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 the Age of Sail, sailing vessels suffered severely from the growth of barnacles and weeds on the hull, called "fouling". Starting in the mid-1700’s thin sheets of copper and approximately 100 years later, Muntz metal, were nailed onto the hull in an attempt to prevent marine growth. One famous example of the traditional use of metal sheathing is the clipper Cutty Sark, which is preserved as a museum ship in dry-dock at Greenwich in England. Marine growth affected performance (and profitability) in many ways.
- The maximum speed of a ship decreases as its hull becomes fouled with marine growth, and its displacement increases.
- Fouling hampers a ship's ability to sail upwind.
- Some marine growth, such as shipworms, would bore into the hull causing severe damage over time.
- The ship may transport harmful marine organisms to other areas.
In an official 1900 Letter from the U.S. Navy to the U.S. Senate Committee on Naval Affairs, it was noted that the (British) Admiralty had considered a proposal in 1847 to limit the number of iron ships (only recently introduced into naval service) and even to consider the sale of all iron ships in its possession, due to significant problems with biofouling. However, once an antifouling paint "with very fair results" was found, the iron ships were instead retained and continued to be built.
During World War II, which included a substantial naval component, the U.S. Navy provided significant funding to the Woods Hole Oceanographic Institution to gather information and conduct research on marine biofouling and technologies for its prevention. This work was published as a book in 1952, the contents of which are available online as individual chapters. The third and final part of this book includes a number of chapters that go into the state of the art at that time for the formulation of anti-fouling paints. Lunn (1974) provides further history.
Modern antifouling paints
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 fibers sticking out from the coating surface. These small fibers move in the water, preventing bottom growth from adhering.
In the 1960s and 1970s, commercial vessels commonly used bottom paints containing tributyltin, which has been banned in the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization due to its serious toxic effects on marine life (such as the collapse of a French shellfish fishery). The Port of San Diego is investigating how to reduce copper input from copper-based antifouling coatings  and Washington State has passed a law which may phase in a ban on copper antifouling coatings on recreational vessels beginning in January 2018. However, despite the toxic chemistry bottom paint and its accumulation in water ways across the globe a similar ban was rescinded in the Netherlands after the European Union's Scientific Committee on Health and Environmental Risks concluded The Hague had insufficiently justified the law. In an expert opinion, the committee concluded the Netherlands government's explanation "does not provide sufficient sound scientific evidence to show that the use of copper-based antifouling paints in leisure boats presents significant environmental risk." 
"Sloughing bottom paints", or "ablative" paints, are an older type of paint designed to create a hull coating which ablates (wears off) slowly, exposing a fresh layer of biocides. Scrubbing a hull with sloughing bottom paint while it is in the water releases its biocides into the environment. One way to reduce the environmental impact from hulls with sloughing bottom paint is to have them hauled out and cleaned at boatyards with a "closed loop" system.
Some innovative bottom paints that do not rely on copper or tin have been developed in response to the increasing scrutiny that copper-based ablative bottom paints have received as environmental pollutants.
A possible future replacement for antifouling paint may be slime. A mesh would cover a ship's hull beneath which a series of pores would supply the slime compound. The compound would turn into a viscous slime on contact with water and coat the mesh. The slime would constantly slough off, carrying away micro-organisms and barnacle larvae.
- "Coating Systems For Underwater Hull Surfaces". NSTCenter. Naval Surface Treatment Center. Retrieved 2016-07-03.
- "Marine Corrosion Explained". MarineCoatingPaint.com. June 29, 2015. Archived from the original on January 8, 2016. Retrieved 2015-10-31.
- Are foul-release paints for you? Coating calculator Archived 2011-07-17 at the Wayback Machine, National Fisherman
- Dafforn, Katherine A.; Lewis, John A.; Johnston, Emma L. (2011). "Antifouling strategies: History and regulation, ecological impacts and mitigation". Marine Pollution Bulletin. 62 (3): 453–65. doi:10.1016/j.marpolbul.2011.01.012. PMID 21324495.
- "Letter from the Secretary of the Navy, Transmitting the Views of the Members of the Board on Construction Relative to the Question of Sheathing and Coppering the Three Battle Ships and the Three Armored Cruisers Authorized by the Naval Appropriation Act, Approved March 3, 1899 (p.17, S. Doc. No. 165, 56th Congress, 1st Session)". Google. U.S. Government Printing Office. February 10, 1900. Retrieved 2016-07-06.
- Stetson, Judith (Feb 23, 2011). "Woods Hole in World War II" (PDF). Woods Hole Historical Museum. Woods Hole Historical Museum. Retrieved 2016-07-06.
- "Marine fouling and its prevention; prepared for Bureau of Ships, Navy Dept". Woods Hole Oceanographic Institution. United States Naval Institute. 1952. Retrieved 2016-07-06.
- Lunn, Iver (1974). Antifouling: a brief introduction to the origins and development of the marine antifouling industry. Thame, UK: BCA Publications. ISBN 0950129917.
- Collins, Andrew P (13 August 2019). "Why So Many Ships Are Red On The Bottom". Jalopnik. Gizmodo Media Group. Retrieved 15 August 2019.
- Ecofriendly boating, antifouling paint
- "Focus on IMO - Anti-fouling systems" (PDF). International Maritime Organisation.
- "San Diego Region - Shelter Island Yacht Basin Copper TMDL". Regional Water Quality Control Board - San Diego. Retrieved 2015-10-31.
- http://www.tradeonlytoday.com/home/511463-washington-state-bans-copper-bottom-paintdid[permanent dead link] the same in May, 2011.
- "EU expert committee challenges Dutch copper-paint restrictions". Free Online Library. 2009. Retrieved 2015-10-31.
- Selecting an anti-fouling paint Archived 2011-09-28 at the Wayback Machine, West Marine
- http://goliath.ecnext.com/coms2/gi_0199-5573950/Fouling-out-coppers-glory-fades.html[dead link] Fouling out: coppers glory fades as new bottom-paint research leads to improved antifouling treatments, National Fisherman, June 2006
- "EPA Reports to Congress on Tributyltin Boat-Bottom Paint" (Press release). EPA. June 20, 1997. Retrieved August 26, 2012.
- http://findarticles.com/p/articles/mi_m0BQK/is_6_10/ai_n15858063/[dead link] Will California terminate copper paint?, BNET, 2005
- Paul Marks (September 2009). "Slimy-skinned ships to slip smoothly through the seas". New Scientist. Retrieved 28 September 2009.
- "Slime-Covered Boats Could Keep Marine Pests at Bay". 28 September 2009. Retrieved 28 September 2009.
- Selecting an anti-fouling paint, West Marine
- Clean Boating Tip Sheet, Selecting a Bottom Paint, .pdf chart, Maryland Dept. of Natural Resources
- Bottom Paint for Racing Boats, Sailing World, 2007
- Are foul-release paints for you? Coating calculator, National Fisherman
- Using Antifouling paint against the Gribble Menace, Teamac Marine Coatings
- "Focus on IMO - Anti-fouling systems" (PDF). International Maritime Organisation.