Why does my boat need winterising?

Winterising your boat protects the engine, plumbing, and structural systems from freezing temperatures that can crack engine blocks and split pipes. It also prevents fuel degradation, internal corrosion, and battery failure during months of inactivity.

The threat of freezing water

Water expands by about 9% when it freezes. This exerts immense pressure on anything containing it. For a Yamaha F200, the yard flushes the cooling system with fresh water to remove salt. Mechanics tilt the outboard down to drain the block. On inboard engines like a Volvo Penta D3, they circulate non-toxic propylene glycol antifreeze through the raw water circuit. A cracked engine block from ice damage is rarely covered by insurance. Replacements easily cost upwards of £4,500.

Domestic plumbing is equally vulnerable. A standard 20-litre Quick calorifier or the plastic valves inside a Jabsco manual toilet split if left full of fresh water over winter. Bronze seacocks like Pegler or DZR brass fittings must be left in the open position (or half-open if ashore) so trapped water inside the ball valve can drain. Shut valves trap water. That water freezes and splits the valve body.

Preventing fuel degradation

Modern fuels degrade rapidly when left sitting. E10 unleaded petrol attracts moisture from the air. This causes phase separation where a corrosive water and ethanol mixture drops to the bottom of the tank. Marine diesel containing FAME biodiesel is highly prone to diesel bug and sludge buildup.

Yards treat fuel tanks with a marine-grade stabiliser like Quicksilver Quickstor or Sta-Bil. They run the engine for ten minutes to draw the treated fuel through the lines and injectors. Mechanics usually fill the fuel tank to 95% capacity. This minimises the air space where condensation forms during temperature swings.

Fighting internal corrosion

Engines sitting idle for six months are prone to internal rust. Mechanics spray fogging oil into the air intake to coat the cylinder walls and valves with a protective film. They also change the engine oil and filter. Used engine oil contains acidic combustion by-products that pit bearing surfaces over time.

Expect the lower unit gearcase oil to be changed too. This check is vital. If a fishing line damages the prop shaft seal, water gets in and turns the gear oil milky. Left untreated, that water freezes, expands, and cracks the gearcase casting. Finally, yards apply marine grease like Quicksilver 2-4-C to the steering ram, throttle linkages, and outboard pivot points. The propeller is often removed. Mechanics grease the splines and re-torque the prop nut (typically to 55 Nm on a 200hp outboard) to prevent it fusing to the shaft.