Why Cold Weather Is Hard on a Marine Engine

There is something quietly melancholy about a marina in January. Halyards tapping, frost on the coachroof, and row upon row of boats sitting idle while the weather does its worst. An engine that runs weekly stays healthy; one that sits untouched for four months is a different proposition altogether. Metal contracts and expands with every freeze-thaw cycle, rubber hardens, damp salt air finds its way into every connector, and batteries quietly give up their charge to the cold.

None of this is dramatic. It is slow, cumulative damage that announces itself on the first bright morning of April, usually with a click rather than a roar. Half a day's work now, while you are still visiting the boat anyway, is the cheapest insurance you can buy.

Coolant: Strength, Level and Condition

Antifreeze does three jobs, and only one of them is about freezing. It lowers the freezing point, raises the boiling point, and carries a package of corrosion inhibitors that protect the inside of your engine from the inside out. Those inhibitors degrade with time, which is why old coolant can look perfectly fine and still be doing nothing useful.

Test the mix with a hydrometer or refractometer before the hard frosts arrive. A 50/50 mix of the correct coolant and water is the usual benchmark, good for roughly minus 37°C, which is more than enough for a British winter. Check the level in the header tank when the engine is stone cold, then look for the tell-tale crusty streaks of dried coolant around hoses, the water pump and the heat exchanger.

• Never mix different coolant technologies, as the chemistry can gel and block the system.
• Inspect the pressure cap; a weak cap lets the system run hot and lose fluid.
• If the coolant is more than two or three years old, drain, flush and refill rather than topping up.

Belts, Hoses and the Raw Water Circuit

Squeeze every hose you can reach. Rubber that has gone hard, cracked or chalky at the ends is telling you it is finished, and a burst hose in a seaway is a miserable experience. Check the alternator and water pump belts for glazing, fraying and correct tension, generally around 10mm of deflection on the longest run.

The raw water impeller deserves particular attention. Most of them fail just after a lay-up, because the vanes that have sat pressed against the cam for months take on a set and then snap when asked to flex again. If you are not planning to use the boat, either change the impeller now or make it the first job at launch, and always carry a spare. If the boat is coming ashore and hard frosts are forecast, drain the raw water side of the system where possible; trapped seawater can freeze and crack a block, a manifold or an exhaust silencer. Close the seacocks and work them open and shut while you are there, so they do not seize solid over winter.

Batteries: The Quiet Killer of Winter

Cold saps a battery's usable capacity, and self-discharge plus a handful of small parasitic loads will flatten it within weeks. A discharged battery left in the frost can freeze, and a frozen battery is scrap. This is the single most common cause of a disappointing first launch of the season.

• Charge fully before lay-up. A rested reading of 12.6 volts or better means healthy; 12.2 volts is roughly half discharged.
• Leave it aboard on a smart charger or maintenance charger, or take it home and store it cool and dry, recharging every couple of months.
• Clean the terminals and smear them with petroleum jelly or silicone grease against damp and corrosion.
• On serviceable batteries, top up with distilled water before charging, never after.
• Check the charging system too, so you are not simply storing a fault for spring.

Fuel Systems and the Dreaded Diesel Bug

Water is the enemy here. As temperatures swing, condensation forms inside a part-empty tank, and diesel bug lives and thrives at the fuel-water interface. Fill your tanks before winter to leave as little airspace as possible, and dose the fuel with a good biocide and conditioner. Drain the water separator bowl and look at what comes out; grit, cloudiness or a dark film are all worth investigating now rather than in a chop off the headland.

Change the fuel filters before lay-up rather than after, so clean fuel is what sits in the system rather than a filter slowly clogging with debris. While you are in the engine bay, change the engine oil and filter too. Combustion acids and moisture sit in old oil over winter and quietly attack bearings and bores. Run the engine afterwards to circulate the fresh oil, and if you can, turn it over briefly by hand or on the starter with the fuel off to spread a little protection around the cylinders.

A Last Walk-Round Before You Leave Her

Before you lock up and head home, do a slow lap of the engine bay with a torch and a notebook. Check the gearbox oil, the stern gland, the anodes and the condition of the engine mounts. Look for any weeping joints, chafe on wiring, or loose hose clips. Blank off the exhaust and air intake against driving rain where practical, but keep some ventilation through the boat to stop condensation, and leave a couple of moisture absorbers in the lockers.

Write down everything you have done and anything you have deferred. Then close the seacocks, grease the seacock handles, and step ashore. A little care now means that when the first warm Saturday of spring arrives, you can be the one motoring out past the moorings while everyone else is down below, swearing at a spanner.