Winterising Your Boat Engine: A Simple Checklist

Why Winterising Matters More Than You Think

UK coastal winters are a peculiar beast. It is not just the frost that catches boat owners out, but the relentless damp, the salt-laden air, and the freeze-thaw cycles that can quietly wreck an engine block. Raw water trapped in a cooling passage expands when it turns to ice, and that expansion has to go somewhere. Often it cracks a manifold, a heat exchanger, or worse. A few hours spent winterising now will save you thousands and a lot of heartache when the first warm weekend of spring arrives.

This checklist is designed for the practical boater on a mooring, in a marina, or on a trailer. You do not need to be a marine engineer. You do need to be methodical, and you need to do the job before the first hard frost settles on the pontoon.

Drain the Raw-Water System First

If your engine uses raw-water cooling – and most outboards, sterndrives, and many inboard diesels do – this is the single most important step. Salt water left sitting in the block, exhaust manifold, and heat exchanger will freeze and expand. Start by closing the seacock. Then remove the raw-water pump impeller. Inspect it, and if the vanes are cracked or set, replace it now rather than in spring.

• Close the seacock and remove the hose from the raw-water strainer.
• Drain the strainer, then open every drain plug on the engine block, exhaust manifold, and heat exchanger.
• Flush the system with fresh water using a hose and muffs, or a flush port if fitted, until the water runs clear.
• Let it drain completely, then leave the drain plugs out or finger-tight so any residual moisture can escape.

Do not skip the flush. Fresh water pushes out corrosive salt, and it also dilutes any remaining raw water so that when you add antifreeze later, you get proper protection.

Change Filters and Freshen the Fuel

Condensation in fuel tanks is a winter problem. As temperatures swing, moisture forms on the inside of a half-empty tank and drips into your diesel or petrol. Change your primary fuel filter and water separator now, and drain any water from the bowl. If you have a secondary filter on the engine, change that too. A clean filter means fewer surprises when you next turn the key.

• Replace the engine oil filter and change the oil. Old oil contains acids and moisture that corrode bearings over winter.
• Fill the fuel tank to the brim to reduce air space and condensation. Add a marine fuel stabiliser, especially for petrol.
• Check the air filter and replace it if it is grimy or damp. Damp filters can freeze and restrict airflow.

For petrol engines, consider fogging the cylinders with a storage oil. It is a simple job – remove the spark plugs, spray a little oil into each cylinder, turn the engine over briefly, and refit the plugs. That thin film protects bores from rust.

Antifreeze: The Right Type and Strength

There are two sides to antifreeze protection. For engines with a closed cooling system – the ones with a pressurised header tank – test the existing coolant with a hydrometer. It should be a 50/50 mix of antifreeze and water. If it is weaker, drain and refill with the correct marine-grade coolant. Never mix different types of antifreeze, as they can gel and block your system.

For the raw-water side, you need to introduce antifreeze after draining. Use propylene glycol, which is far less toxic than ethylene glycol if any escapes into the water. Pour it into the raw-water strainer while the engine is running on a flush supply, or use a dedicated winterising kit. You want to see coloured antifreeze coming out of the exhaust before you stop the engine. That confirms the whole raw-water path is protected. Remember to leave the seacock closed afterwards.

Batteries, Belts, and Outdrive Checks

A flat battery in spring is a miserable way to start the season. Remove your batteries and store them ashore in a cool, dry place. Charge them fully, then give them a top-up charge every month or so. If you leave them aboard, disconnect the negative terminal and make sure they are fully charged – a discharged battery can freeze and crack its case.

• Check all belts for glazing, cracks, or looseness. A slack alternator belt will not charge properly in the cold.
• Inspect the outdrive or sterndrive. Drain the gear oil and look for water or metal flakes. Refill with fresh oil.
• Check anodes. If they are more than half wasted, replace them now so they protect the drive over winter.
• Grease steering cables, throttle linkages, and any exposed pivot points.

A Final Walk-Round Before You Leave Her

Once the engine is winterised, turn your attention to the boat itself. Leave a little ventilation – a slightly open hatch or a dedicated vent – to reduce condensation below. Place moisture absorbers in lockers and under bunk cushions. If you are on a mooring, check your lines and fenders for chafe. If you are ashore, make sure the trailer or cradle supports the hull properly and that the drain plug is out.

Finally, write a note of what you did. Next autumn, that note will be your best friend. Winterising is a rhythm, and once you have done it a couple of times, it becomes second nature. Your engine will thank you with an easy start and many more seasons of reliable coastal cruising.