Boat Maintenance Living Aboard a Boat

Spring Has Sprung a Leak: Cleaning and Preventing Mildew

Friday, March 28, 2014S.V. CAMBRIA

With record breaking rainfall and lower than average temperatures, Cambria has been wet this winter. 

Really wet. 

Really, really wet. 

And that means one thing – mildew.  Or is it mold?  Either way, I’m tired of fighting it and have been searching for a more permanent solution to help keep the problem at bay.  At this point, I’ll try anything so I started with the causes:

þ     moist environment
þ     poor lighting
þ     poor ventilation 
þ               place to live (a host)

Short of moving away from the Pacific Northwest or drilling ventilation holes in all of our cabinetry, there’s not a lot more we can do about the causes, so I quickly moved on to removing it.

For years I’ve been using a bleach solution to clean mildew and was happy with the result until David told me that he read an article online that presented a compelling argument against its effectiveness:  bleach is unable to penetrate surfaces and kill the roots of mold and mildew, leaving it to grow back. Turns out he was right.  Although there are hundreds of people out in cyberspace who think bleach is the cat’s meow for cleaning mildew, the consensus seems to be that it only works well on non-porous (solid) surfaces like tiles, glass and porcelain. 

But there are more obvious reasons to avoid bleach – it’s harmful, not only to the material being cleaned, but to yourself.  It can damage your skin and the fumes can irritate your lungs and eyes, particularly when using it in places that aren’t well ventilated.  Considering our problem occurs in areas of the boat that are difficult to reach and where there’s limited airflow, it was time to try something else:

Borax:  Borax isn’t toxic or corrosive, which is a huge plus, but I didn’t find it to be very effective either.  After mixing a tablespoon of with a cup of water (one cup per gallon ratio), I tried it out on a section of woodwork along the hull with no result.  It did, however, work just as well as anything when it came to cleaning a smooth, painted surface like the bilge. 
Vinegar:  Vinegar is a natural and safe mild acid so there’s no need to dilute it with water.  Unfortunately, it was equally ineffective as borax for removing mildew on woodwork but it did clean painted surfaces.   
Tea Tree Oil: Last winter we left Kanberra Gel (a tea tree oil mixture)  around the boat to help keep mildew from growing while we were visiting family over the holidays.  It’s difficult to say whether they worked or not.  We didn’t have a mildew problem when we got back, but we generally don’t whenever we’re not living on the boat.  It did, however, smell fresher which was promising; but at $18 for a 2 ounce pot, it’s not cost-efficient.  So I decided to give the problem areas a direct hit by mixing one teaspoon of tea tree oil with a cup of water and came up with similar results to the borax solution and vinegar. 
It was time to move on to a product a fellow cruiser had recommended: 

Mold Control:  Concrobium Mold Control claims to get rid of existing mold and prevent re-growth without using harmful chemicals (concrobium is just a name they invented).  According to their website, it works by killing the mold spores at their roots, leaving behind an invisible antimicrobial barrier to prevent future mold growth.”  The instructions are easy to follow:  apply, dry and re-apply as needed.  In some cases (like ours), it may require a little more elbow grease.  The Mold Control worked really well on wood (with some rubbing) but not on our vinyl hulliner or surfaces with texture.  It was also much gentler on my skin, eyes and lungs than bleach (though I still recommend wearing gloves, protective glasses and a mask for GPs). 

Will we use Mold Control again?  Yes and no. 

When we tied up for the winter in October, I cleaned the entire boat with Mold Control before we left Washington for the holidays, resorting to bleach only on the hulliners (a textured vinyl material).  We came home to a very clean, mildew free boat.  But, as I’ve already mentioned, that isn’t unusual.

A painted stringer before using Mold Control and after (with the help of a toothbrush).

It’s now two months later and I’m in the process of preparing the boat for the season, which means putting away the piles of clothes, boat bits and toys that I removed to keep things away from the hull (why is it all of our best storage is in the coldest, dampest places?).  Unfortunately, this required more cleaning but not as much as previous years even though February and March were colder and wetter than normal.  I was also happy to see that re-growth had only occurred in the areas I’d used bleach with the exception of one spot in the v-berth where a squab had been touching the hull.  So not only does Mold Control clean well, it appears to live up to its promise to help keep mildew at bay.  
All in all, I like the product, so why would I hesitate to buy it again?  Simply because the active ingredient in Mold Control is sodium carbonate (0.95% mixed with 99.05% “other ingredients).  The next time we’ll just buy the powder at the local hardware store or online, mix it with water, and make it ourselves.

But is Mold Control the only answer? 

No.  Clearly I didn’t exhaust all of the possibilities and there are a lot more ideas out there about how to clean and prevent mildew, but I’m happy with the result and am going to stick with what works for us.  My preference is to get rid of the bleach altogether; but because mildew gets trapped in textured surfaces, it looks like bleach will have remain part of my cleaning regimen.  At least now it’s on a limited basis and in areas of the boat that have better ventilation!   

After two months, the mildew returned to the hulliner (left).
I gave Mold Control one more try (center) before resorting to bleach (right).

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  1. I have found the best way to prevent mildew is to keep the humidity on my boat low. I run a dehumidifier all winter, it sits on the galley counter and the drain tube runs into the sink. It has a range of humidity settings, but I keep it at 50%. I have read that mildew needs in excess of 70% to start growing. After using it for five years we have not had any mildew problems.

    1. Definitely agree there . . . low humidity is the answer. The boat generally sits at 35 to 40% during the winter months but we live aboard and even a dehumidifier can't keep up with the condensation our bodies and cooking produce (boy, do I wish it could!). We take some time out to visit family over the holidays and always come back to a dry, mildew-free boat. But the real problem occurs when we're at anchor because we try to stay out from May until November.

    2. Ah yes, we sweaty bodies always muck things up :-) The dehumidifier is way to bulky and noisy to run while living aboard and sucks too much juice at anchor. If someone came up with a 12v, low amp, quiet, small, dehumidifier that would be so awesome.

    3. That would be brilliant! And I think we'd be the first in line to buy one.

  2. There are now Peltier effect dehumidifiers, which are extremely low power draw. Here's an example: But, as you might expect, they have correspondingly low moisture removal rates...

    For a zero-draw dehumidifier, try the calcium chloride pots. But they are messy.

    Like Bob above, we run a small 110v (1.6 amps, 15 pints/day) dehumidifier all winter long on Eolian. It helps A LOT!

    s/v Eolian

  3. I've a trio of re-chargeable Eva-Dry dehumidifier's off the shelf from Fishery Supply,yet available from common distributors like Ace... once the blue crystals shown in a view window become water logged with moisture.. the unit get's dried out by plugging in.. warms it all.. till moisture is out and crystals return to dry blue .... ready for another few weeks, down here in Monterey,Co.Ca. .. i periodically shift them from the socks,to the books shelf to the hanging apparel etc... they have my confidence !