How to Combat a Dirty Bum-Consciously

By TWS resident Marine Ecotoxicologist Sam Gaylard

As any liveaboard sailor would know, what you put on your bum below the surface can make all of the difference; not only to the speed of your vessel and the maintenance required but also to the marine environment around you. Environmental Toxicologist, Sam Gaylard has spent the last 20 years working as the lead marine scientist informing the South Australian Government about how pollutants impact the local marine systems. Below he shares with us the science behind our boats' behinds and some scientifically proven tips on how we can be more environmentally conscious in this area of boat maintenance.

dirty bottom time for new antifoul

Why do we Antifoul?

From an ecological point of view, space is at a premium. If we leave a bare space available in the marine environment, it will be colonized quite quickly by a range of different organisms. Over time, this growth builds up and creates drag on the boat. So when we leave a boat in the water over time, we need to stop that natural growth. Low drag allows us to go faster, be more efficient in the given conditions, use less fuel/fewer carbon emissions, getting to the top mark quicker, or arrive at the anchorage in time for sundowners. In the commercial shipping world, the monetary savings in fuel are in the order of USD $3 billion a year globally [1].

Antifouling also prevents long-term damage to the hull by organisms that can fix onto the hull by secreting special adhesive that etches into the hull material. In timber and fiberglass boats, this can let moisture in. In steel and aluminium boats, this can start corrosion. Timber boats are also at risk from boring organisms (shipworms) that will slowly eat away the structure of the boat. This damage adds cost in the dry dock and can be catastrophic in extreme cases.

Finally, antifoulants help prevent the spread of invasive marine organisms. This is where an organism can attach to a hull and be transported across oceans to arrive in a new location. If the conditions are suitable the organism can take hold, spread, and decimate the local ecology and aquaculture industries. For example, up to 78% of the introduced marine species introduced into Hawaii have been brought by vessel biofouling [2].

Antifoulants – what are they really?

Fouling occurs in a process, where proteins in water form a slime layer on the hull. This is very quickly colonized by bacteria and diatoms (type of microscopic algae). Once the slime layer is established, secondary fouling can occur dominated by barnacles and worms. Once this process starts, it will start a cycle where the fouling on the hull will promote further fouling, and so on - creating that all too familiar 'hull garden' we seem to cultivate at the end of each cruising season.

Ever since humans have traveled or traded by boat, fouling has been a problem. As early as 1500-300 BC, the early Phoenicians (ancient northern Africa) used lead and copper sheets on their wooden hulls to prevent fouling as a form of ANTI-fouling. This proved quite effective but it wasn't until the late 18th century that coatings containing copper, arsenic, and mercury were increasingly applied to vessels as paints [3]. As we know using antifoul paint is still the most common form of protection against biofouling but there are also new technologies popping up on the market that use alternative measures.

But let’s just take a step back in time and look at some technological advances into the world of antifouling history and the impact of mistakes we have made along the way.

The TBT story

In the 1960s a product called tributyltin (TBT) was introduced as an antifoulant with dramatic results. Vessels were clean for long periods of time and were able to extend their dry dock period to over 5 years. Mariners couldn’t be happier.

Under the water, however, the effect the antifoulant was having on the environment was just as dramatic. By the mid-1970’s oyster farms were collapsing in France, caused by widespread shell deformities and complete reproductive failure [4]. This was not an isolated occurrence, severe impacts were observed on oysters, mussels, whelks, and other mollusks seen throughout major shipping regions worldwide.

It was discovered that TBT is highly toxic, particularly to molluscs with effects seen in concentrations above 1 nanogram per litre (this number is so small that is very hard to visualize, 1 ng/L is equal to one part per trillion, or 1 drop in 68 million litres). TBT acts as an endocrine disruptor, essentially mimicking hormones in the organism. It causes a condition called imposex, which is the masculinizing of female molluscs, which can result in sterilization, leading to population collapse [5]. TBT accumulates in sediments where it is relatively persistent, that is the chemical is still present, decades after it entered the water and importantly, still toxic, and it also accumulates in organisms with increasing concentrations in their tissues over time [6].

On recognizing the impacts, governments started banning the use of TBT. In 2008 the International Convention on the Control of Harmful Antifouling Systems on Ships came into force to signatory countries, which prohibits the use of harmful organotins in antifouling paints used on ships, and effective stopping ships with TBT coatings from stopping at their home ports [7]. Unfortunately, in some countries, a lack of enforcement means that TBT antifoulants are still produced, including within the US under the name Seahawk Paints, and distributed through the Caribbean and Central America [8].

Traditional Antifoulants Paints

As discussed antifoulant paints are toxic by design. They contain highly toxic active ingredients such as copper and booster biocides, with varying concentrations based on their intended use. Formulations will vary depending on the hull material, the likely water temperature, and the speed of the vessel.

These paints work either through a slow leaching process (ablative) which releases the active ingredient into the thin layer of water around the hull, repelling organisms for obvious reasons. Or the coating is a “hard” paint that relies on toxicity when an organism touches the surface, Coppercoat is one such hard paint that fits in this category- a two-component copper/epoxy mix.

So, what is the science behind their impact on the natural environment?

From an environmental perspective, all antifoulant coating particles will be released into the water, inevitably settle into the sediments and accumulate very slowly. Environmental contamination as a result of antifoulant paints is concentrated around shipping harbors and marinas where boats congregate and sit over time. These areas are also generally areas of reduced dilution/or reduced water movement so these contaminants are not flushed out. In some cases, ecological impacts on sediment-dwelling organisms can be seen [9]. Conversely, in areas with lower vessel concentration and higher dilution such as the boat moorings at the offshore reefs or coastal anchorages, contamination can be at background levels, suggesting that both densities of vessel and flushing rates are critical in ecological impacts [10].

These marinas and shipping harbors are not the areas with the greatest contamination but of course, it is the areas surrounding slipways where maintenance work is undertaken. The large number of vessels that use these facilities concentrates the paint residues and when paint dust, paint chips, and contaminated washdown runoff drain uncontrolled into waters where they accumulate to very high concentrations. Concentrations in sediments at the bottom of slipways can exceed toxicity thresholds by a factor of 1,000x [11].

While many countries have strict regulations about runoff from slipways, these regulations are relatively new. Many slipways have been in the same location for decades before regulations. In many ways, legacy contamination at marinas and shipping harbors is a tradeoff that we accept when we are part of the community that own boats that require these facilities. This contamination is part of the cost that needs to be managed when facilities need to be dredged or closed.

New Technology “environmentally friendly” antifoulants

With the realization that long-term contamination and its effects on the marine environment are not sustainable, the search for less toxic antifoulants is ramping up. Significant research into new types of antifoulants includes advanced chemistry formulations, biological enzymes from bacteria within slow-release paints, amphiphilic coatings that retain a microlayer of water around the hull, and coatings that mimic biological attributes. There are countless potential active ingredients but very few make it to the point of coating development and commercialization. This is a long and expensive process.

Unfortunately, there is plenty of marketing spin and many producers want to keep the details behind the workings of the antifoulant secret in the hope of making millions of dollars. Additionally, many of these products are very new to the market and there are few early adopters to test them out in real-world situations, so real data on effectiveness is limited.

New Biocidal Technologies

Advanced chemistries that still use a biocidal or antimicrobial action are generally less toxic than traditional metal-based compounds and have shorter persistence in waters. Igarol and Diuron have been around for many years and there is some data in the public domain about off-site impacts and persistence. These two products are organic herbicides so are particularly toxic to plants. Igarol is quite persistent in waters and very persistent in sediments but will reduce over the period of months to years. Diuron is much shorter-lived in the environment with half-lives in the period of days to weeks, whereas copper is permanent [12]. These compounds are widely used, sometimes as a booster biocide in traditional metal-based antifoulants.

Development of products is progressing, with new active ingredients emerging on the market. Products that contain Econea and capsaicin are becoming available but long-term data on persistence in the environment and non-target toxicity is often poorly investigated, and rarely in the public domain [12]. These paints often require a combination of active ingredients to be able to have both longevity and effectiveness. Additionally, care needs to be taken with in-water cleaning as the coatings can be damaged relatively easily. Further research is needed to understand the toxicity and impact of breakdown products, non-target impacts, and the potential for bioaccumulation and persistence in sediments.

Non-Biocidal coatings

Non-biocidal coatings are a very new type of antifoulant coating. They work by either a detachment principle where fouling accumulates on the hull, but with frequent movement, the fouling will easily be removed. The second mode of action preventing attachment. These use non-biocidal methods to prevent attachment, including irregular surfaces that restrict the adhesion of fouling or acoustic frequencies to discourage growth.

Detachment coatings

The more common of these two types use the detachment principle. Silicon-based paints as antifoulants are becoming more commonplace. These coatings are fouling release coatings rather than true antifoulants. This means that growth will occur on the vessel, but with vigorous use, the fouling will release off the hull. Sounds like a win, just a smooth slippery coat of silicon-based paint and sail loads right?!

Online accounts suggest that these ARE relatively effective, but fouling is dependent on using the boat regularly and at speeds higher than +9knots. I don’t know about you but we rarely see numbers higher than 7 or 8 knots on the chart plotter! On top of this, these coatings are quite difficult to apply to require specialty primers and if damaged, they can be hard to repair and can reduce their longevity.

In the scientific literature, reviews suggest that some of the critical organisms (diatoms) responsible for the slime layer are not removed at speed, therefore mechanical hull cleaning is required regularly to stop secondary (macro) fouling [13]. The majority of happy customers appear to be either in cold water locations or from motorboats that get higher hull speeds.

Acoustic Treatments

Nonbiocidal coatings that prevent attachment have been tested for many years, including the Soviet navy. With regard to acoustic treatments, there has been many tests at various frequencies with results ranging from partially effective to not effective at all including some acoustic treatments that actually increase fouling [14]. Ultrasonic devices hold the most promise for a practical antifoulant with some results indicating promise, even on large ships but over a relatively short time (4 months). There is little information on the dissipation of the sound away from the hull, which gives rise to questions about non-target impacts. Any audible acoustic sound used in the marine environment has the potential to mimic or mask natural sounds that may be used by animals to navigate, spawn, settle or communicate [14]. Obviously, more data is needed to understand to only the effectiveness of systems but optimizing transducer placement and the potential for non-target impacts.

Irregular Surface Coatings

People have noticed that certain animals that live in marine waters do not get fouled. Sponges and sea urchins have large surface areas with very little fouling. A novel surface coating that mimics sea urchin spines applied through a wrap can now be coated on vessels with Dutch design Finsulate. This is claimed to prevent attachment of fouling on the surface. Similar to other non-biocidal coatings it appears as though frequent cleaning will still be required. Only time will tell whether these coatings are effective.

The Future of Antifoulants

There is a wealth of research underway including nanotechnology designing non-stick coatings, enzyme-based paints, and new and emerging biocides to name a few. So, it is worth keeping an eye on the ever-growing market in this field.

What are some ways we can be more environmentally conscious around the antifouling process realistically ?

  1. Instead of accepting the boatyards suggestion for "the REALLY good stuff” you might like to consider Coppercoat. Coppercoat technically is considered as non leaching and has the added benefit of lasting up to 20 years between recoating, resulting in substantially less waste, runoff and resources generated with regular applications of regular anitfoul paint. For those on a tight budget Coppercoat might be out of reach- in this case your best option it to seek out an ablative paint with less copper and boosters such as diuron- this will reduce the long term effects of contaminates your paint releases into the water.

  2. If you can, always use a licensed slipway that has an interception drain with treatment for runoff. Talk to your slipway about what they do, firstly this will ensure you are making the right choice, but secondly, it will highlight to the slipway that these are factors that are gaining your business. Generally, facilities with travel lifts are better than railway slipways as they can control runoff.

  3. Avoid using careening poles at all costs. Often boat owners will justify to themselves that scraping old antifouling off and repainting their boat is acceptable as it's only one boat. Paint chips release biocides and metals into waters and can accumulate in sediment. Even if one boat does this in an unrestricted area, there can be impacts and contamination. When hundreds or thousands of people have this attitude, significant environmental impacts can occur, even at individual locations [15].

  4. Contrary to this though it is still important to complete in-water cleaning on a regular basis for the lifespan of your antifoul and to prevent the transport of invasive species between ports. The potential impact of translocation of marine invasive species is far greater than liberating a small volume of biocides into the surrounding waters. Be sure to check your local guidelines about in water cleaning and if cleaning is allowed, then doing so in an approved location will ensure that the risk from contamination is managed. If you do clean your hull and there are no approved locations, then look for a well-flushed area that is safe to dive. For those with highly ablative coatings avoid doing this too frequently as you will quickly discover you wont have any paint left!


While at this point in time, the use of new technology environmentally friendly antifoulants is not widespread we need to be very careful that in the desire to reduce contamination in the marine environment, we do not promote more marine invasive species incursions. At this point in time, I think that non-biocidal coatings have significant limitations on effectiveness and should be thoroughly researched before application. We need to keep the perspective that the environmental cost of not using antifoulants is likely to be far greater than the impact that they cause.

  • With increased drag on vessels adds significantly to the fuel cost, which contributes to climate change through burning fossil fuels. We see in the commercial shipping world, the monetary saving in fuel is in the order of USD $3 billion a year globally [1].

  • Any incursion of a marine invasive species will significantly alter local ecology, aquaculture, power station cooling waters and add to infrastructure costs. This has a far greater impact on the environment than isolated areas of metal and biocidal contamination.

Sam Gaylard

Sam has been sailing for over 30 years in both social and high performance racing all size boats from dinghies, high powered sportsboats through to carbon fibre race machines. For the past 2 years Sam and his family have been living aboard their Lyons 47 in Adelaide and cruising up the east coast of Australia diving the vast kelp forests of southern Australia and the warm tropical coral in the north. Sam trained as an environmental toxicologist specializing in how pollution impacts on marine systems and has been working as a leading marine scientist for South Australian government for over 20 years. If that is not enough, Sam is also working on a PhD focusing on how we can improve environmental decision making.

Follow along with Sam and his family adventures


1. Townsin, R., The ship hull fouling penalty. Biofouling, 2003. 19(S1): p. 9-15.

2. Davidson, I., G. Ruiz, and S. Gorgula, Vessel biofouling in Hawaii: current patterns of a potent marine bioinvasion vector and potential management solutions. Prepared for the Department of Land and Natural Resources (DLNR), Coordinating Group on Alien Pest Species (CGAPS), and the Hauoli Mau Loa Foundation, 2014.

3. Dafforn, K.A., J.A. Lewis, and E.L. Johnston, Antifouling strategies: History and regulation, ecological impacts and mitigation. Marine Pollution Bulletin, 2011. 62(3): p. 453-465.

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5. Smith, P., Selective decline in imposex levels in the dogwhelk Lepsiella scobina following a ban on the use of TBT antifoulants in New Zealand. Marine Pollution Bulletin, 1996. 32(4): p. 362-365.

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13. Nurioglu, A.G. and A.C.C. Esteves, Non-toxic, non-biocide-release antifouling coatings based on molecular structure design for marine applications. Journal of Materials Chemistry B, 2015. 3(32): p. 6547-6570.12.

14. Legg, M., et al., Acoustic methods for biofouling control: A review. Ocean Engineering, 2015. 103: p. 237-247.

15. Breitwieser, M., et al., Biomonitoring of Mimachlamys varia transplanted to areas impacted by human activities (La Rochelle Marina, France). Chemosphere, 2020. 243: p. 125199.

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