The purpose of applying antifouling bottom paint is to prevent hard biofouling organisms like mussels, barnacles, and tubeworms, and soft growth like weeds and algae (slime!) from attaching to your hull below the waterline.

There are several different antifouling coatings available, with new ones being formulated all the time, making it very difficult to choose the paint that’s right for your boat and your budget. Some coatings work better in different locations, including fresh or saltwater. Some are more environmentally friendly than others, and some cost more based on the type of biocide and the protection they provide.

The key antifouling ingredient in bottom paint is some type of biocide for deterring hard marine growth. Some antifouling paints also include an algaecide for preventing soft growth.

Biocide basics: For centuries, some form of copper has been used successfully to prevent the attachment of barnacles and other marine growth on hull bottoms. Today, the type of copper most commonly contained in antifouling paint is either cuprous oxide or cuprous thiocyanate. Cuprous thiocyanate has the added benefits of being a lighter copper in terms of color, so it produces brighter, truer paint colors. Also, paints using cuprous thiocyanate use half the amount of copper used in conventional, heavier cuprous oxide paints.

Because copper in the concentrations used in antifouling paints is a potential threat to some marine ecosystems, there are now equally effective, eco-friendly, antifouling paints that contain a metal-free biocide called Econea™ (main ingredient is tralopyril). Econea’s main benefit is that it dissolves after it is shed into the water, leaving no byproduct to harm marine life. Its secondary benefit is that it allows for the creation of brightly colored paints, unlike bottom paints containing cuprous oxide as the biocide.

Algaecide basics: If you use your boat in an area where slime, algae, and grass growth on the bottom of your hull are common, you need an antifouling paint that also contains an algaecide to prevent this soft growth. In the past, some antifouling paints contained an algaecide called Irgarol, which is no longer being used. Irgarol was discontinued because of its persistence after release into the water, and harmful effects on marine life. Zinc pyrithione (Zinc Omadine™) is now the slime-fighting agent used in many antifouling bottom paints. Its effectiveness is similar to Irgarol, but it dissipates quickly and harmlessly into marine or freshwater ecosystems. Zinc pyrithione is found in chemical solvent-based and water-based antifouling marine paints.

Chemical solvent-based or water-based antifouling paint? Another consideration is the type of solvent in the paint. Solvents keep the biocide, algaecide (if included), and pigment suspended in the paint so they’re distributed evenly in the coating as it’s applied to the hull surface. In the past, most antifouling paints contained harsh chemical solvents, and required the use of chemical solvents for thinning and cleanup. Those harsh chemical solvents contain toxic VOCs (volatile organic compounds) that evaporate easily into the atmosphere. To help meet environmental regulations in some areas of the country, and the world, there are now water-based paints, that also use water for thinning and cleanup. Many of these water-based antifouling paints contain the biocide Econea, and zinc pyrithione as the algaecide, because both dissolve in the water after release.

Won’t a water-based antifouling paint just dissolve and leave the hull unprotected while the boat is in the water? The short answer is no. As the water-based paint cures, the water evaporates, leaving behind a protective film of biocide and/or algaecide, and pigment.

Teflon and vinyl bottom paints: Teflon paints are expensive, dry quickly, cure to a very thin film, and are ideal for freshwater areas where algae is a problem. Vinyl bottom paints are hard bottom paints that can be burnished smooth for racing, and are especially effective in saltwater, though not as effective as ablative paints for battling biofouling organisms. Both Teflon and vinyl bottom paints are used more to enhance performance, as in race boats, instead of antifouling.

Proprietary two-part bottom coatings: These products are new and expensive, using patented chemical formulas to create a hard and very slippery biofilm that doesn’t allow living organisms to attach to the boat.

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