Anti-fouling paint – a category of commercially available underwater hull paints (also known as bottom paints) – is a specialized category of coatings applied as the outer (outboard) layer to the hull of a ship or boat, to slow the growth and/or facilitate detachment of subaquatic organisms that attach to the hull and can affect a vessel’s performance and durability (see also biofouling). Anti-fouling paints are often applied as one component of multi-layer coating systems  which may have other functions in addition to their antifouling properties, such as acting as a barrier against corrosion on metal hulls that will degrade and weaken the metal, or improving the flow of water past the hull of a fishing vessel or high-performance racing yachts.
In modern times, antifouling paints are formulated with cuprous oxide (or other copper compounds) and/or other biocides—special chemicals which impede growth of barnacles, algae, and marine organisms. Historically, copper paints were red, leading to ship bottoms still being painted red today.
“Soft”, or ablative bottom paints slowly slough off in the water, releasing a copper or zinc based biocide into the water column. The movement of water increases the rate of this action. Ablative paints are widely used on the hulls of recreational vessels and typically are reapplied every 1–3 years.
“Hard” bottom paints, or “nonsloughing” bottom paints, are made in several types. “Contact leaching” paints “create a porous film on the surface. Biocides are held in the pores, and released slowly.” Another type of hard bottom paint includes Teflon and silicone coatings which are too slippery for growth to stick. SealCoat systems, which must be professionally applied, dry with small fibres sticking out from the coating surface. These small fibres move in the water, preventing bottom growth from adhering.