As any boat owner knows, some sort of antifouling system is necessary in order to keep the hull beneath the water line free of organisms that might otherwise grow there, a condition known as biofouling. These organisms create drag, and thus negatively impact both speed and fuel consumption. They can also grow in the water intakes, potentially getting into the pipes and creating corrosion and blockages.
One way to prevent biofouling is to store the boat out of the water and/or periodically clean the hull. For many boat owners, however, this option is impractical. Modern-day antifouling consists either of some sort of coating on the hull or an internal system that may be either electrical or ultrasonic.
Hull coatings come in two varieties: paints in which a copper compound such as copper oxide is suspended along with some other biocides, or epoxy resins containing copper particles.
The copper oxide in antifouling paint reacts with seawater to produce cuprous ions, which are toxic to seaweeds and barnacles. The presence of a layer of cuprous ions will usually prevent these organisms from attaching to the hull. Other biocides in the paint serve to keep the hull free of algae and slime. Because these biocides leech out of the paint, it becomes less effective over time. Consequently, the hull must be repainted with some frequency. Some paints must be reapplied every year, while others may last two or more seasons.
The semi-permanent coatings, which consist of epoxy resins containing copper particles, are both more expensive than paint and more durable. It may be necessary to wash them off and give them a light sanding once a year, but most of them will last for over 10 years. They work on the same principle as the paints do, but because the epoxy matrix is harder, they last longer.
Ultrasonic antifouling systems use a wave generator coupled with external transducers mounted either to the sea chests or strainers. An electrical signal from the wave generator excites piezoelectric crystals in the transducers, creating a high-frequency ultrasonic barrier. This barrier both repels organisms and prevents them from adhering to the hull. These systems are expensive, there have been mixed reviews as to their efficacy, and most manufacturers still recommend putting a hard-coat antifouling resin on the hull in addition to the ultrasonic unit.
Electrical systems use a cathode fixed to a keel along with anodes attached above it to create sodium hydroxide, hydrogen, chlorine, and oxygen bubbles through the electrolysis of seawater. These then react to form sodium hypochlorite, chlorine, and hydrogen which serve to poison any organisms that may cause fouling. Like the ultrasonic systems, they require a power source to work.
All of these systems, with the exception of the ultrasonic systems, release chemicals into the water that may be problematic for marine life. Further research is ongoing, both in terms of the impact of current systems and the development of new ones.