Ask any yard what damages boats most, and hardly anyone answers « rough weather at sea ». The expensive damage happens in a quiet corner of a compound between November and March, to a boat that nobody has looked at since the last weekend of the season. Winter does not need waves to be destructive. It needs water, time and a cover that was fitted in a hurry.
The five, in order
- Clean and dry completely, inside as well as out.
- Drain and winterise before the first hard frost, not after it.
- Stabilise the fuel and run the engine so treated fuel reaches the system.
- Remove the battery, charge it, keep it somewhere cool and dry.
- Cover it properly, with ventilation and enough tension to shed snow.
- 1 What actually goes wrong over a winter
- 2 Tip 1. Clean it as if you were selling it
- 3 Tip 2. Drain everything, then winterise the engine
- 4 Tip 3. Deal with the fuel before it deals with you
- 5 Tip 4. Take the battery out
- 6 Tip 5. Cover it properly, and ventilate it
- 7 Then go and look at it
- 8 Common winter storage questions
What actually goes wrong over a winter
Three mechanisms account for nearly all off-season damage, and understanding them makes the tips below feel less like a chore list.
Freezing water expands. Any water left in an engine block, a raw water circuit, a livewell or a freshwater plumbing run can crack the housing it sits in. This is the single most expensive winter failure and it is entirely preventable.
Trapped moisture breeds. A sealed boat with damp carpets and closed lockers becomes a mould incubator. What survives the winter is usually not the boat but the smell.
Chemistry keeps working. Salt keeps corroding, fuel keeps degrading and a discharged battery keeps sulphating, all without anybody touching the boat.
Tip 1. Clean it as if you were selling it
Wash the hull down before it goes into storage rather than after it comes out. Algae, salt film and grime left on the gelcoat over several months are considerably harder to remove in spring, and salt in particular keeps drawing moisture out of the air onto metal fittings all winter.
Inside, take out everything that is not fixed down: food, drinks, soft furnishings, lifejackets, tools, paperwork. Anything organic will attract rodents, and anything absorbent will hold damp. Vacuum, wipe down, then leave lockers, hatches and the fridge propped open so air can move.
The mistake that costs most here: putting the cover on over damp carpets. A boat that is 90% dry when you close it up stays 90% wet for four months.
Tip 2. Drain everything, then winterise the engine
This is the job with a deadline attached. Get it done before the first hard frost, because one cold night is enough to crack a block.
Drain the engine’s raw water circuit, the bilge, livewells, freshwater tanks, the water heater and any washdown pump, then add the appropriate marine antifreeze where your engine manufacturer specifies it. Change the oil and filter at the same time, at the end of the season rather than at the start, so the acids and moisture in used oil do not spend the winter sitting against bearing surfaces. Fogging oil into the cylinders protects against internal rust while the engine sits.
Follow your engine manual’s procedure specifically. Raw water cooled, freshwater cooled and closed-circuit systems all winterise differently, and getting it wrong on an unfamiliar system is a good reason to have the yard do it. If you are at all unsure, a professional winterisation is far cheaper than a cracked block.
Tip 3. Deal with the fuel before it deals with you
Petrol degrades in storage, and petrol containing ethanol can absorb moisture from humid air and eventually separate, which no engine will run on cleanly.
Add a fuel stabiliser at the dose on the bottle, then run the engine for several minutes so treated fuel reaches the lines, filter and carburettor or injectors. Stabiliser sitting in the tank while untreated fuel sits in the fuel system does half a job.
Opinions differ on tank level. A nearly full tank leaves less air space for condensation to form in, which is the usual advice for boats stored outdoors in damp climates. Some owners in warmer or drier conditions prefer to store nearly empty and start the season with fresh fuel. Check what your engine manufacturer recommends, because that guidance is model specific.
Replace the fuel filter and water separator element as part of the lay-up rather than in spring, so the system goes into storage clean.
Tip 4. Take the battery out
A battery left connected in a cold boat will slowly discharge through the parasitic loads it is still feeding, and a discharged lead-acid battery both sulphates and becomes more vulnerable to freezing.
Disconnect it, take it off the boat, charge it fully, and store it somewhere cool, dry and frost-free. Top it up periodically over the winter or use a maintenance charger designed to hold a charge without overcharging. Clean the terminals before it goes away and again before it goes back.
The mistake that costs most here: leaving a battery on the boat over a whole winter and expecting a jump start to fix it in April. It usually will not, and you find out on the first good weekend of the year.
Tip 5. Cover it properly, and ventilate it
A cover has two jobs that pull against each other: keep water out, and let moisture escape. Get one right and ignore the other and you trade rain damage for mould damage.
Use a cover made for your boat, ideally custom fitted. Fit it taut, with a frame or support poles underneath so snow and rain run off rather than pooling. A pool of water on a slack cover becomes surprisingly heavy and will stretch, tear or collapse the cover onto the boat.
Then ensure air can move. Shrink wrapping is excellent protection but must include vents. Under any cover, leave lockers ajar and consider desiccant containers in the cabin, replaced through the winter as they fill.
Indoor or outdoor: what the choice actually changes
| Aspect | Indoor storage | Outdoor storage |
|---|---|---|
| UV and weather | Removed as a factor | Cover quality does all the work |
| Freeze risk | Reduced, not eliminated unless heated | Full, winterise thoroughly |
| Ventilation | Easier, hatches can stay open | Must be designed into the cover |
| Access for checks | Usually straightforward | Awkward once shrink wrapped |
| Cost | Highest, particularly climate controlled | Lower, plus the cost of a good cover |
Then go and look at it
The sixth thing, which nobody puts in a list of five, is visiting. Check the boat two or three times over the winter: cover still taut and undamaged, no water pooling, no droppings or chewed wiring, no condensation running down the inside of the windows, support stands and trailer tyres still where they should be.
Everything on this page is preventive, and prevention only holds if somebody verifies it occasionally. A tear in a cover found in December is a repair; the same tear found in April is a re-upholstery job.
Common winter storage questions
Do I need to winterise if I live somewhere that rarely freezes?
The freeze protection becomes optional, the rest does not. Fuel still degrades, batteries still discharge, salt still corrodes and damp still breeds mould in a mild climate. Adjust the antifreeze step, keep the other four.
Should the boat be stored on its trailer or on stands?
Either works if the hull is properly supported. On a trailer, check that the bunks or rollers support the hull along its designed load points and take some weight off the tyres if it will sit for months. On stands, that is the yard’s responsibility and worth confirming rather than assuming.
Is shrink wrapping better than a fabric cover?
Shrink wrap seals better and sheds snow well, but it must be vented and it is single use. A good fabric cover is reusable, easier to lift for a mid-winter check, and depends more on how well it is tensioned.
What about the trailer itself?
It is easy to forget and expensive to ignore. Grease the wheel bearings, check the tyres including the spare, test the lights and lubricate the winch and coupling before storage, not on the morning of the first launch.
Getting ready to recommission in spring?
Most of what you undo in March is engine work, and it goes far more smoothly on an engine that has been looked after all along.
Last updated: 4 August 2026. Winterisation procedures differ substantially between raw water cooled, freshwater cooled and closed-circuit engines: follow your engine manufacturer’s published procedure, and use a professional service if you are unsure. Storage yard rules, insurance conditions during lay-up and required safety equipment vary by country and by facility, so confirm what applies to you locally.