How do you winterise a boat to prevent frost damage?
I'm preparing to lay up my 34-foot sailing cruiser for the winter here in the UK. The yard is hauling her out next week, but I've never fully winterised a boat myself before. What exactly do I need to do to the engine, water systems, and batteries so nothing freezes or rots?
Answer
Winterising a boat comes down to removing water where it can freeze and split metal or plastic, and managing moisture to prevent mould and corrosion. You must flush the engine with antifreeze, drain the domestic plumbing entirely, and isolate your fully charged battery banks.
Winterising the Inboard Engine
Start by warming up your Volvo Penta, Yanmar or similar inboard. Change the engine oil and filter. Acidic used oil pits bearings over winter. Next, winterise the raw water cooling circuit.
- Close the raw water seacock and remove the intake hose from the valve.
- Place the hose end into a bucket containing five litres of a 50/50 mix of fresh water and non-toxic marine antifreeze (such as Star brite Propylene Glycol).
- Start the engine and run it until coloured antifreeze exits the exhaust, then shut it down immediately.
Never use automotive ethylene glycol antifreeze for this process. It is highly toxic to marine life and illegal to discharge. Finally, slacken the alternator belt to take tension off the bearings. Spray the engine block with a marine corrosion inhibitor like Boeshield T-9.
Fuel and Hull Preparation
Fill your diesel tank to 95% capacity to minimise the air gap where condensation forms. Water in the fuel breeds diesel bug. Dose the tank with a proven biocide such as Marine 16 or Grotamar 71. Run the engine for ten minutes to pull treated fuel through the lines and into the injectors. If lifting the boat ashore, remove the speed log transducer to prevent the paddlewheel being crushed by lifting strops. Fit the blanking plug. Ensure all seacocks are closed except the cockpit drains, which must remain open to let rainwater escape.
Protecting Domestic Water Systems
Frozen pipes split, usually at the most inaccessible plastic push-fit connector. Empty your main water tanks completely by running the taps until they spit air. Turn off the pressure pump. Disconnect the plumbing at the pump outlet. Blow through the lines using a dinghy foot pump to clear residual water from the galley, heads and transom shower. Pour a cup of non-toxic antifreeze down every sink and shower drain to protect the P-traps and sump boxes. For marine toilets, flush a litre of marine antifreeze through the bowl and out the discharge line. Leave a small amount in the bowl to keep the rubber joker valve lubricated.