Experienced skippers know how important it is to keep their bottoms clean by maintaining an effective layer of antifouling paint. Without it, their boats face invading armies of barnacles, mussels, seaweed, algae and other marine pests. The first ones arrive about five minutes after the boat is launched, or so it seems. Others keep on coming, and soon a once silky-smooth bottom becomes an underwater jungle of seagoing flora and fauna. Antifouling, or “bottom paint,” is your defense against uninvited marine guests.
There are seemingly endless choices of antifouling paint on the market, so how do you choose the right one? Should you pay extra for a multi-season “ablative” paint? Are there environmentally friendly options? Sounds complicated, but fortunately picking a bottom paint isn’t rocket science once you know what’s really in the can.
Pick Your Biocide
Antifouling paint’s mission is to kill, or at least repel, invaders, and paints from the reputable manufacturers all do the job just fine. To repel those invaders, paint manufacturers use biocides. In essence, all manufacturers use the same ones. The manufacturer of the paint is less important than the biocide and the usage for which the paint is formulated. Once you pick your biocide, choosing a paint is much simpler.
Copper-Based Paints
Cuprous oxide is the classic biocide—it’s why old-timers called antifouling paint “copper bottom” paint. Cuprous oxide gradually releases copper ions into the water, repelling barnacles and other unwanted critters. A separate algaecide, frequently zinc pyrithione (trade name Zinc Omadine), is often added to the paint to combat
slime and algae. Copper has been keeping bottoms clean for centuries. The British Navy started copper-sheathing some ships in the 1760s. In 1863, James G. Tarr and Augustus H. Wonson of Gloucester, Massachusetts, received a patent for a copper-oxide-based antifouling paint, using tar as a binder. It probably wasn’t very easy to brush on, but it did the job.
Copper bottom paint still does the job. Interlux Fiberglass Bottomkote NT, for example, is a single-season, cuprous-oxide antifouling with modern upgrades. It uses the company’s Dual-Resin Technology to combine the advantages of a hard paint with those of an ablative. The paint’s hard surface will gradually polish off during use. Interlux says this will minimize the build-up of dead paint after years of recoating paint that eventually must be removed. Bottomkote NT is easy to apply, so it’s suitable for do-it-yourselfers. The cost is roughly $170 per gallon.
Copper-Free Paints
Cuprous oxide antifouling cannot be applied to aluminum hulls. The copper reacts galvanically with aluminum, and aluminum will lose that battle. Folks with aluminum boats usually choose antifouling with a biocide of metal-free tralopyril, marketed as Econea, frequently reinforced with Zinc Omadine as an algaecide. Econea is an effective biocide that can be used on any hull, and on underwater metal, too. It works differently than copper at repelling marine organisms, but the result is the same: a clean bottom. Econea releases no toxic metals into the water, so it’s better for the environment than cuprous oxide—or less bad, anyway. Two states, Washington and California, have suggested, although not enacted, regulations limiting the use of copper biocide, and a few Northern European countries have already enacted restrictive regulations, so Econea and other metal-free biocides might be the antifoulings of the future.
Pettit’s copper-free Hydrocoat ECO is a water-based antifouling containing Econea and Zinc Omadine that’s suitable for all hull materials. It’s a self-polishing ablative so it will be effective for multiple seasons and will fight fouling even if the boat is not used much. You only recoat the bottom when the paint gets thin. Professionals apply an initial two coats of contrasting colors, so it’s easy to see when the topcoat is wearing down. Because it is water-based, Hydrocoat ECO doesn’t have the strong solvent smell of traditional paints, and clean-up is easy. It sells for about $343 per gallon.
Total Boat’s Krypton antifouling is solvent-based and not a water-based paint. It also relies on Econea biocide backed by Zinc Omadine. It’s a single-season paint, so it is less costly than multi-season ablatives. It’s priced at about $230 per gallon. It can also be used to paint outdrives, props and other underwater metals.
A Different Copper
Some antifouling paints contain a third biocide, cuprous thiocyanate. According to the paint manufacturers, cuprous thiocyanate doesn’t promote galvanic corrosion with aluminum hulls because the paint’s polymer matrix insulates the metals from each other. Nevertheless, most manufacturers recommend protecting aluminum with an epoxy barrier coat before applying antifouling (fiberglass hulls should also be barrier-coated, to prevent water absorption into the laminate). When exposed to salt water, cuprous thiocyanate reacts with the sodium and releases copper ions that repel invaders.
Interlux Trilux 33 combines cuprous thiocyanate and Biolux controlled-release biocide. It’s formulated for primed aluminum hulls, but works fine on fiberglass, steel and wood too, and on underwater metals. It’s a multi-season, slow-polishing ablative paint that will hold up on high-speed powerboats. The price is about $378 per gallon.
Cuprous thiocyanate is often blended with other biocides to make the paint more effective. Sea Hawk’s ColorKote, for example, blends Econea, cuprous thiocyanate and Zinc Omdine. The manufacturer says it’s compatible with all hull materials, works in salt, fresh and brackish water, and is California Air Quality Compliant for low levels of VOCs. ColorKote is self-polishing and ablative and can be applied over most existing bottom paints. It comes in six colors but should the precise hue of your bottom be important, the six can be blended to make 46,656 custom colors, according to the manufacturer. Cost is around $395 per gallon.
Of course, picking the right bottom paint isn’t just about the biocide. It’s also about how you use your boat, where you keep it and how much of an up-front investment you want to make. Before spending big bucks on antifouling, do some research. The manufacturers’ websites have product guides where you answer a few questions and they’ll point you towards two or three of the most suitable paints for you.
Make use of local knowledge, too. Marine pests are very localized, and the paint that works great in one harbor might not be so great a few miles down the coast. Paint that’s effective at floating docks near a tidal flow might not work so well in a relatively stagnant boat basin. Ask the locals. The guy behind the counter at the chandlery is a good place to start. He’ll probably know what biocide is effective in your waters.
For most boaters, a multi-season, ablative paint is the answer. Ablative paints are formulated to wear away gradually and release more biocide as the boat moves through the water. They’re good for boats that are used frequently. If you don’t use your boat often—and most people don’t—a self-polishing ablative paint offers the same benefits but doesn’t require so much water movement. Ablative paints don’t need recoating every year, only when they wear thin. They also don’t build up layers of dead paint on the bottom. Although more expensive up front, ablative paints will save money over time.
Other boaters want to minimize their cash outlay, and single-season paints cost the least. They still use the same biocides, so they will be equally effective as multi-season paints. The question is, do you pay now, or do you pay later? Single-season antifouling paints will need recoating every season, and the paint will probably cost more next year. They also leave behind dead paint that eventually builds up and has to be stripped, which is a costly procedure.
Some folks have special needs. Sailboat racers, for example, want their bottoms as smooth and slick as possible, and will wet sand and polish them during the season to get a few tenths of a knot more speed during a race. Speed-crazy powerboaters might do the same, and maybe get a little extra fuel economy, too. These folks might choose a hard paint that will stand up to frequent cleaning and abrasion without wearing through.
No matter what marine pests you face, no matter how fecund your local waters are, there’s an antifouling paint for you. Finding it, among the seemingly infinite choices, takes just a little knowledge, a few minutes of research, and a pleasant stroll through the paint department of your local chandlery. It all starts by picking the right biocide.
Keep Your Metal Clean
Most people paint their props, struts, rudders and, sometimes, propshafts with the same antifouling paint they use on the rest of the hull, often just slapping it on without appropriate preparation or primers. More finicky skippers rely on special-brew foul-release coatings. Biocide-free, these multi-layer coatings are based on silicone, ceramic or other slippery compounds, and make the metal so slick that growth can’t stick to it. Leave the boat sitting for weeks and you might still get a layer of growth on the props and rudders, but according to the manufacturers when you go for a ride most of it will slough off. An added advantage is improved propeller efficiency thanks to the hard, smooth surface created by the coating.
The foul-release product most people have heard of is silicone-based Propspeed, but there are others. PropGlide is one, Glidecoat’s ceramic Prop Optimizer is another. They can all be applied to almost any metal and will usually last for a couple of years, depending on how you use your boat, but they can be costly. A 200-milliliter Propspeed kit—enough to coat a four-bladed prop up to 24 inches diameter—costs about $320. Covering all the underwater gear of a typical mid-size powerboat requires a full liter of Propspeed, which will cover 36 square feet, enough for twin 24-inch props, shafts, struts, rudders and trim tabs. That will cost around $600 in most chandleries. PropGlide works similarly for roughly two-thirds the cost.
Some boat owners go another route and coat their underwater metals with a zinc-based paint that both inhibits corrosion and keeps barnacles and growth away. It won’t last like Propspeed, but it costs much less. Pettit says its Prop Coat Barnacle Barrier contains 93 percent zinc, will self-clean when you run the boat, and is easy to apply. Sand the metal back to a shiny surface, spray or brush on three coats and you can recoat after five minutes. Barnacle Barrier comes in a 20-ounce aerosol can for about $40, or you can get a quart for about $125.
April 2025
