Boat Hacks – Marine Growth Edition

It’s mid-July here on Florida’s Gulf coast, which means daytime temperatures are routinely approaching those found on the surface of the sun.  It’s peak air conditioning season, and our poor overworked system runs nonstop from dawn till way past dusk, steadily losing ground the entire time. While it’s a comfortable 72° (22° C) early in the morning, the temperature onboard can peak as high as 78° (25.5° C) by dinnertime before finally turning the corner and starting to decline again.

Unlike your typical home cooling system, most onboard AC systems are water cooled. A pump sucks up some of whatever water the boat is floating in (seawater, in our case) and uses it to cool the system’s condenser. If you understand anything about how an air conditioner works, you’ll know what that means. If not, the short explanation is that all the heat the air conditioner sucks out of the air has to go somewhere, and that somewhere is the seawater. Naturally, proper care and feeding of the AC system is priority one at the moment. This means regularly cleaning both the air filter and the seawater strainer.

Now the problem with using seawater to cool the AC condenser is all the microorganisms it contains. If you had any idea about the quantity of jellyfish eggs, oyster larvae, baby barnacles, and fish semen that every quart of ocean contained, you’d never swim in it, much less get any in your mouth, nose or ears. Most of those microorganisms are just drifting around looking for a safe place to homestead so that they can put down roots and grow up to be bigger organisms and make their own eggs and babies. Unfortunately, many of them do so inside the air conditioner’s heat exchanger. Over time, this buildup of marine growth will foul the condenser, reducing your air conditioner’s ability to make cool refreshing air.

If you read the owner’s manual for your system, you’ll see that it says that you should periodically (annually?) flush the unit’s seawater system with a mild acid solution. This is supposed to kill any marine growth that may be forming and dissolve the barnacle farm that sprouts wherever there’s seawater touching a solid surface. You basically fix up a bucket of muriatic acid solution and use a small pump to circulate it through your unit’s heat exchanger.


Personally, I think it’s similar to flossing after every meal. We all know we should, but who honestly ever does it? I’m guessing it’s the pretty rare sailor who has a bucket, circulating pump, jug of muriatic acid and some suitable hoses sitting around waiting for its once-a-year use.

Now we boaters have a solution to deal with marine growth on the underwater portions of our boats. It’s called bottom paint. Marine bottom paint is a special type of paint that contains toxins to kill off or keep under control all the little critters floating around in the water. The most commonly used toxin in bottom paint is copper. Each gallon of bottom paint contains several pounds of ground up copper, which forms a layer on the bottom of the boat that tells the little ocean critters, “Move along, you’re not welcome here, go try your luck elsewhere.” It occurs to me that if it were possible to get some copper inside the air conditioner’s heat exchanger, maybe it would help slow down the buildup of growth inside the condenser, and reduce the need to acid flush it, which quite frankly isn’t being done anyway.

While turning this over in my head one day, I recalled how when we used to live in Washington (the state, not the city), everyone was constantly fighting moss and mildew growth on their roofs. At its best it was unsightly, at its worst it made some roofs look like putting greens with their luxuriant mossy coats. But an easy fix to the problem was to attach some zinc strips along the peak of the roof. You could buy a roll of it at the local hardware store. As it rained, a tiny bit of the zinc would dissolve and flow down the roof, killing the mildew and moss. I guess they are zinc intolerant.

So here’s where the boat hack comes in. I thought the same basic idea might work with copper. So I bought a package of copper plumbing fittings at Lowe’s. It cost about three dollars for ten ½” slip couplings. Then when I did my monthly cleaning of the system’s seawater strainer, I dropped a couple in the strainer basket. The strainer is plastic (nylon I believe), but even if it were stainless steel, copper is much higher on the galvanic series, so it shouldn’t hurt the strainer. (Don’t worry if you don’t know what that means. It’s technical. Unless you own a boat. Then you should be very worried!)  As the seawater flows through the strainer, it should pick up some dissolved copper. Not much, just some ions, but it shouldn’t take too much to make the inside of the heat exchanger less hospitable to ocean critters.

Will it work at reducing marine growth inside the condenser? I have no idea. But at about 30 cents per copper fitting, it’s a cheap experiment. I’ll monitor the fittings at each monthly strainer cleaning to see if I can notice any reduction in their size, indicating that they are slowly dissolving. Please let me know if you think I’ve overlooked something.

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4 thoughts on “Boat Hacks – Marine Growth Edition

  1. Paul

    Interesting idea. I don’t THINK it will work for a couple of reasons but one experiment it worth 1000 theories! First, the copper in anti-fouling paint is usually not elemental (ground up) copper. It is a salt or oxide of copper, which has a much higher solubility than metallic copper. Second, the copper in anti-fouling paint is in direct contact with organisms it is trying to kill so the availability is higher. Lastly, the toxicity of copper probably (not sure about this though) depends on the speciation of the copper. That is, the form of the copper and whether it is bound or not. I am not sure that metallic copper dissolving in salt water will produce the most toxic species. Copper complexes easily with organic compounds forming non-toxic species. I think it is only copper 2+ that is toxic. But the experiment is so cheap, it is certainly worth doing. I enjoyed your posts involving the electrical power storage issues.

    I chartered my first “big boat” this summer. Looking forward to getting out there in the relatvely near future also. Keep living the dream!

    1. Robert Post author

      I’m not really thinking it’s going to make a dramatic difference, but again, I’m intrigued by how rainwater running over a simple strip of zinc was apparently able to retard the growth of moss and mildew on our roof. I do believe that if this does actually do some good, it will primarily be in retarding the growth of barnacles. I don’t think it will do much about the biofilm slime layer. Even copper-based bottom paint doesn’t resist slime growth, it takes the addition of a chemical biocide. But I think it requires sunlight for biofilms to develop, which obviously isn’t present inside the air conditioning condenser. So who knows. I figured it was worth a try though, as I really don’t see a downside.

      There will be another chapter or two of More Power Scotty coming up soon. I still have to talk about how we reconfigured the charging system and the tools we’re using to monitor it all. Stay tuned!

      1. Bryan

        I going to try a simple solution of pennies and an old piece of shaft zinc in the strainer basket. Might hit it from a couple of chemical angles. Thoughts?

        1. Robert Post author

          I know copper has antifouling utility (it’s poisonous to marine growth), zinc not so much. It’s really more for galvanic corrosion control. As for using pennies to provide some copper, you’d need to make sure they’re pretty old pennies. For quite a while now (you’d have to research how long) pennies have just been a thin copper cladding over a base metal base (probably zinc). So try and find some 20 year old pennies if you want some copper. Or just drop a dollar at the hardware store to get a handful of solid copper plumbing fittings. So far I’ve been getting what appear to be pretty good results. Last two strainer cleanings, I’ve had the usual thick coating of green slime, but no sign of any baby barnacles.


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