Why mould grows
Mould needs moisture, oxygen and nutrients. There is plenty of moisture. During the winter months relative humidity is usually above 90 per cent, so everything in the boat gets damp however well it is ventilated. In spring the sun warms the air under the cover, and the moisture condenses on cold surfaces lower down in the boat.
What you can control is the nutrients and the air: clean surfaces and air that moves.
Before covering
- Take home cushions, mats, bedding and clothes. Dry them first.
- Empty the fridge and food lockers.
- Clean all surfaces, including the ceiling and walls by the galley. Mould grows on dirt, food splashes and grease.
- Mop up water in the bilge, lockers and cockpit.
- Prop open hatches, cupboards and floorboards so air can get in.
Wood: don't oil in the autumn
The Swedish Cruising Club has found that wood treated with teak oil in autumn often goes mouldy over winter. Teak oil for indoor use contains vegetable oils that mould feeds on.
- Clean the wood and leave it untreated over winter.
- If you want to protect it, varnished surfaces are a safer choice than oil.
- Bleach kills mould but bleaches the wood and is bad for the environment.
The canvas
- Take the canvas off and store it at home. Canvas makers advise against leaving it on the boat over winter.
- Rinse with fresh water and a medium-soft brush. Strong cleaners wear the fabric and make the windows cloudy.
- Let it dry completely before rolling or folding it. Never fold a damp canvas.
Ventilation, moisture absorber or dehumidifier
| Method | Works well when | Keep in mind |
|---|---|---|
| Vents in the cover and open hatches | Always, as a baseline | Openings should let air out but not let rain in |
| Moisture trap or desiccant | Smaller spaces, places without power | Replace or empty as instructed |
| Electric dehumidifier | The boat is in an enclosed space | You need permission from the site. Consider the fire risk |
Many clubs prohibit heating and long-term power use in boats ashore. Ask before you plug anything in.
If mould has already appeared
- Air the boat out thoroughly before you start.
- Wear gloves and a respirator when wiping off mould, and wash with a mild cleaner.
- Let the surfaces dry completely.
- Cushions and textiles that still smell after washing may need replacing.
Frequently asked questions
Should the boat be open or closed over winter?
The cover should keep out rain and snow but have openings so the air is exchanged. Inside the boat, hatches and lockers should be left open. Completely sealed solutions often cause more condensation.
Does teak oil help against mould?
No, teak oil for indoor use can cause more mould. Mould feeds on the vegetable oils. Clean the wood and leave it untreated over winter.
Can I use a dehumidifier in the boat on land?
It depends on the location. You need permission from whoever runs the storage. Some clubs, such as Nacka Båtklubb, ban all heating in boats.
Can I wash off mould with bleach?
It kills the mould but bleaches wood and canopy fabric and is bad for the environment. A mild cleaner and drying are gentler.