The key trade-offs in selecting an antifouling coating for your boat’s bottom involve effectiveness, durability and longevity, all measured against cost in time, labor and dollars.
For example, if you haul and lay up your boat at the end of the season for, say, six to eight months, then you may find that the most effective antifouling paint is also the least expensive—because your short use cycle precludes you from realizing the full value of paints with greater longevity. If, however, you keep your boat in the water year-round, then hauling to clean and paint (and relaunching) become added costs that can be avoided with more expensive paints that remain effective for two or three years.
The point is that true cost involves more than what you pay per gallon for the paint. To determine value, start with local knowledge. If you keep your boat in a given area most or all of the time, then find out what works best for other boaters. Local conditions (water salinity, temperature and pH levels) greatly affect which antifouling coatings work best and last the longest.
And, of course, there are saltwater versus freshwater needs; if the major fouling organisms in your waters are grasses and slime, then you need minimal, if any, copper-based biocide in your antifouling coating.
Decide up-front whether you or a professional will apply the antifouling product. Ever-tightening environmental regulations allow certified professionals significantly greater latitude in the types and formulations of coatings they can use. (However, even for professionals, the regulations are different for yachts over 82 feet.)
You’ll also need to choose between water-and solvent-based coatings. Because water-based coatings do not throw off volatile organic compounds, they are better for the people applying the coatings.
And, you’ll want to research fouling organisms in your waters: barnacles, mussels, algae, slime, whatever. Depending on which organisms are the most prolific, you may decide to use an antifouling coating containing a copper-based biocide. However, copper biocides are the hardest on the environment. Several manufacturers offer coatings that contain multiple biocides, covering most or all problems while minimizing the copper.
Your boat’s hull material is also a factor. For instance, if you have an aluminum hull, then you’ll want to avoid antifouling coatings that are heavily loaded with copper-based biocides, which can galvanically attack the aluminum substrate. There are ways to use nonconductive epoxy primers and barrier coats to ameliorate the problem—primed and barrier-coated steel can better tolerate copper-based antifouling—but I’d still avoid the copper-based biocides if at all practical. Fiberglass and related composites give you the most flexibility in choice of biocide. And wooden hulls should, in my experience, use a copper-based biocide to discourage Teredo worms, which can turn a wooden hull into a real mess.
You’ll hear a lot of talk about ablative versus hard antifouling coatings. Ablative coatings tend to have their outer surface slough off when your boat is moving through the water. These coatings have biocide dispersed throughout them, so this sloughing action brings fresh biocide to the surface. Hard (or nonablative) coatings do not slough away, so there is a higher probability that their antifouling properties will diminish or deplete sooner. The latter might not be a problem if your boating season is relatively short, but hard coatings will build up over time with each seasonal recoat, so they must be sanded down periodically to avoid excessive buildup.
Then again, some of the hardest of the hard coatings can be burnished to reduce friction, reportedly making for a fast bottom on racing sailboats.
Most biocides do not work by poisoning the fouling organisms, but rather by creating an environment that those organisms (especially barnacles) don’t like. So, try not to feel too guilty about sending the plague of little critters downstream to make someone else’s life miserable. Those boaters probably didn’t take the time to heed the good advice offered here.