Mold and Mildew on a Boat

Controlling mold and mildew on a boat comes down to two things working together: airflow to reduce moisture, and treating surfaces with the right products to kill spores, clean up the staining, and discourage new growth.

Mold peaks in winter on boats, not summer. In summer the hatches are open, heat dries surfaces quickly, and air moves constantly. In winter you close everything up, condensation builds on cool surfaces, and mold thrives.

Unfortunately, no matter the season, you will probably need to clean mold every two to four weeks on a boat. Products that claim to prevent it entirely are designed for house conditions. Prevention helps. It doesn’t eliminate it.

The good news: with the right products and a simple routine, you can keep it from taking over.

The Biggest Myth: Bleach Kills Mold

When confronted with mold, most people reach for bleach. I did too, for years. And wondered why the mold kept coming back with absolutely no reduction.

Then I learned: bleach removes the stain. It does not kill the spores. The mold looks gone, and then it comes right back.

The same goes for most wipes and most things marketed as a “mildew stain remover.” They clean up the visual evidence. The underlying problem remains.

What Actually Kills Mold

To kill mold spores, you need one of four things:

• White vinegar, straight and undiluted
• Borax, mixed into hot water as a strong solution
• The Practical Sailor formula (recipe below)
• Concrobium (generally found in the paint department in hardware and big box stores, not with cleaning supplies; also available on Amazon)

Concrobium works well but is much more expensive than the other options, so I use it only where it is particularly better. In most cases, the cheaper alternatives work just as well.

Don’t mix vinegar and borax together in the same solution. Vinegar is an acid and borax is a base. They neutralize each other and you end up with something far less effective than either one alone.

The Practical Sailor Formula

Practical Sailor magazine tested homemade mold treatment formulas and found them to perform as well as commercial products. Our team member Pam has been using this one with good results:

• 1 quart hot water
• 2 tablespoons baking soda
• 2 tablespoons borax
• 1 tablespoon trisodium phosphate (TSP)

This costs about a penny per ounce. Apply with a cloth or sponge, not a spray bottle — the mixture settles and clogs the nozzle.

TSP is caustic. Wear rubber gloves and avoid splashing near your face. TSP is also high in phosphates, which drive algae blooms in harbors and anchorages. Don’t let rinse water drain overboard. Wipe surfaces down and dispose of the used solution on land — modern wastewater treatment plants are designed to remove phosphates, so a shoreside drain is fine.

Two-Part Prevention

Controlling mold on a boat comes down to two things working together:

• Airflow reduces moisture so mold has less to feed on.
• Treating surfaces with one of the mold-killers kills existing spores, cleans up the black staining, and leaves a residue that discourages new growth for two to three weeks.

These aren’t alternatives. You need both.

Airflow

Open hatches whenever conditions allow, and run fans to keep air circulating throughout the boat.

A wind scoop pulls fresh air down below so your fans are moving clean outdoor air, not just recirculating what’s already there. Our favorite is the Breeze Bandit 4-Way Wind Scoop, which catches wind from any direction automatically, even as the boat swings at anchor. For more on fans and other ventilation options, see Boat Ventilation.

In particular, there needs to be airflow around fabric items. Don’t store them right against the hull. Condensation forms on the hull, fabric soaks it up, stays wet, and mold grows right in it.

Many cruisers raise their mattress off the hull using interlocking rubber grids (Amazon) or the Froli System for exactly this reason.

PortVisors let you keep ports open even when it’s raining, which makes a real difference for maintaining airflow in wet weather.

If you have shore power, a dehumidifier can help, but only if you keep the boat sealed up so you’re not constantly pulling in more humid air. Friends who’ve tried it found it wasn’t much more effective than good ventilation, except when the boat was in storage with nobody going in and out.

Treating Each Surface

Treating surfaces does three things at once: kills existing mold spores, cleans up the black staining, and leaves a residue that discourages new growth for two to three weeks.

Hard Surfaces

Ceilings, floors, counters. Use a rag wrung out in white vinegar, the borax solution, or the Practical Sailor formula. Go over the area two or three times until your cloth stops picking up color. Don’t rinse it off but instead let it dry on the surface.

Fabric and Upholstery

Anything that cannot be put in a washing machine. Use a rag wrung out in white vinegar or a borax solution in the hottest water you can stand. Rub it in, blot it out, let it air dry. This usually pulls most of the black staining out as well.

You can also use the Practical Sailor formula on fabric, but test for colorfastness on a hidden area first.

Washable Fabrics

Sheets, towels, clothing. Wash in the hottest water possible with borax added as a detergent boost. At a laundromat, add borax to a hot wash cycle. On the hook with no machine available, boiling items in a strong borax solution on the stove works well. If you can’t find borax, substitute white vinegar.

Even items that don’t look visibly affected can be harboring mold. Washing every 7 to 10 days in hot water with borax makes a real difference.

The Bilge

Start by keeping it as dry as possible. If you can’t keep it dry, add about half a gallon of white vinegar every week or so. That’s enough to kill whatever is growing down there.

One exception: skip the vinegar if you have an aluminum boat, as the acid will corrode the metal over time.

Paper and Books

There’s no good answer here. Store them away from the hull and away from any area where drips can reach them.

Tight Spots

Locker corners, headliner gaps, behind equipment. These are the areas with the least airflow, which is exactly where mold gets its worst grip.

Concrobium spray is worth the extra cost here because you can reach places a cloth or sponge simply can’t get to, and it tends to outperform white vinegar where there is no airflow.

Whatever you are using, wear rubber gloves. You’re protecting your hands both from the mold itself and from the cleaning solutions.

The Locker Problem

Lockers are one of the most overlooked parts of mold control. On Barefoot Gal I once counted 41 lockers and drawers. That number stopped me cold.

I started emptying and wiping out two per day — at that pace I got through all of them in three weeks. That made a huge difference in the number of mold spores “hidden” on the boat. Be sure that whatever solution you are using dries completely before returning items to the locker.

Mold and mildew control isn’t especially complicated. More than anything, it requires diligence. Stay on top of it and mold stays a nuisance, not a crisis.

Want Help Dealing with More Aspects of Living on a Boat?

Mold is just one of dozens of things that catch new liveaboards off guard. My course The Basics of Living on a Boat covers the practical reality of day-to-day life aboard, so you spend less time surprised and more time enjoying it.

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