Frost gets all the attention when people talk about leaving a boat in France over winter. It should not. In ten years of laying up on this coast the thing that has actually cost me money and weekends has not been ice, it has been damp. A black bloom of mould across the headlining, a chart table that smells like a cellar, a mattress you cannot save. Damp is patient and it works while you are not there.
Understanding why it happens makes the fixes obvious, so bear with me for one paragraph of physics.
A glassfibre hull is a giant cold plate. When warm, moist air inside the cabin touches it, the air cools, and cool air cannot hold as much water as warm air. The water has to go somewhere, so it condenses on the cold surface. A single cold night can put cups of liquid water onto the inside of a hull that looked perfectly dry the evening before. That is the dew point in action: chill any air far enough and the moisture in it falls out as liquid. Mould then needs only that moisture and a still, unventilated space, and a winter boat provides both for free.
The number to remember
Aim to keep relative humidity inside the boat between 50 and 60 percent. Below 50 and you are spending energy for no extra benefit and risking shrinkage in joinery; above about 65 percent mould spores find conditions they like. A cheap hygrometer left on the chart table, the kind that costs under 20 euros, tells you whether you are winning. I leave one aboard and photograph it through the window when I visit. If it reads 55, I relax. If it reads 75, I know something has gone wrong with the ventilation or a hatch has leaked.
There are three levers you can pull, and the best winters use all three.
Lever one: move the air
Stagnant air is mould's best friend. The cheapest, most effective single thing you can do is create a through-draught. Crack a forward vent and an aft vent so air can track through the boat rather than sitting in pockets. Solar-powered cabin fans that run off a small panel cost in the region of 40 to 80 euros and turn over the air without any shore power at all. Open lockers, lift floorboards, prop the heads door, leave drawers ajar. Anywhere air cannot circulate is where you will find the first black spots in spring.
Lever two: take the water out
A dehumidifier does the heavy lifting where shore power is available. The type matters more than the brand. A compressor dehumidifier pulls a lot of water fast but loses its efficiency badly once cabin temperature drops below roughly 10 to 15 Celsius, exactly the conditions a winter boat lives in. A desiccant dehumidifier uses a slowly rotating wheel of silica or zeolite and keeps working in the cold, which is why it suits an unheated French lay-up far better. It uses more electricity per litre extracted, but it actually extracts.
If your marina forbids leaving an appliance running unattended, or you have no shore power because the boat is on the hard, fall back on passive moisture absorbers. The refillable calcium-chloride tubs cost a few euros and pull water out of the air into a salty brine you tip away on each visit. One tub per cabin is the rough rule, and I budget on emptying them monthly. They are not glamorous but they buy you time between visits.
For boats kept in the water rather than ashore, the damp problem compounds with bilge condensation and the simple fact that the hull sits in cold water all winter. The dedicated piece on keeping your boat afloat winter france covers the afloat-specific angles, and if you are running any kind of cabin heat, the article on heating damp french winter afloat explains why a heated boat can paradoxically grow more mould if you get the ventilation wrong.
Lever three: raise the temperature, carefully
Warm air holds more moisture, so gently lifting cabin temperature lifts the dew point and keeps condensation off the hull. A low-wattage tube heater or a thermostatically controlled greenhouse heater set to come on around 5 Celsius does this for pennies. The danger is heating without ventilating. Warm a sealed boat and you simply load the air with more moisture, which then dumps onto the coldest surfaces the moment the heater cycles off. Heat and ventilate together, never one without the other.
Hunt down the sources
Drying the air is half the battle. The other half is stopping water getting in. Before you leave, find and fix the leaks. Run a hose over the deck and watch inside for drips at the chainplates, the windows, the stanchion bases and the deck fittings. A weeping window over winter will defeat any dehumidifier you care to buy. Lift and stand all cushions on edge so air reaches both faces, and if you can take the mattresses and soft furnishings home, do it. They are the things mould ruins first and they are the hardest to clean.
Strip the fridge, prop its lid open, and empty every food locker. A forgotten onion or a half-bag of flour will scent the whole boat by February.
The cold-spots you forget
Mould does not bloom evenly. It picks the coldest, stillest places and starts there, so once you know where those are you can target your effort. The worst offenders on most boats are the same: the underside of the foredeck where the anchor locker chills the laminate, the corners of lockers against the hull below the waterline, behind the backrest cushions in the saloon where air never moves, and the inside of any uninsulated metal, the windows, the chainplates, the engine. These are where the air first reaches dew point and where the first black spots appear.
Two practical habits help. Pull cushions a hand's width off the hull with a spacer so air can track behind them, or stand them on edge entirely. And wipe down the cold metal surfaces with a thin film of a corrosion inhibitor before you leave, which both protects the metal and gives mould a less welcoming surface. A boat I shared a pontoon with lost an entire set of teak-faced lockers to mould one winter purely because they sat hard against an uninsulated hull with the cushions jammed in front of them, sealing the air in. The owner did everything else right and still lost them to the one spot he could not see.
Visiting the boat over winter
If you can get to the boat every few weeks, half an hour of attention transforms the outcome. Open everything up, let the boat breathe on a dry day, empty the moisture-absorber tubs and the dehumidifier reservoir, check the hygrometer, and run your nose around the lockers. A musty smell is the early warning that arrives before the visible bloom, and catching it then means a wipe-down rather than a refit. If you genuinely cannot visit, lean harder on a desiccant dehumidifier that drains continuously overboard or into a large reservoir, and accept that you are managing risk rather than eliminating it.
What spring should look like
Get all of this right and you step aboard in April to a boat that smells of nothing in particular, with a hygrometer reading in the 50s and not a black spot on the headlining. Get it wrong, as I did that first miserable year, and you spend the first sunny weekend of the season scrubbing dilute bleach into the deckhead instead of sailing.
If you are still deciding whether to lift out or stay afloat, the costs comparison in wintering ashore france yards costs is worth a read, because a boat under a proper cover ashore is often easier to keep dry than one sitting in cold water all winter. Either way, the principle does not change: move the air, take the water out, and stop the leaks before you walk down the pontoon for the last time in October.
