A boat that sits in its berth between visits gets damp for one reason. Warm, moist air meets cold steel, glass or fibreglass and the water comes out of the air onto it. You stop it by letting less moisture in, keeping the fixed vents clear, drying the air with a dehumidifier or moisture traps, and keeping the temperature as even as you can. This guide covers each of those, the safety limits that come with them, and what to check every time you come back.
It picks up where our winterising checklist leaves off. That deals with frozen pipes, batteries and carbon monoxide before the first frost, while this is about the slow damage damp does through the months the boat waits for you.
Why boats get damp in winter
Warm air holds more water than cold air. When it cools against a surface, the excess condenses as droplets, and a boat has plenty of cold surfaces close together. Uninsulated steel, single-glazed windows, mushroom vents and a hull sitting in water a few degrees above freezing.
The moisture comes from ordinary things like cooking (Yachting Monthly works out that burning 100g of butane produces about 155ml of water, before the steam from the pan), breathing overnight in a small cabin, wet coats and boots, unflued gas or paraffin heaters, and rain getting in around windows and hatches.
Temperature swings make it worse. Practical Boat Owner points out that a cabin which warms and cools every day deposits moisture on every exposed surface. It suggests relative humidity (how much water the air holds against the most it could hold at that temperature) of 30 to 50% is ideal, that untreated iron rusts above 50%, and that mould grows from around 68%. A cheap hygrometer left aboard tells you where you stand.
Ventilation: the vents you must never block
Most inland boats have fixed ventilation, high and low, sized under the Boat Safety Scheme (BSS, the safety standard for boats on Canal & River Trust and Environment Agency waters) to give fuel-burning appliances the air they need. On inland craft those vents should be permanent and not closable, and the BSS notes they also cut condensation.
It is tempting to tape over a draughty vent in December. Don't. A blocked vent can starve a stove or cooker of air and produce carbon monoxide (CO), the colourless, odourless gas that can kill. The Canal & River Trust's winter advice says the same, add secondary glazing film to windows if you like, but never cover a ventilation grille.
What you can do is help air reach the places it misses:
Clear cobwebs, leaves and dust from vents and louvres.
Prop open lockers and wardrobes, and stand cushions and mattresses on edge.
Take bedding, towels and clothes home, as the Trust suggests, and never leave wet gear aboard.
Draw the curtains, low winter sun warms the cabin by day and the moisture condenses again as it cools.
Check the leak points too. Window rubbers, hatch seals and the base of roof fittings. Condensation forms in cold spots in any weather, a leak shows up after rain and usually leaves a tidemark.
Dehumidifiers on shore power
If your berth has power, a dehumidifier is the most effective way to keep a closed up boat dry. The type matters more than the brand.
Compressor dehumidifiers chill a coil so water condenses on it. Their output falls as the air cools. Homebuilding magazine puts the drop off below about 15°C, with many stopping as the coil ices near 5°C.
Desiccant dehumidifiers pull air through a drying wheel, then heat the wheel to release the water. They keep working close to freezing, and Practical Boat Owner says the air they put out is about 10°C warmer than the air they take in. The trade-off is higher power use.
For an unheated boat in a UK winter, desiccant is the usual choice. Close windows and hatches while it runs, because a dehumidifier wastes its effort drying a stream of outside air, but leave the BSS fixed vents exactly as they are. Stand it level in the galley, leave internal doors open, run the drain hose to a sink that empties overboard, and use the humidity setting rather than running it flat out.
Some owners add a thermostatic tube or oil-filled greenhouse heater just to keep frost off; Practical Boat Owner gives 60 to 100 watts as typical. Avoid fan heaters, which run up large bills and add to the fire risk. The same marina and insurance check applies.
No shore power? Moisture traps and airflow
Without power, ventilation and passive moisture traps do the work. Most traps use calcium chloride crystals, which draw water from the air and dissolve into a brine that collects in the base. They are cheap but limited:
A trap that has turned to liquid has stopped working, so replace them on every visit.
The brine is corrosive. Stand traps in a tray, away from wiring and wooden floors.
They suit lockers, wardrobes and the engine bay better than a whole open cabin.
Don't leave a 12V dehumidifier or fan running from the domestic batteries without solar or other charging, a flat battery in a frost is its own problem.
Heating the boat when you visit
Steady warmth beats a blast of heat. Keeping a stove burning evenly rather than letting the boat go cold and heating it fast, which sends condensation streaming down the windows. The Trust suggests running the stove hot enough to open the windows for a while, which drives damp out.
Avoid portable gas heaters, which add water vapour and use up the cabin's oxygen, and never use the cooker to heat the cabin. A flued stove or diesel blown air heater sends its fumes outside, and some diesel heaters can run on a timer before you arrive.
What to check each time you come back
Test the CO alarm with its button as you step aboard, as the BSS recommends.
Open up and air the boat for a few minutes.
Check the hidden places: behind cushions, under mattresses, inside lockers against the hull and around window frames.
Empty or swap the dehumidifier tank and moisture traps, and note how much they caught.
Look for new drips or stains around windows and roof fittings after rain.
Deal with mould early. Clean and dry small patches. Mould that keeps coming back points to an airflow, insulation or leak problem, not one to paint over.
Mould is not only cosmetic. UK government guidance says damp and mould mainly affect the airways and lungs, with children, older people and anyone with a health condition most at risk. It recommends mould and mildew products over bleach, and a mask, gloves and goggles while you clean.
A berth that makes winter easier
Our guide to types of moorings compares them, and how to find a long-stay mooring covers what to ask before you sign. Check the mooring has the facilities you need to maintain the health of your boat.
Find a berth with shore power
Search moorings by waterway, boat size and facilities such as shore power and water.
Sources and further reading
The organisations below are the definitive source for their own rules and advice, which they update. Check them before you rely on anything here.
This article is general information, not legal or financial advice. Where money, safety or occupancy rights are at stake, take your own advice, and follow the manufacturer's instructions and a qualified marine electrician for anything left running on board.