If you leave your boat in the water, especially salt water, for any extended period of time, all kinds of aquatic creepy-crawlies are gonna start growing on the bottom of the boat—not a good thing. When this happens, we say that the bottom’s fouled—nautical-speak for a bad situation.
One way to prevent marine wildlife from colonizing on your boat is to use antifouling paint on the parts of the hull that are continually submerged—essentially, from the waterline down.
Antifouling paint contains biocides—toxins, usually a form of copper—that the mini-creatures that love your boat absolutely hate. In addition to biocides, many high-end antifouling paints also feature ingredients to keep algae/slime from taking up residence on your hull.
The kind of boat you have and how you use it dictates what type of bottom paint is best for your situation.
Hard Paints
If you have a fast powerboat and you want the boat’s bottom to be as slick as it can be, then a hard antifouling paint, with contact leaching (migrating) () properties might be the best choice for you. This type of paint dries to what appears to be a hard finish, but under intense magnification, you’d see that the surface is somewhat porous and chock full of biocides.
As you zoom along, the biocides leach out of the bottom paint to keep plants and animals away from your boat. The upside to this kind of antifouling paint is the boat bottom stays smooth and free from growth—for a reasonable period of time—maybe a full boating season. After a while, the level of available biocides decreases to the point of offering little or no protection. In addition, if you haul your boat out of the water during the season, the antifouling properties of the paint go away and you’ll need to repaint the bottom before putting it back in the water.
Paints That Wear Away
Another approach to protecting your boat’s bottom is to use antifouling paints (containing biocides/algaecides) that are designed to wear away a little bit at a time as you use your boat throughout the season. Available in various levels of solubility (self-polishing, controlled solubility, and ablative, to name a few) these coatings are saturated with ingredients to keep aquatic beasts at bay. As the top layer wears off, a fresh layer of biocides/algaecides is revealed to keep up the good fight—kind of like rubbing a bar of soap—scrub all you want, the soap keeps comin’—same process with these paints.