Antifouling paint is a vital part of any boat's efficiency and health – especially if applied to a well-prepared hull (see: How to prepare and apply antifouling). But when it comes to choosing the right antifouling, the mass of options on the chandler's shelves can seem baffling!

Understanding marine fouling

Foul propeller

From the slightest slime, to full growth and barnacles, every bit of growth will slow your boat down.

Marine fouling develops from plankton, which is often particularly concentrated in estuaries, where sewage disposal and fertiliser run-off provide rich sources of nutrients. Fouling on the immersed sections of the hull and around the waterline may be either animal or plant based. For instance, the soft branching organisms that grow in dark areas under the hull are usually assumed to be weed, but their polyps have minute tentacles that capture prey.

All very interesting, but for the boat owner, a foul hull means a slow boat and reduced fuel efficiency. It’s a battle boaters have been fighting for years and there is an array or products on the market.

Types of antifouling

Regular antifouling paints contain water-soluble biocides that are slowly released to reduce the rate of growth of marine life. Antifouling paints containing biocides are classed as biocidal products and are regulated like pesticides, with an on-going trend increasingly strict regulation. Key issues when evaluating the safety of antifoulings include effects on non-target organisms, build up in the food chain and safety during application.

Hempel classic antifouling

Regular antifouling paints like this one from Hempel contain water-soluble biocides.

It's worth noting that some countries, even within the EU, have their own specific regulations, and antifoul bought in one territory may not be legally applied or used in another, especially in environments that are considered to be sensitive. It's therefore important to be sure the product you intend to apply is legal both in your home country and any that you intend to visit.

Modern antifoulings are copper based, with copper oxide (CuO2) the most important biocide by far. This is more soluble in water than plain copper, and copes well with animal fouling. Some products also use copper thiocyanate (CuSCN), which enables brighter colours to be created, but is less effective against animal fouling if used alone.

In addition, some products use organic biocides keep slime at bay, which copper oxide is not very good at preventing. Once slime starts to form it harbours other organisms that can get a hold on the hull. Perfecting the exact formula of organic biocides used can make a marked difference to performance, with manufacturers typically exerting a considerable amount of effort in this respect.

International Interspeed antifouling

Do you need a hard antifouling or an eroding one? It will depend on your boat usage.

Hard antifouling

Hard types of antifoul use biocides that dissolve very slowly in water, so they gradually dissolve as the season progresses. The paint in which the biocide particles are suspended dries to a hard finish, which enables periodic scrubbing during the season to keep the bottom in perfect condition.

This type is ideal for racing yachts that are kept afloat and for fast powerboats. For the latter category especially, attempting to skimp on antifouling is nearly always a false economy: even a small amount of growth results in significantly increased drag and therefore a double whammy of impaired performance combined with much higher fuel bills.

Eroding antifoulings

These use a paint that is very slightly soluble in water, so microscopic fragments of it are constantly falling away to exposing fresh biocide. This has the advantage that the build up of layers of antifoul is reduced and tends to be cheaper than hard antifoulings, but the drawback is it cannot be scrubbed. Eroding types are most suitable for cruising yachts and displacement motorboats.

Inland waterways antifouling

Boats kept in freshwater need to be protected against different organisms.


Boats kept in freshwater need protection against different species of fouling compared to those moored in a saltwater environment. At first sight, this can present a dilemma for those who move their vessels from inland waterways to the sea and back again. However, there's a simple solution: use the product for the area in which the vessel spends most time. If you move a boat from the sea to inland waterways, for instance, it will take several weeks for freshwater species to start becoming established and during this time any existing growth from the saltwater environment is likely to die.

Metal boats and outdrives

The high copper content of antifouling can create problems with underwater metal surfaces, especially with aluminium vessels and outdrives, so it's important to choose a product that's suitable for these applications, and to select compatible primers.

Frequency of application

How often antifouling needs to be re-applied depends on a wide variety of factors. These include the type of vessel, the purposes for which it's used, local fouling conditions, and the length of time the boat will remain afloat. Many raceboats are dry sailed for good reason - it's almost the only practical way to ensure a perfectly clean bottom throughout the year. With the increasing number of dry stack facilities available, this is also a viable option for many daysailers and powerboats up to around 9 metres in length.

However, this is not a practical solution for all boat owners. Most competitive racing fleets that stay afloat throughout the season organise a fortnightly scrubbing programme to ensure they remain free of all growth, even though they use the best antifouling available.

Although other boat owners may not need to be as fastidious in their approach, this gives a measure of the scale of the problem they face. Traditionally, cruising yachts have applied a couple of coats before being launched in the spring. This is often sufficient generally to last until autumn, although those in areas of high fouling may start to see slime forming after only a few weeks afloat. Conversely, in areas with little fouling a single coat may well be fine for a boat that is in commission for a relatively short period of three or four months.

A long-term antifouling solution?

A different approach is taken with long-lasting products formed of epoxy resin mixed with a large amount of ultra-fine copper powder, or copper nickel alloy. Although initially considerably more expensive than conventional antifoulings, these have the potential to give protection for 10 years or more, although slime may need to be scrubbed off periodically. This process also exposes a fresh layer of copper on the surface. Read Coppercoat: a worthy alternative to traditional antifouling?

A yacht mid season after scrubbing awaiting a new coat of antifouling

Boat dried out, after scrubbing a hard antifouling mid-season.

Still not sure which one to pick? Read 5 antifouling paint solutions: which is the best?. Ready to apply your antifouling? Check out our guide How to prepare and apply antifouling.