Short answer: An antifouling coating is a paint applied to the underwater hull and appendages to limit marine growth such as slime, weed and barnacles. Fouling increases drag and fuel use, reduces speed and can carry invasive species between regions. Most antifoulings release a biocide, usually copper-based; biocide-free foul-release coatings instead use a slippery surface. On yachts, antifouling is commonly renewed every one to three years, depending on the product and how the yacht is used.
What Is Antifouling?
Antifouling is the outer layer of the underwater paint system. Its job is to stop or limit organisms attaching to the hull, propellers, shafts and other submerged parts. Fouling can develop quickly, depending on local conditions and how much the yacht moves.
“Antifouling paint” means the same thing. The term “antifouling system” is wider: it covers any coating, paint or surface treatment used to control fouling, which is the definition used in the IMO Anti-Fouling Systems (AFS) Convention. See the glossary entries for antifouling paint and the AFS Convention.
Antifouling matters for three reasons:
- Performance: fouling increases drag, which raises fuel use and lowers speed.
- Maintenance: a fouled hull needs more cleaning, hides damage and makes underwater inspection harder.
- Environment: fouling can transfer invasive species between regions, which is why some countries now check hulls on arrival.
Types of Antifouling
There are four main types of antifouling: self-polishing (ablative), hard, hybrid and biocide-free foul-release coatings. They work in different ways (International):
| Type | How it works | Best for | Watch out for |
|---|---|---|---|
| Self-polishing copolymer (SPC) or ablative | The binder slowly wears away in a controlled way, releasing biocide and exposing fresh surface | Yachts that move regularly but also spend time at anchor | Film thickness reduces over time, so the coating must be renewed |
| Hard (contact-leaching) | Biocide leaches out while the paint film stays largely in place | Faster boats and hulls that are cleaned or raced | The spent layer can build up over repeated coats |
| Hybrid | Combines features of ablative and hard coatings | A balance of durability and self-cleaning | Follow the maker’s compatibility rules |
| Foul-release (biocide-free) | A low-friction, usually silicone-based surface weakens how organisms attach, so growth releases as the yacht moves | Yachts that run regularly at the speed the product requires | Less effective on yachts that sit still for long periods; follow the maker’s primer and tie-coat system |
Most biocidal antifoulings use copper, usually cuprous oxide, as the main biocide. The product choice follows the manufacturer’s data sheet for hull material, speed and use.
How to Choose Antifouling for a Yacht or Superyacht
Choose antifouling to match the yacht’s profile, not just a product preference. The main factors are:
- Operating pattern: time at anchor or in the marina versus time underway.
- Speed: select a coating rated for the yacht’s operating speed; foul-release performance also depends on movement and use.
- Water conditions: warm, nutrient-rich water means higher fouling pressure.
- Docking interval: how long the coating has to last before the next haul-out.
- Existing coating: compatibility with the layers already on the hull.
- Hull material: on aluminium hulls, use only products the manufacturer approves for aluminium, because some copper-based antifoulings can cause corrosion (International).
- Local rules: in the EU, biocidal antifoulings fall under the Biocidal Products Regulation as product-type 21 and require authorisation, subject to the regulation’s transitional provisions.
On superyachts, antifouling planning looks more like commercial practice. Yachts of 24 m and above on international voyages fall under the AFS Convention’s declaration or certificate rules, docking intervals are usually planned around class surveys, and the coating is applied as part of a specified underwater paint system, often inside a contained area to control dust, overspray and runoff.
How Often Should a Yacht Be Antifouled?
Yachts are commonly antifouled every one to three years. The interval depends on the product, the water and the operating pattern. New Zealand’s Ministry for Primary Industries notes that boat antifouling paints generally last one to two years, and some foul-release systems for yachts are specified for three years (Hempel). Some commercial ship coatings are designed for docking intervals of up to five years.
Check the antifouling at every haul-out and survey. Early renewal or cleaning may be needed if the yacht has been laid up, used in warm waters or operated outside the profile the product was chosen for.
Check the antifouling at every haul-out and survey.
How Antifouling Is Applied
Antifouling is applied as part of the underwater paint system, usually during a haul-out or dry docking. A typical sequence is:
- Wash and inspect. The hull is washed and the old coating is checked for adhesion, build-up and damage.
- Decide on preparation. If the old coating is sound and compatible, it is cleaned and lightly abraded. If it has failed or built up too thick, it is removed back to a sound layer or to the substrate.
- Repair the substrate. Corrosion, damage and, on GRP hulls, osmosis are dealt with first, along with any fairing.
- Apply primers and tie coat. Anticorrosive primers protect the hull; a tie coat helps the antifouling bond to the layers below.
- Apply the antifouling. The number of coats and the dry film thickness follow the coating specification.
- Respect the timing. Each product has temperature-dependent minimum overcoating times and minimum and maximum times before launching, set out in its technical data sheet. Some products allow several months between application and launch; others much less.
Antifouling only performs as well as the preparation and layers beneath it.
Running gear and appendages, such as propellers, shafts and stabilizer fins, may need different products from the hull. See paint system application and paint inspection.
Antifouling Rules and Biofouling Requirements
Antifouling is regulated by the IMO AFS Convention, by biocide law where the product is sold and used, and increasingly by biofouling rules at the port of arrival:
- AFS Convention. The convention, in force since 17 September 2008, bans organotin biocides such as TBT. Cybutryne was added from 1 January 2023: it may no longer be applied, and existing cybutryne coatings must be removed or sealed at the next antifouling renewal, at the latest 60 months after they were applied, with some exceptions.
- Declaration or certificate. Yachts and ships of 24 m or more in length but under 400 GT on international voyages need a Declaration on Anti-fouling Systems signed by the owner or an authorised agent, with supporting documents. Those of 400 GT and above need an International Anti-fouling System Certificate. For how this fits into the docking process, see dry docking.
- EU biocidal products. In the EU, biocidal antifoulings are regulated under the Biocidal Products Regulation (EU) No 528/2012 as product-type 21.
- Biofouling management. IMO’s 2023 biofouling guidelines (MEPC.378(80)) recommend a biofouling management plan and a biofouling record book. They are voluntary at international level, but national rules can be binding.
- Arrival checks. New Zealand requires arriving vessels, including yachts, to meet clean-hull requirements; for longer stays, only a slime layer and goose barnacles are allowed, and approved alternative ways to comply exist. Australia has required pre-arrival biofouling reporting since 15 June 2022.
In-Water Hull Cleaning
Cleaning the hull in the water can extend the life of an antifouling between haul-outs, but it has limits. Cleaning a biocidal coating can damage it and release biocides and organisms into the water. Many ports and marinas restrict in-water cleaning or require permission. In April 2025, IMO approved guidance on in-water cleaning of ships’ biofouling (MEPC.1/Circ.918). Check local rules before cleaning, and follow the coating maker’s guidance on what cleaning the product can take.
When Antifouling Becomes a Bigger Job
Antifouling stops being a routine item when:
- The old coating is failing over large areas or has built up too thick
- The yacht has been idle long enough for heavy fouling or coating damage
- Underwater repairs, corrosion or impact damage are already open
- Appendages, shell openings or repair areas need different preparation
- The owner wants a change of system, for example from biocidal to foul-release
In these cases antifouling becomes part of a wider underwater package with its own budget and schedule. See yacht refit cost.
Plan Antifouling Before the Docking Window
Plan yacht and superyacht antifouling before the haul-out, including preparation, application and curing time. Inspection, repairs, primers and coats all have to fit the time the yacht is out of the water, so decide on the system, check compatibility and order materials in advance.
Containing the work area keeps dust and overspray under control during coating work.
For underwater coating work as part of a refit, see yacht painting at KRM Yacht, learn more about superyacht refit, or request a refit quote.
Frequently Asked Questions
What is antifouling?
Antifouling is a coating applied to the underwater hull and appendages to limit the growth of slime, weed, barnacles and other organisms that increase drag and fuel use.
What does antifouling paint mean?
Antifouling paint is paint designed to stop marine organisms attaching to a hull. Most types release a biocide; foul-release types use a slippery surface instead.
What are the types of antifouling coating?
The main types of antifouling coating are self-polishing or ablative, hard, hybrid and biocide-free foul-release.
How often should a yacht be antifouled?
Commonly every one to three years. Check it at every haul-out and renew earlier if needed.
Can new antifouling go over old antifouling?
Apply new antifouling only over a sound, compatible coating. Use a manufacturer-approved tie coat where specified, and remove unsound layers.
What is the antifouling convention?
The IMO International Convention on the Control of Harmful Anti-fouling Systems on Ships (AFS Convention), in force since 2008, bans harmful antifouling substances such as organotin compounds and cybutryne, and sets declaration and certificate requirements for ships on international voyages.
Can copper antifouling be used on an aluminium hull?
Only products that the manufacturer approves for aluminium should be used, because some copper-based antifoulings can cause corrosion on aluminium hulls.
Sources
- IMO: Anti-fouling systems
- IMO Resolution MEPC.331(76): cybutryne amendments to the AFS Convention
- IMO: Biofouling
- IMO Resolution MEPC.378(80): 2023 Biofouling Guidelines
- Regulation (EU) No 528/2012 (Biocidal Products Regulation)
- MPI New Zealand: Biofouling management
- MPI New Zealand: Hull fouling and keeping your boat’s hull clean
- Australian Department of Agriculture: biofouling requirements notice 269-2023
- International Yacht Paint: Types of antifouling
- Hempel Yacht Pro catalogue 2022–2023
