How to Prevent and Remove Mold and Mildew on Your Boat

Look for drips and stains around portlights, windows, hatches, stanchion bases, cleats, rails and windlass, and (on sailboats) chainplates and the mast step. Check after a heavy rain or melting snow. Make sure cockpit scuppers and drains are clear; clogged drains can send water into the cabin. A leak through a cored deck can also rot the core, so don't ignore it.

Leaking deck hardware and portlights usually need to be removed and rebedded with the right marine sealant. If you see water coming in below the waterline, at a through-hull, or the bilge pump runs more than usual, treat it as urgent: see our seacocks and bilge pump guides, and call a pro. If the boat is taking on water faster than the pump can handle, call TowBoatUS, Sea Tow, or the USCG on VHF 16 or 911.

Pump and sponge the bilge dry. Wipe up condensation. Take wet towels, foul weather gear, life jackets and lines out, dry them ashore, and bring them back only when fully dry.

For storage, remove bedding, mattresses, pillows, clothing, towels, paper goods and food. These soak up moisture and feed mold. Stand cushions on edge so air reaches all sides, or take them home.

Prop open lockers, drawers, cabinet doors, the head and the engine compartment (if safe) so air can circulate. Under shrink-wrap or a full cover, consider leaving cabin doors open if security allows.

Passive vents (cowl vents, louvers, vent plates) only work if air actually moves. Solar-powered vents work without shore power and can run all day, some all night on a battery. A common rule of thumb is to change the cabin air about once an hour; larger boats may need one vent pulling air in and one pushing it out. Shrink-wrap and winter covers should have vents built in. If you visit on a sunny winter day, open the boat up for a few hours, then close it again.

• Moisture absorbers: calcium chloride tubs or hanging bags are cheap, safe and need no power. Put them in lockers and the cabin, and empty or replace them when full. Keep them where they can't tip over; the collected liquid is salty and can stain.
• Electric dehumidifier: works well on a boat with reliable shore power, but only through a properly installed marine shore power system (see our shore power guide), never a household extension cord run through a hatch. Choose a unit with automatic shutoff, route its drain hose to a sink or the bilge, and check it on every visit.
• Heaters: warm air causes condensation on cold surfaces like hatches and portlights; ventilation still matters.

A small hygrometer helps you see whether your plan is working. The EPA's home guidance is to keep relative humidity below 60 percent, ideally 30 to 50 percent, and to dry anything that gets wet within 24 to 48 hours, before mold can take hold. A boat won't always reach those numbers, but they are a useful target.

Small patches on hard surfaces are a normal DIY job. The US EPA suggests that areas larger than about 10 square feet (roughly 3 feet by 3 feet), or mold after a lot of water damage such as a sinking or flooding, call for professional cleanup. Also call a pro if mold is inside headliners, cored panels or air-conditioning ducts.

Start with a marine mildew cleaner made for gelcoat and follow the label. For tougher spots, BoatUS suggests:

• Vinegar: 3 parts white vinegar to 2 parts water, sponged on; stronger if needed.
• Bleach: diluted with about 3 parts water, on hard, nonporous surfaces only, with plenty of ventilation.
• Borax: about half a cup in a gallon of hot water, as a cleaner and preventive.

Use one at a time, never mixed, and rinse well between products. Test each on a small hidden area first.

• Vinyl seats and headliners: use a marine vinyl cleaner or mildew remover made for vinyl. Skip bleach; it can weaken stitching and damage the finish (see our canvas and vinyl guide).
• Acrylic canvas: follow the fabric maker's mildew method (Sunbrella has a diluted bleach-and-soap recipe for its fabrics).
• Life jackets: never use bleach; it can damage the fabric and the flotation inside. Use mild soap, rinse and air dry.
• Wood: bleach works poorly on wood and can damage finishes; use a cleaner made for that wood and finish.
• Carpet and cushion foam: if mold has grown deep inside, replacing is often the only real fix.

A coat of paste wax on gelcoat inside lockers, the anchor locker and below-waterline compartments won't stop mold, but it lets you wipe it off with a rag. Mesh or grate liners under stowed gear keep air moving underneath, which is where mold usually starts.

After cleaning and drying, spray or wipe a mold and mildew retardant on compartment surfaces, liners and the undersides of hatches. Most only last up to about 90 days, so put a reminder in BoatMend to reapply.

A smell that comes back right after cleaning may not be mold at all. Permeated head hoses, a dirty holding tank vent, or stagnant bilge water smell similar. See our marine head and holding tank guide.