Engine winterisation: a checklist for inboard diesels and outboards
A few hours of work in autumn can save you an expensive repair in spring. A properly laid-up engine is protected from corrosion, frost and stale fuel. The exact procedure depends on the model, so always follow your manufacturer's manual; the checklist below covers the generally accepted steps.
Change the engine oil and oil filter before the lay-up, not in spring. Used oil contains acids and combustion residues that can attack internal parts during months of standstill. Warm the engine up first so the old oil drains more completely. This is also a good time to replace the fuel filters and check the air filter.
The cooling system needs protection from frost. On engines with a closed freshwater circuit, check the antifreeze concentration and renew the coolant if needed. The raw-water side is usually flushed with fresh water and then filled with a suitable antifreeze mixture or drained, as the manual says. Remove the impeller for the winter or at least inspect it: a rubber impeller left in place for months can take a set, and a worn one is a common cause of overheating.
Do not leave the tank half empty for months: condensation forms inside, and water in diesel encourages microbial growth. Many owners fill the tank almost to the top, add a fuel stabiliser and run the engine briefly so treated fuel reaches the whole system. For petrol outboards, follow the manufacturer's advice on stabiliser or running the carburettor dry.
On an outboard, change the gear oil in the lower unit. If the old oil looks milky, water has got in and the seals should be checked. Flush the cooling passages with fresh water, grease the fittings and store the motor upright.
Batteries lose charge in the cold. Charge them fully and disconnect them, or take them home and keep them topped up with a suitable maintenance charger. Clean the terminals while you are at it.
If you are unsure about any step, leave the job to a qualified marine engine workshop, ideally one that knows your make. You can find engine services and boatyards near your marina in the NautMap catalogue.