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!

There are many tasks that have become easier and more cost effective over time thanks to improvements in technology. However, that’s not true of antifouling paints. Even though most manufacturers are investing plenty of research in improved formulations, the problem is that the ultimate protection for the bottom of your boat is incompatible with the health of the marine environment as a whole. Put simply, keeping too many boats in one location over a long period of time means that high-strength antifouling products not only keep the bottom of a boat clean, but they also become responsible for slowly wiping out marine life in a wider area.

Foul propeller

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

Understanding marine fouling

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.

Freshwater antifouling

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.

Antifouling for 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 antifouling 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.

Five boats, five antifouling solutions

Below is an outline of what antifouling paint you might want to choose for different types of boat and patterns of use. It’s important to recognise that the optimum type changes not just for different styles of boats, but also how often the boat is used and the purpose for which the vessel is used.

1. 33ft sailing cruiser

Boat used: minimum three weekends a month May to September

This boat is one of the easier ones to protect, so a mid-priced eroding product such as International Cruiser Uno or Hempel Cruising Performer will have a reasonable chance of doing the job with acceptable efficiency. There are a number of reasons for this: firstly, if slime forms towards the end of the season, then on a cruising sailboat it’s critical to neither performance nor fuel economy. Secondly, the very fact that the boat is used regularly will help the eroding antifouling paint to do its job, thereby minimising the build up of slime in the first place. Thirdly the relatively short (five month) season means that prolonged protection is not required.

International Paints Cruiser Uno antifouling paint

International Paints Cruiser Uno.

2. 36ft racing yacht

Boat use: One or two cross Channel or similar races per month, over a seven month season.

This is a much tougher scenario, as the boat is used less frequently over a longer season, which gives more time for slime to build up between outings. In addition, any slime that does form will quickly take the edge of the boat’s performance and while losing 0.2 knots won’t matter much to a cruising yacht, for a racing boat it can make a half hour difference on a 100-mile race.

For this boat a quality hard racing antifouling paint such as Nautix A4, Hempel Hard Racing or Flag Performance Extra will give excellent short term protection and allow the boat to be lifted to clean the hull before your most important races.

Hempel Hard Racing antifouling paint

Hempel Hard Racing.

3. 42ft long distance cruiser

Boat use: continuous cruising in warm waters for 10 months per year.

If you’re in a position to clean the bottom of the hull from the water, then a product such as Coppercoat would be worth considering and may eventually repay its high up-front cost several times over. While this isn’t as good at preventing slime as conventional antifouling paints with a relatively high organic biocide content, particularly for boats that don’t move much. However, if you’re regularly on the move and tend to anchor and swim in the kind of places where it’s easy to give the bottom of the boat a quick wipe periodically this may be a good choice.

Alternatively, a high-performance eroding antifoul such as International Micron Extra 2 or Hempel’s Ocean Performer will keep marine life at bay for longer than lesser products.

Coppercoat antifouling paint

Coppercoat antifouling.

Fast planing powerboat: choose the best antifouling paint

The performance and fuel economy on a fast planing powerboat such as this Fairline Phantom 40 is affected by even small amounts of marine growth on the hull.

4. 40ft planing motor yacht with 23-knot cruising speed

Use: one weekend per month, plus a two-week summer 400 mile cruise

Just as the racing yacht needs to avoid the build up of slime, the same is true of fast power boats. Even a small amount of growth will increase the amount of power needed to get on the plane, significantly increasing friction, and therefore fuel consumption, at speed.

Most eroding antifouling paints are not recommended for speeds above 17 knots, so this factor is likely to limit you to the hard racing types – as with racing yachts it’s worth choosing a top quality product as the relatively modest extra cost will easily be covered by fuel savings. In addition, you’ll want the hull and propellers to be as clean as possible for your summer trip, so schedule a lift out and pressure wash as close to the start of this as you can.

Nautix A4 T.Speed antifouling paint

Nautix A4 T.Speed.

5. 25ft RIB or sportscruiser

Use: Bank holiday and occasional long weekends from Easter to September, plus a handful of day trips during a two week summer holiday.

For a boat like this the arguments against even bothering with antifouling get stronger each year. Given that it’s next to impossible to keep the bottom of a boat that’s only occasionally used clean, and this type of boat is very sensitive to any layer of slime, for most owners it would be better to admit defeat. Instead of keeping the boat afloat, book a slot in a dry stack.

High performance RIB: which is the best antifouling?

It may be best to duck the challenge entirely and simply keep this boat out of the water when not in use.

This means you won’t need to pay for moorings, or for annual antifouling, yet the bottom will always be clean when you want to use the boat. In addition, given that most drystack facilities provide a degree of protection from the elements, it will also be dry and clean each time you want to go out.

For more information and advice on antifouling paint, see: How to prepare and apply antifouling.

Written by: Rupert Holmes
Rupert Holmes has more than 70,000 miles of offshore cruising and racing experience, in waters ranging from the North Sea to the Southern Ocean and Cape Horn. He writes about all aspects of boat ownership and marine travel, including destinations, seamanship and maintenance, as well as undertaking regular new boat and gear tests. He currently sails around 5,000 miles per year and in the past couple of seasons has cruised from the UK to the Azores, as well as winning his class in the 2014 two-handed Round Britain and Ireland Race. He also owns two yachts, one based in the Mediterranean and the other in the UK.