The underwater hull of a ship is always immersed in seawater and is subject to marine growth. Marine growth, or biofouling, is comprised of marine organisms, such as tubeworms, mussels, barnacles, and algae that are present in marine water. The accumulation of these organisms adds weight to the hull and increases frictional drag through the water. This results in a reduction in the ship’s speed, maneuverability, and an increase in fuel consumption and ship operating costs. Biofouling removal is required by mechanical means. The ideal antifouling coating would be environmentally friendly, prevent marine growth, and have a long service life.
In general, the underwater hull coating system consists of an anticorrosive coating and one or more antifouling topcoats. The anticorrosive component of the coating system protects the substrate from the seawater, while the antifouling paint is necessary to prevent or reduce biofouling.
MIL-PRF-24647, Paint System, Anticorrosive and Antifouling, Ship Hull is the performance specification that provides requirements for underwater hull anticorrosive and antifouling paint systems. The specification is available for download from the ASSIST Quick Search website.
Underwater hull coating systems are categorized into one of the following coating types:
- Type I – Paint systems having topcoats that contain biocides, other than copper, which ablate or self-polish. Copper content must be less than three percent by weight.
- Type II – Paint systems having topcoats that contain copper or other biocide not cited in Type I which ablate or self-polish.
- Type III and Type IIIa – Paint systems having topcoats that are foul-release coatings which do not contain biocide.
- Type IV – Paint systems having topcoats that contain any biocide which do not ablate or self-polish.
Ablative or self-polishing antifouling coatings prevent marine organisms from attaching sufficiently to the coating surface. This type of antifouling paint contains biocides. As the top layer dissolves, a smooth, fresh layer of biocide is exposed. The rate of depletion is controlled, allowing a uniform transition through each layer of the coating. The rate at which these coatings dissolve is accomplished by a chemical reaction between the marine water and a chemical binder contained within the coating matrix.
Foul-release coatings do not contain biocides that could leach into the marine environment. This type of antifouling coating provides a slick surface that makes it difficult for marine organisms to attach to the hull. Those organisms that are able to attach can be easily removed because their ability to adhere to the hull is diminished. Biofouling is released by ship movement through the water.
Although paint systems with topcoats that contain biocide which do not ablate or self-polish (Type IV) are specified in MIL-PRF-24647, they are no longer available in the Qualified Products Database.
Underwater hull coating systems must comply with several environmental regulations. An Environmental Protection Agency (EPA) Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) antifouling paint registration number is required for all antifouling topcoat paints that contain a biocide. A toxicity evaluation is also required by the Navy and Marine Corp Public Health Center. Per MIL-PRF-24647, the grade of the coating system is determined by the volatile organic compound (VOC) content of its ready-to-apply condition.
The service life of an underwater hull coating system depends on its ability to adhere and perform without failing due to blistering, flaking, depletion by excessive ablation, or loss of antifouling capability. The application of the underwater hull coating system is defined by the expected service life interval (3, 7, or 12 years, or a minimum of 2 years for high-speed vessels attaining 40 knots or greater).