An aging "dock rat" once told me that cheap latex house paint works just as well as expensive marine antifouling paint on your boat's bottom: You apply the latex and a month or so later it is completely fouled with algae, grass and barnacles - exactly like expensive marine antifouling.

The quip is, of course, ridiculous. However, it does spotlight an important truth. Unless you choose a marine antifouling paint carefully and assure that it is applied correctly, you might just as well save yourself a stack of cash by using ordinary house paint.

Manufacturing a yacht bottom coating that combats marine fouling and is at the same time practical to apply, reasonably durable, and not prohibitively expensive, is a highly complex business. Paint and chemical companies such as Interlux, Pettit, and Z-Spar, have devoted very significant research and development budgets to the problem, with the result that there is today a wide variety of antifouling bottom coatings from which to choose.

 Unfortunately, as the number of available options has grown, so has consumer uncertainty and anxiety levels. Indeed, many boaters and yachtsmen now face the marine paint counter with trembling indecision, while horrific visions of creeping marine crud dance in their heads.

Whoa! Relax. Whether you have a Hatteras motor yacht, a Grand Banks trawler, or Bertram convertible sport fishing boat, you can ease your bottom paint anxieties by taking a look at what antifouling coatings are all about, how they're formulated, how they work (and fail to work), why they cost as much as they do, and how to choose one best suited to your needs.

Yacht antifouling coatings are specialized formulations whose primary function is to

retard or prevent the growth of marine animals and plants on a vessel's underwater

surfaces. These coatings accomplish this by poisoning the organisms involved and/or providing an environment that is otherwise unfavorable for the attachment and growth of said heinous marine animals and plants. The constituents of all paints, including

antifouling paints, divide into three major categories: 1) a medium or binder, 2) pigment, and 3) solvent.

The medium (or "binder") creates the basic coating film. Subsequent to application, it converts by chemical reaction from liquid form to a solid film. Pigments are suspended in the medium, and add specialized qualities, such as abrasion resistance or antifouling

properties, to the coating film. Solvents provide viscosity (flow) control to the mixture, and prevent the liquid medium from converting prematurely to a solid film.

In an antifouling paint, water-soluble toxic pigments (referred to in the industry as "toxicants") are held in a medium or binder that enables them to be released on a controlled basis into the water immediately surrounding your boat or yacht's underwater hull surface. It is this toxic, waterborne layer that discourages the growth of fouling organisms on the hull and underwater gear.

Nowadays, antifouling paints divide into two broad categories: 1) hard paints, and 2) ablative coatings. Ablative coatings break down further into a) soft sloughing paints, 3) b) controlled-solubility copolymers and controlled-depletion polymers, and c) self-polishing copolymers.

1. Hard antifouling paints are filled with biocides as part of the pigment, which leach out of the coating when it is immersed in water.

 These antifouling coatings begin life with a strong punch, but continually lose strength as the biocides are leached out of the paint film, until the level of biocide release becomes so low as to make the antifouling coating useless. Because the paint film of hard paints remains pretty much intact even after its contained biocides are depleted, repeated re-coatings, year after year, result eventually in an overly thick antifouling paint coating that tends to crack and peel and, in general, become a nuisance.

2. Ablative antifouling coatings are more efficient than hard coatings at delivering biocide to the water interface layer around a yacht's submerged hull. Ablative paint films literally wear away (erode) as water flow passes over the paint film, There are, however, several different ways in which an ablative coating can wear away.

a) Soft sloughing antifouling paints are formulated with a natural osin-based medium, and are also filled with biocides that leach out of the coating over time in use. The soft sloughing paints are the least expensive in terms of initial materials costs, but they are also the least durable, and tend to lose their effectiveness in a relatively short period of time. These soft paints do, however, have the advantage that, by the time their antifouling effectiveness has been expended, the paint film has mostly sloughed away,. Consequently, with these paints, there is little film build up over multiple recoatings. 

A disadvantage of these paints is that they generally must be launched within 48 to 72 hours of being applied, or they may lose their antifouling capabilities. As well, it is not generally advisable to allow these kinds of antifouling paints to dry out for extended periods of time during a haul outs for maintenance or repair work..

b) Controlled-solubility copolymers and controlled-depletion polymers employ a medium that is partially water-soluble. This means that the paint film literally dissolves, albeit slowly, as water passes over it. This action continually exposes new layers of biocide to the water, as the biocide at the paint film surface becomes depleted. 

As a result, these antifouling coatings are very effective in discouraging growth, since the strength of their biocide release does not wane significantly until the sum total of contained biocides is effectively depleted.. These paints also display the advantage of automatically reducing their residual paint film thickness in use, which avoids the insidious buildup of depleted coatings over time.

c) Self-polishing copolymer antifouling coatings contain an acrylic copolymer that reacts chemically with seatwater at the exposed surface of the paint film. As with other ablative coatings, this reaction results in the constant exposure of a fresh paint layer, with full-strength contained biocides, during the entire working life of the coating. However - and this is a key point - because the ablation is the result of a chemical reaction, and not the result of water passing over the paint film surface, the biocide exposure rate is constant, whether or not the yacht moves slowly or fast, or remains static at dock.

The softer, less durable antifouling paints are generally less expensive in terms of materials first cost. The cost of haul, launch, and application, remain pretty much constant for all types of antifouling coatings. Therefore, the long-term cost of using a cheaper, soft antifouling paint may not work out to be that much less over the long run. If your boat or yacht rarely moves, and if she stays in the water continually between bottom paintings, then some cost saving may be realized with the less expensive soft antifouling paints. 

But if you run your boat a lot, and especially if at higher speeds, you will almost certainly find that one of the controlled solubility, controlled depletion, or self-polishing copolymer products will suit your needs better and more cost-effectively.