2 min read · Updated 30 June 2026
Antifouling is the paint that keeps a boat's bottom clean and fast. Applied below the waterline, it stops the marine growth — slime, weed and barnacles — that would otherwise roughen the hull, create huge drag, and slow the boat. For a racing yacht especially, a clean bottom is worth real speed, which is why the underwater surface gets so much attention.
Why boats foul
Any hull sitting in the water quickly attracts fouling — first a film of slime, then weed and barnacles. Even a thin layer creates enormous drag relative to a smooth surface, because the hull's efficiency depends on a clean, fair surface slipping through the water (see hull design). A fouled bottom means slower passages, higher fuel use, and — for a racer — a serious performance loss.
How antifouling works
Antifouling paint contains biocides that discourage growth from settling and taking hold. It's applied to the prepared, cleaned underwater hull, usually when the boat is hauled out, and keeps the bottom cleaner and the boat faster between haul-outs.
Hard vs ablative
There are two broad types:
- Hard antifouling — sets to a tough film that stays put and can be burnished and wet-sanded to a very smooth finish. Favoured by racing boats for that smoothness, though it builds up with repeated coats and its biocide can fade over time.
- Ablative (self-polishing) — designed to wear away slowly as the boat moves, constantly exposing fresh biocide and shedding growth. Suits cruising boats and avoids heavy paint build-up.
The choice depends on how the boat is used.
How often, and racing focus
Antifouling is typically renewed at the annual haul-out, though it varies with the paint, the waters (warmer water fouls faster) and how much the boat is used. It's usually combined with cleaning and inspecting the hull, keel and fittings, and looking after the gelcoat.
For racing, the priority is smoothness: racers use a hard antifouling sanded and burnished to a fine finish and keep the bottom scrupulously clean, wiping off slime before racing. Some dry-sailed boats, kept out of the water between races, use different finishes — but a smooth, fouling-free hull is a universal priority for speed. For the vocabulary, see the sailing terms glossary.
Frequently asked questions
- What is antifouling?
- Antifouling is a specialised paint applied to the underwater part of a boat's hull to prevent or slow the growth of marine organisms such as weed, slime and barnacles. Left unchecked, this growth — called fouling — roughens the hull, dramatically increasing drag and slowing the boat, and can damage fittings. Antifouling paint contains biocides that discourage growth from settling, keeping the bottom cleaner and the boat faster and more efficient between haul-outs.
- Why does a boat need antifouling?
- Because a hull sitting in the water quickly attracts marine growth, and even a thin layer of slime or a scattering of barnacles creates enormous drag that slows the boat and increases fuel or effort. For a racing yacht a fouled bottom is disastrous for performance; for a cruiser it means slower passages and higher fuel use. Antifouling keeps the underwater hull smooth and clean, protecting both speed and the hull itself.
- What is the difference between hard and ablative antifouling?
- Hard antifouling sets to a tough film that stays put and can be burnished and even wet-sanded to a very smooth finish, which is why racing boats often use it, but its biocide can become less effective over time and it builds up with repeated coats. Ablative or self-polishing antifouling is designed to wear away slowly as the boat moves through the water, constantly exposing fresh biocide and shedding growth, which suits cruising boats and avoids heavy paint build-up. The choice depends on how the boat is used.
- How often should you antifoul a boat?
- Antifouling is typically renewed when the boat is hauled out for its regular maintenance, commonly about once a year, though this varies with the type of paint, the waters the boat sits in, and how much it is used. Warmer waters and boats that sit still for long periods foul faster and may need more frequent attention. Many owners combine antifouling with the annual haul-out, cleaning and inspection of the hull, keel and fittings.
- Do racing boats antifoul?
- Yes, but with a strong focus on smoothness, because for a racing yacht the state of the underwater surface is worth real speed. Racers often use a hard antifouling that can be sanded and burnished to a fine, smooth finish, and keep the bottom scrupulously clean, wiping off any slime before racing. Some high-performance and dry-sailed boats, kept out of the water between races, may use different finishes, but keeping the hull smooth and fouling-free is a universal priority for speed.