The Fog Is Yours: A Cabin Drying Plan for UK Boats
Photo by Photo by K. K. on Unsplash on Unsplash
Mould does not normally break into a boat. It gets invited in, given a warm bunk and a steady supply of damp air, then left to multiply while we congratulate ourselves on having fitted a shiny little heater. On a UK boat in November, the enemy is rarely a leak alone. It is the water we make ourselves: breathing, boiling the kettle, cooking, drying oilskins, storing wet towels and leaving bedding pressed against a cold hull.
The answer is not to turn the cabin into a heated soup bowl. Heat without ventilation and moisture removal just gives mould a more comfortable climate. The goal is to shift water out of the boat, or stop it getting aboard in the first place.
First, stop topping up the fog
Before buying another gadget, walk around with a critical eye and a bin bag. Wet oilskins hanging in the cabin are a reservoir. So are damp towels, sodden deck shoes, tea towels, sponges and that mysterious puddle in the bilge. Get wet gear out of the accommodation when you can. If it must stay aboard, hang it in a cockpit locker or a well-ventilated space, not over the bunk.
Wipe condensation off windows and hatch frames before it runs into cushions. Fix the obvious leaks — hatch seals, chainplates, locker drains — but do not assume a leak explains everything. A dry boat can still get damp from cooking, breathing and cold surfaces.
Ventilation is not a draft, and it is not always the answer
There is a stubborn myth that the cure for cabin damp is to crack a hatch all winter and let the boat breathe. Sometimes that helps. Sometimes you are simply importing wet Atlantic air into a cold cabin and giving it a nice place to condense.
Ventilation moves moisture. It only removes it when the air outside is drier than the air inside. So measure rather than guess. A cheap digital hygrometer is one of the most useful tenner-saving bits of kit on the boat. Put one at bunk level and one in a locker. Relative humidity around 50–60% is a sensible target. Much below 40% is over-drying; much above 65% is mould weather.
Use cross-flow when conditions suit: open a forehatch and a companionway hatch briefly, especially on a dry, breezy morning. Keep passive vents clear. Do not seal the boat like a Tupperware box and expect a dehumidifier to fight the entire harbour.
The dehumidifier rules
A dehumidifier is useful, but only if it is allowed to do its job. Close the boat up. Running one with hatches open is like heating the pontoon. Use the continuous drain if you have one, so you are not emptying a tiny tank every ten minutes.
Compressor dehumidifiers work better in warmer cabins; desiccant types tend to cope better with cold conditions. Size it for the cabin volume, not the whole marina. If you are on shore power, use a proper lead, an RCD and a bit of common sense. Do not trail extension leads across a wet cockpit. If the boat has no power, do not pretend a dehumidifier is an option.
Heat alone is not drying. It raises the air's capacity to hold moisture, but that moisture is still in the boat. Pair gentle heat with air movement and a dehumidifier, or leave the heating off and rely on ventilation when the outside air is genuinely drier.
Moisture traps have a narrow job
Refillable silica or calcium chloride traps are handy in lockers, under berths and in small enclosed spaces. They are not a whole-cabin solution. Check them regularly, keep them upright and accept that a full trap is water you have successfully captured — not a magic spell against damp.
Bedding and cushions are the quiet culprits
A mattress is a sponge with a duvet on top. Body warmth and breath pass through bedding and condense against a cold bunk base. Lift the mattress every weekend, or at least every visit. Use a breathable underlay rather than a plastic sheet, and take bedding home when the boat is laid up. Cushions should stand on edge, not lie flat against the hull. Open locker doors so air can move.
The weekend routine
On arrival, open up and read the hygrometer. Wipe condensation. Check the bilge. Air the bedding. If the outside air is drier, ventilate hard for twenty minutes. Before leaving, dry surfaces, lift the mattress, stand cushions on edge, empty the bin and remove wet towels. If shore power is available, leave a dehumidifier on a timer with the boat closed. If not, leave the boat ventilated only when the forecast is dry.
The winter routine
Visit fortnightly if you can. Run a short drying session: gentle heat, air movement and a dehumidifier if you have power. Do not leave unflued gas heaters running. Keep locker doors open. Check moisture traps. Remove textiles where possible. If mould appears, treat it properly and find the moisture source rather than just painting over the evidence.
The rule is simple: if it is wet, take it home. If it is damp, air it. If it is closed, dry it. If it is cold, move the air. Do that and the fog stops being yours.
