Removing mould on board and keeping it away
Cleaning mould off a boat is the easy half. Most people manage it in an afternoon. The half that decides whether you do it again next spring is what you change afterwards, because mould does not return by chance. It returns to the same place for the same reason.
First, protect yourself
Open every hatch before you start. Wear gloves and an FFP2 mask: disturbing a mouldy surface releases spores, and a closed cabin concentrates them. If someone on board has asthma or a known allergy, they should stay off the boat until it is aired.
If the affected area is larger than roughly half a square metre, or if it sits inside the structure rather than on it, stop and get a professional in. That is a remediation job, not a cleaning job.
What to use on what
On hard surfaces such as berth boards, locker interiors, GRP and painted timber, a 70 to 80 % alcohol solution works well. It kills the growth and evaporates without leaving moisture behind, which matters on a boat. Alcohol vapour is flammable: switch off the cooker and heater, no open flame, and keep the hatches open while you work.
On fabric and cushion covers, wash at the highest temperature the label allows. If it does not come out, it will not come out. Replace the cover rather than escalating the chemistry.
Household vinegar is often recommended and is fine on non-porous surfaces, but it is water-based: dry the area properly afterwards or you have added to the problem you are trying to solve.
What to avoid
Chlorine bleach bleaches the stain without reliably reaching what is under the surface, and on a boat it attacks fittings, sealant and textiles. It makes the mark disappear, not the mould. Never combine it with vinegar or any other acid: the mixture releases chlorine gas.
Steam cleaners put warm water into exactly the layer that has too much of it already. On upholstery in a closed cabin, that is a step backwards.
Sanding a mouldy timber surface indoors spreads spores through the whole cabin. If it has to be sanded, do it off the boat.
Dry it out properly before closing up
After cleaning, get the moisture out. Lift every cushion onto its edge, open lockers, and if shore power is available run a dehumidifier for a day or two. Warm air alone is not enough: it has to be able to leave.
Do not put cushions back flat until both they and the boards under them are genuinely dry. A damp cushion set down on a damp board restarts the cycle before you have left the marina.
Then change the reason it grew there
Mould needs moisture, and under a berth it gets it from condensation between the mattress and the board. As long as those two surfaces lie flat against each other, that moisture has nowhere to go, and the cleaned spot is the spot it will reappear on.
A 3D spacer fabric between mattress and board keeps a ventilated layer open under sleeping load, so the humid air leaves at the edges instead of settling. It goes in without tools and, unlike a dehumidifier, it keeps working while the boat sits shut all week.
Clean with alcohol on hard surfaces and a hot wash on covers, skip the bleach and the steam, and dry everything before you close up. Then open a ventilated layer under the mattress, otherwise you have booked the same afternoon again for next season.
Frequently asked
- How do I remove mould from a boat?
- On hard surfaces such as berth boards, lockers and GRP, use a 70 to 80 % alcohol solution. It kills the growth and evaporates without adding moisture. Wash fabric covers at the highest temperature the label allows. Wear gloves and an FFP2 mask, and open every hatch first.
- Can I use bleach or a steam cleaner on boat mould?
- Better not. Chlorine bleach lightens the stain without reliably reaching what is under the surface, and it attacks fittings, sealant and textiles. A steam cleaner puts warm water into exactly the layer that already has too much of it.
- Why does mould keep coming back in the same spot?
- Because the reason it grew there has not changed. Under a berth, that reason is condensation trapped between the mattress and the board. As long as those two surfaces lie flat against each other, the cleaned spot is the spot it reappears on.
- When should I call a professional?
- If the affected area is larger than roughly half a square metre, or if the growth sits inside the structure rather than on it. That is remediation work, not cleaning.