterisation.

Regular maintenance of the whole fuel system is more important nowadays due to the high bio content of fuel.

The perceived wisdom used to be to top the tank off to prevent condensation and help prevent the growth of diesel bug, but the high bio content in fuel has changed that. Marine 16, which produces fuel additives and diesel treatment systems, no longer recommends topping the tank.

“All the oil majors collectively agree you should not store diesel longer than six months – but let’s face it, that is totally impractical on a boat. More so as by the time it takes to get to you, the end user, it is often up to two months old anyway,” explains Marine 16’s, Peter Weide.

“Modern diesel is highly unstable, it’s been refined to within an inch of its life to remove the sulphur and now has a minimum of 7% plant oil along with animal and cooking fats. It degrades very quickly producing sludge that ends up on the bottom of your tank.

“The stability improver and antioxidant we blend into Marine 16 Diesel Fuel Complete (DFC) helps slow some of the degradation – but it cannot be stopped.”

Keep it running smoothly

People often forget about the gearbox but it’s vital to check that the parts that need to run smoothly are well lubricated.

Most gearboxes are cooled down by the raw water feed to the engine.

The aluminium casings can and do corrode, as can the intercooler pipe inside them.

Milky oil on the dipstick is the first sign.

TOP TIP The oil level is checked with the dipstick unscrewed on the Volvo Penta MS2 gearbox.

My MS2 Volvo Penta gearbox has gone through several iterations of oil recommendations.

First, we used multigrade engine oil, the same as the engine, then automatic transmission fluid; now it is GL5 fully synthetic gear oil.

Whichever you choose, check it complies with the manufacturer’s recommendations, although remember you do not need to buy the original equipment manufacturer’s oil.

Ordinary vehicle oil will be fine as long as it meets the manufacturer’s specifications.

Powering down

Batteries need to be ‘put to bed’ as well. My personal view is that I don’t leave them connected to a shore-fed mains charger.

Smart chargers can be pretty stupid sometimes! When I leave my boat, I disconnect the mains power plug.

Do I want to be connected via the earth connection to the pontoon while I’m not there? No, thank you!

Do I want to leave my batteries connected to a smart charger unattended? No, thank you.

This is because if one cell is going bad it fools the smart charger into overcharging.

I have solar panels on the handrails which are swung up to match the angle of the sun to get optimum charging.

If they’re left in the down position they work less efficiently, but if I leave my Victron solar controller connected I’ve found the batteries are kept in prime condition.

The solar doesn’t provide enough power to cook them if things go wrong while I am not there.

This system worked well in Pwllheli as well.

Most lead-acid batteries can be opened on the top, even batteries which are allegedly sealed.

The case will have 10p-sized screws on the top of the cells that can be removed to check the fluid level.

Top them up with distilled water if needed. It’s best to remove one of the terminal clamps if leaving the batteries unconnected.

This prevents parasitic devices from slowly draining them.

But do check that your auto bilge pump is connected directly! Don’t leave batteries in any torches or electronic devices; they can and do leak.

Remove and store them nearby ready for next time.

Keep it fresh

Once a year I put household bleach in my water tanks and boat plumbing systems – about a quarter of a cup in each 40gal and 60gal tank.

Don’t use perfumed bleach. I use the cheapest 5% concentrated household bleach I can buy.

Bleach this weak decomposes rapidly into chlorine and sodium when mixed with fresh water and with seawater, so the amount of chlorine entering the sea when water is discharged is negligible.

Milton, used to clean babies’s bottles is also good. Whatever you use, ensure your house water tanks are sterilised.

There is an argument for disconnecting the suction side of the house plumbing and running the pump for a while to get air in the pipes; this will allow for expansion if they freeze.

To sum up, if you are winterising these are the steps you need to take:

• Raw water system flush with antifreeze
• Remove and check the water pump impeller
• Change the oil and filters as appropriate
• Check the alternator V-belt tension
• Attend to your batteries and house water system
• On some engines such as my Volvo Penta MD22, the overhead camshaft and fuel injection pump are driven by a cambelt. This needs to be checked, which will be covered later in the engine series, along with checking the alternator/pump drivebelt tension.

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