Winterizing a yacht: what to get done before the cold sets in, and what not to do

It's not winter that ruins a boat. It's the water left inside in autumn: in the cooling circuit, in the heat exchanger tubes, in the head, in the deck shower hose. When it freezes, it splits open things that cost far more than the entire winterizing job.

Here's the system-by-system order to follow, and separately, what can safely wait and what can't be put off under any circumstances.

First, decide where the boat will winter

Half the work depends on this.

Afloat. Less fuss, but the boat stays under load: from ice, wind, and wake from passing vessels. You need working bilge pumps, reliable mooring lines, and someone who checks in on board every two weeks.

Ashore. More expensive and more trouble, but the hull gets a rest, the bottom dries out, and in spring you can calmly inspect the propeller, rudder blade, and seacocks.

In a heated boathouse. Expensive. Justified for wood and for boats with delicate finishes.

Below is the general order of work. Where the wintering option changes things, that's noted separately.

The engine: where things split most often

The raw-water circuit

This is exactly where water gets left behind. The order is as follows:

1. Close the seacock, disconnect the hose from the raw-water strainer.
2. Put the hose into a container of cooling system antifreeze.
3. Start the engine at low revs and let it draw the antifreeze through itself until colored fluid comes out of the exhaust.
4. Shut it down as soon as it does.

This way the antifreeze reaches everywhere raw water used to be: the pump, the heat exchanger, the water lock, the exhaust hose.

Choose antifreeze rated for a temperature at least ten degrees below your coldest expected night. Cutting corners here makes no sense: the price difference is the cost of one dinner out, while a cracked heat exchanger costs as much as a good vacation.

The impeller

A rubber impeller left compressed over winter often develops cracks at the base of the vanes by spring. There are two approaches: remove it and store it somewhere warm until spring, or leave it in place and replace it when commissioning.

It's simpler to remove it. While you're at it, count the vanes — we covered why that matters separately.

Oil and filters

Change the oil in autumn, not spring. This isn't a whim: used oil is acidic, contains water and combustion byproducts, and leaving it in the engine for half a year means poisoning the bearings all winter. Change it, run the engine for ten minutes, shut it down.

Change the fuel filter in autumn too.

Fuel

Leave the tank full. An empty tank collects condensation on its walls over the winter, the water settles at the bottom, and in spring you end up with water at the fuel pickup and colonies growing in the diesel.

Add biocide if the boat is kept somewhere warm. In the cold, microorganisms go dormant, but in the south winters are mild.

Fresh water and the head

Everything that holds water must be either empty or filled with non-toxic antifreeze.

• Tank and pipes. Drain them, pump antifreeze through every tap, including the cockpit shower — the one most often forgotten.
• Water heater. Must be drained; it has its own drain plug.
• Head. Pump antifreeze through it until the water is fully displaced. Empty and rinse the holding tank.
• Bilge pumps. Make sure no water remains in the hull.

Antifreeze for potable water systems is different — non-toxic, usually pink. Don't confuse it with engine antifreeze.

Batteries

A discharged battery freezes at minus ten, a charged one withstands minus forty. Hence the rule: batteries must winter fully charged.

• If shore power is available, leave a charger on in maintenance mode.
• If there's no shore power, charge fully, disconnect the ground, and come back to top up the charge every couple of months.
• Lead-acid batteries self-discharge noticeably faster than lithium ones. Over a winter without a top-up, an old bank can drop into deep discharge and never recover.

Removing the batteries and taking them home makes sense if the boat is left unattended and the batteries are new and expensive.

Hull and deck

Bottom. Wash it right after haul-out, while the fouling is still wet: dried fouling takes three times as long to remove. Check for blisters — that's osmosis, and it's better to find out about it in autumn, when you have half a year to deal with it.

Seacocks. Autumn is the only convenient time to inspect the through-hull fittings from outside. What to look for and when to replace them.

Deck. Remove everything that holds water and dirt: cushions, lines, sails. Tape or cover vents so air can still circulate but water can't get in. A boat sealed completely airtight grows mold inside over the winter.

Cover. Pull it taut so water runs off instead of pooling. A puddle freezes, the ice tears the cover and presses down on whatever the cover rests on.

Sails and rigging

Take the sails down, dry them fully, and stow them dry. A damp sail left in a bag over winter will grow mildew, and that doesn't wash out.

Remove the running rigging, rinse it in fresh water, and dry it. Inspect the standing rigging for rust streaks and broken strands; in autumn there's time for this, in spring there isn't.

Three mistakes that cost dearly

First: sealing the boat up completely. Without ventilation, mold grows inside over the winter, and cushions, upholstery, and lockers turn damp. Air needs to circulate: hatches cracked open under the cover, vent cowls, dehumidifiers in the cabin.

Second: leaving water in place because "it won't freeze anyway." The Mediterranean gets sub-zero nights in winter, and one is enough. A cheap jug of antifreeze versus a cracked heat exchanger isn't really a choice.

Third: putting off the oil change until spring. The most common mistake and the least visible one. The engine sits for half a year in acidic used oil, and the consequences show up later, not obviously.

What can wait without harm

Polishing the topsides, refreshing the varnish, painting the bottom, replacing anodes — all of this can safely be done in spring, before launch. No need to spend time on it in autumn.

The main rule: in autumn, do what protects against water and frost. Everything else can wait for spring.

What to record in case a part is needed in spring

The autumn inspection is the best time to gather information on what needs replacing: you're already reaching into every corner anyway, and there's time before spring to track down the part calmly.

• A close-up of the engine nameplate — manufacturer, model, serial number. How to read and photograph it.
• The removed part next to a ruler, from both sides.
• The stamp on the component itself — the pump, heat exchanger, and thermostat each carry their own part numbers.
• The mounting location from the reverse side, to understand the layout.

With this set of information, the part can be found within a day, even when the dealer says the item has been discontinued.

In short

Run antifreeze through the raw-water circuit, change the oil and fuel filter, fill the tank to the neck, drain and protect the fresh water and head systems, leave the batteries charged, wash the bottom right away, and provide ventilation inside. One day of work in autumn, and in spring you launch the boat instead of repairing it.