Mold and Mildew on a Boat

The most effective way to control mold and mildew on a boat is ventilation to prevent it, and white vinegar, borax, or Concrobium to kill it. Bleach does not kill mold spores. Neither does Lysol or any wipe-based product. They remove the stain but leave the spores behind, and the mold comes right back.

I’ve been fighting mold on boats for more than 17 years, cruising over 14,000 miles from the Sea of Cortez to the Bahamas. When I asked readers what they wanted more help with, the number one answer was mold and mildew. This is everything I know.

One thing to accept going in: you will need to clean mold and mildew every two to four weeks. Products that claim to prevent it entirely are built for house conditions, not boats. On a boat, dampness and condensation are constant. Prevention reduces the problem. It doesn’t eliminate it.

Prevention: Ventilation Is Your Best Weapon

Moving air dries surfaces before spores can take hold. That’s the whole theory.

In practice it means:

• Open hatches whenever conditions allow
• Use a wind scoop in an open hatch to channel even a faint breeze below decks. Our favorite is the Breeze Bandit 4-Way Wind Scoop — the four-chamber design catches wind from any direction automatically, so airflow keeps going even as the boat swings at anchor
• Run fans along with the wind scoop to keep air moving throughout the boat, not just under the hatch. For more on fans and other ventilation options, see Boat Ventilation
• Open lockers so air circulates inside them
• PortVisors let you keep ports open even when it’s raining

Reduce Moisture Sources

Good ventilation works best when you’re also reducing the moisture that feeds mold in the first place.

Don’t store fabric items right against the hull. As condensation forms on the hull, the fabric soaks it up, stays wet, and mold grows right in it. That same moisture then gets pumped back into the cabin air. Many cruisers raise their mattress off the hull using interlocking rubber grids (Amazon) or the Froli System.

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.

What Does NOT Kill Mold (Common Myths)

This is the part that surprises most people.

Bleach does not kill mold or mildew. It removes the stain. It does not kill the spores. The mold grows back.

The same is true of anything labeled as a mildew stain remover. It cleans up the visual evidence. The underlying problem remains.

Other things that don’t kill spores: wipes of any kind, soap, and Lysol. They can remove surface mold from hard surfaces, but they leave living spores behind.

What Actually Kills Mold

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

• White vinegar (straight, undiluted)
• Borax (mixed into hot water as a strong solution)
• Concrobium (find it in the paint department of big box stores, not the cleaning aisle; also available on Amazon)
• Very hot water or steam

Wear rubber gloves whenever you’re cleaning mold. You want to protect your hands both from the mold itself and from the cleaning solutions.

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.

Which one works best depends on the surface. Here’s how I approach each.

Cleaning Mold by Surface

There is no one single way to clean mold and discourage it from returning. It depends on the surface.

Hard Surfaces

For ceilings, floors, counters, and similar surfaces, use straight white vinegar or a strong borax solution in hot water. Go over the area two or three times until your cloth stops picking up color. Don’t rinse — let the solution dry on the surface. I’ve gone back and forth on whether vinegar or borax does a better job. Honestly, both work.

Soft Surfaces

For upholstery, fabric-covered walls or ceilings, life jackets, and anything too large for a washing machine, use straight white vinegar, a strong borax solution in the hottest water you can stand, or a steam cleaner. Rub the solution into the material and blot it out. Let it air dry. This usually pulls most of the black staining out as well.

Washable Fabrics

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.

Sheets, towels, and clothing are serious mold breeding grounds, even when they don’t look visibly affected. Washing them every 7 to 10 days in hot water with borax makes a noticeable difference.

Paper and Books

This is the one area where I don’t have a good answer. There’s no reliable way to clean mold from books and documents. The best you can do is store them away from the hull and out of any area where drips might fall on them.

The Bilge

Keep the bilge 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. Steel hulls are fine.

Lockers

Locker interiors are one of the most overlooked parts of mold control. On Barefoot Gal I counted 41 lockers and drawers. Once I realized that, I made a habit of emptying and cleaning one per day. At that pace I covered all of them in six weeks. Not as often as ideal, but far better than only cleaning them when things got visibly bad.

A Homemade Formula Worth Trying

Concrobium works, but the cost adds up when you’re using it as often as a liveaboard needs to. Practical Sailor magazine tested two homemade formulas and found them to perform as well as commercial products.

Formula 1: 1 quart hot water, 1 tablespoon baking soda, 2 tablespoons washing soda, 2 tablespoons TSP (trisodium phosphate)

Formula 2: 1 quart hot water, 2 tablespoons baking soda, 2 tablespoons borax, 1 tablespoon TSP

At about a penny per ounce, either is worth a try. Pamela on our team has used Formula 2 with good results. One practical note: the ingredients settle quickly and need frequent shaking, and the mixture tends to clog a spray bottle. Apply with a cloth or sponge rather than trying to spray it.

Also, a note on TSP: it’s caustic and can irritate skin and eyes. Wear your rubber gloves when mixing and using either formula, and avoid splashing it near your face.

TSP is also high in phosphates, which drive algae blooms in harbors and anchorages. Don’t let the 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 should be fine.

An Honest Take on Concrobium

I’ve tried Concrobium on just about every surface on the boat. My conclusion: for most surfaces, vinegar or borax does the same job for less money. Where Concrobium genuinely earns its place is in spots you can’t reach with a cloth or sponge. Being able to spray into a locker corner or up into a headliner gap is useful in a way that a rag and vinegar just can’t replicate. So it stays in my toolkit, but it’s not my first choice for every surface.

Winter Is When Mold Is Worst

Here’s something I didn’t expect when I first started cruising: mold peaks in winter, not summer.

In summer, the hatches are open, heat dries surfaces quickly, and air moves constantly. In winter, you keep everything closed up. Condensation builds on cool surfaces. There’s no heat to dry things off. Mold thrives.

If you’re fighting mold hard right now and it’s cold out, that’s not a coincidence. It does get easier when the weather warms and you can open the boat up again.

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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