How To: (And Why To) Clean the Bottom of Your Boat

How To: (And Why To) Clean the Bottom of Your Boat

Go Overboard

Mike Smith on why you need to get some dive gear and get under your boat.

Sometimes it pays to get wet. I don’t mean stand out in the rain—most of us have more sense than that. I mean put on fins, a mask and snorkel and swim under your boat to find out what’s happening below. You can check the anodes, run your fingers over the prop blades and rudders to feel any damage, scrub the slime off the bottom, and scrape off barnacles before they cause trouble. If you have scuba gear, that’s fine, but you can also free-dive this project, since mostly you’ll just be looking things over. And it’s a great excuse for a swim on a hot summer day.

If you find serious problems—a damaged prop, for instance—you’ll probably be smarter to call in a professional diver, or book a short haul at the boatyard. Changing a prop underwater isn’t easy, and you probably don’t have the tools to do it yourself anyway. Pro divers with the qualifications and equipment to prop-swap on even a mid-sized boat don’t work for peanuts, so it’s often cheaper just to haul the boat. With a bigger yacht, one large enough that not many yards have enough lifting capacity, even a short haul costs tall green—a team of experienced divers can be the better choice. You do the math.

Buy Some Gear

Step one is to swim under the boat and look around—anybody who can swim and hold their breath can do that. You’ll need some equipment, stuff that you might already have on board but which is, I’m guessing, not the best; Most people buy inexpensive fins, masks and snorkels at a big box store or online—the masks leak, the fins are too small, etc. Chuck that stuff in the dumpster and buy new gear at a dive shop that carries top-quality equipment that you can try before you buy. Figure on spending $150 to $200 for all three items: a good mask alone can run up to $100, a nice pair of full-foot fins $50 and up—you can even spend $50 on a snorkel.

Add some lead. When I was a sailboat captain, at least twice a year I changed the prop underwater—we had a low-drag two-blade for racing, a three-blade for cruising. (This was back in the Dark Ages before feathering props were available.) Usually I did this by free-diving; I was pretty good at holding my breath back then, but it still took a lot of dives. But what made the job easier was wearing a weight belt. A little lead around your waist means you don’t have to fight as hard to overcome your natural buoyancy to stay underwater.

How much weight you need depends on your body—skinny folks need less, bigger guys more—and whether you’re wearing a wetsuit and other gear. I was younger and thinner 50 years ago, and worked underwater in just a swimsuit, so wore only a few pounds on a belt with a quick-release buckle. Today I’d probably need a couple of kettlebells lashed around my waist. Start with a little weight and work up. You want just enough to make it easier to submerge, not so much that it drags you down.

It’s dark under the boat. Take a light to peer into through-hulls, and always have a sharp knife; you’ll need it for cutting off stray rope, fishnets, etc., that are wrapped around the prop. Carry a tool for whacking the anodes to see if they crack; sometimes they look good but are eaten away under the surface. A small hammer will work, but it’s another thing to carry, and to drop; instead, try the pommel of your dive knife. Don’t cut yourself on the blade, which should be razor-sharp.

Rather than rely on your memory, carry a simple, point-and-shoot waterproof camera. If you find something wrong, a photo will be worth more than a thousand words when you describe it to your yard manager or the professional diver you’re hiring for the repair. Get a camera with a flash since the lighting isn’t always great under a boat. Shoot your pictures before you clean the bottom; afterwards, floating debris will reflect your flash back into the camera and the pictures won’t be as clear. I have a Panasonic Lumix that I bought 12 or 13 years ago, and it still works fine, above water and below, even though in digital camera years, that’s the equivalent of drawing pictures on wax tablets. New cameras are a lot better.

Going Overboard

Step one to inspecting your bottom is to move your boat out of the marina, ideally to a calm anchorage with a sandy bottom, and deep enough that you don’t stir up sand with your flippers. There’s always a risk of stray electric currents in marina waters, so get away from shore power. Pro divers often work around boats in slips, but why take the chance? Stray current is more dangerous in fresh water: Your body, with its higher salt content, is a better conductor than the water you’re swimming in, so you become the path of least resistance. You’re safer in salt water, but be careful anyway. Shut down your own genset when diving around the boat, too, just in case.

Fly your flags. It’s federal law to fly a diver’s flag, red square with a diagonal white stripe, near the divers in the water, and the International Code Flag Alpha, a white and blue swallowtail, from the highest point on a vessel involved in diving operations. If you’re working under the boat, fly both flags. One person in a hundred will recognize the Alpha flag, but most boaters know the diver’s flag, so buy the biggest one you can find and make sure everyone around can see it. The flag should have a stiffener that keeps it “flying” even when there’s no wind. The diver’s flag warns other boaters to stay well clear, up to 300 feet away in open water; good luck with that, but at least you’re obeying the law. Divers should stay within 100 feet of the flag.

Diver’s Flag

Once under the boat, do your inspection first, while the water’s still clear. Visit the bow thruster and every through-hull, using your light to look for critters living inside. Check the running gear, tap the shaft and rudder anodes, wiggle everything to see if there’s excessive play. If you have pod drives, check their anodes too. If you have outboards, check the anodes mounted under the brackets, which are hard to see from above, and be sure the bonding wire is still intact. Cut any fishing line, rope strands, etc., off the props. Photograph everything so you have a record to add to the maintenance log.

Use a thin tool, maybe an old screwdriver, to excavate barnacles out of through-hulls and the bow thruster, and a stiff putty knife/scraper to remove them from everywhere else. (Try not to drop your tools; ideally, tie a lanyard to each one and wrap it around your wrist.) Barnacles are tough to remove by scraping, especially when you have to pop to the surface for air every minute or so. If you find dense communities of them clogging through-hulls, strainers, drives and underwater gear, bite the bullet and call in a pro diver, one with a CaviBlaster, or some similar device. The CaviBlaster is like a power washer that works underwater, but rather than pressure water, it uses what the manufacturer, CaviDyne LLC, calls “hydrodynamic cavitation” to blow the critters away.


In the old days, we used to clean the slime off the bottom with a wad of coarse burlap—you’ll probably prefer a brush or heavy-duty scrub pad. Your boat will seem a lot bigger after repeatedly diving down to the keel and scrubbing your way back up. If you’re clever, you’ll Tom Sawyer this and recruit your kids, spouse, friends, etc., to help out. Before long the algae, gurry and tiny critters you swipe off the bottom will not only turn the water cloudy but get into your hair and any orifices they can find; experienced bottom-scrubbers wear dive hoods to keep them out of their ears. In warm water where slime grows quickly you’ll want to clean the bottom once a month or so, so spending a few bucks for a hood is smart.

Some folks think investing in a professional bottom cleaning service is even smarter. A diver with a scuba tank will do the job faster and better. He or she will have better equipment than just an old deck brush, too—maybe a Nemo Hull Cleaner, a cool tool that I’m looking for an excuse to buy. The Nemo cleaner is essentially a 12-inch diameter brush chucked into a waterproof cordless electric drill. You pump some air into the drill body to pressurize it so no water leaks in—it’s submersible to 164 feet. The manufacturer says once the brush starts spinning, it self-adheres to the surface so you simply move it along. I’ll bet it does a lot better job than a brush does. It’s expensive—$1,470 according to Nemo’s website, including an 18-volt battery and two brushes, hard and soft—so unless you’re planning on cleaning a lot of bottoms, it makes more financial sense to hire somebody who already has one. I want one anyway.

When I started writing this, I looked back at my old captain’s life of swimming around under the boat, scrubbing the bottom, trying not to drown while changing a prop, and other pleasures with nostalgia. Now, I’m not so sure. I think in the future I’ll restrict my underwater activities to inspection and photography; things that are fun and relaxing. And maybe I’ll do a little snorkeling, too. But when it comes to the hard work, I’ll call in a professional. I have enough sense to come in out of the rain, and out from under the boat. So do you, I’ll bet.

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This article originally appeared in the August/September 2024 issue of Power & Motoryacht magazine.