Antifouling coatings comprise of a soluble or partly soluble resin system that contains a mixture of biocide(s) effective against a broad range of fouling organisms. Antifouling coatings primarily differ by the resin system used, often referred to as the ‘delivery mechanism’ and the type and level of biocide(s) used. The solubility of the resin system and the efficacy of the biocides used are the key parameters in determining the overall efficiency of the coating. Simply put the resin system ‘polishes’ away in service delivering the biocides to prevent fouling settlement.
Antifouling coatings currently make up around 80-90% of the fouling control market for marine shipping, with the remainder of the market comprising of non-polishing foul release (both biocide containing and biocide free) along with hard ‘scrubbable’ coatings and a biocide free polishing system.
The selection of an antifouling coating is clearly a complex task with over 140 products to choose from, which is further complicated as a scheme then needs to be designed that is specifically tailored to the expected ship specific operational and environmental factors.
When designing an antifouling scheme, the Dry Film Thickness (DFT) required is directly related to the expected vessel speed, activity and seawater temperature along with the intended in- service period i.e. 36 months, 60 months etc. In simplistic terms, the required DFT of an antifouling coating increases with increasing speed, activity, seawater temperature and longer in-service periods.
If the vessel eventually trades at speeds and/or activities and/or seawater temperatures that are significantly less than the antifouling scheme design, the following are the likely consequences:
- Antifouling will have been applied that is in excess of what is required for the scheme life, which is a waste of upfront paint costs, and;
- At the following dry dock there will be a significant DFT of antifouling remaining on the hull, which may cause problems with excess thickness build up and subsequently cracking and delamination/detachment of the hull coatings, which can manifest itself as the coating dries out on entering the dry dock.
However, if the vessel trades at speeds and/or activity and/or seawater temperatures that are significantly higher than the scheme design, the antifouling coating is likely to ‘polish through’ prematurely before the end of the designed in-service period. Polish through of the antifouling coating will expose the tie coat which will not provide any fouling protection and hence exposed tie coat will quickly foul even if the vessel is static for only a few days under normal port operations.
The photograph shown below (Figure 1), shows a striped pattern of non-fouled coating/fouled coating. The non-fouled areas are in way of spray overlap areas where additional antifouling paint has been applied. However, the areas between the overlaps have polished back to the tie coat and, as a result, have fouled.
To understand how much of a problem premature polish through of the antifouling coatings is on marine vessels, you need to look at data. Safinah an independent coating consultancy has a unique in-house database of coating condition assessments documented from drydock supervision activities during dating back to 2010.
Based on Safinah’s data from drydock hull projects conducted between 2015 – 2024, the data showed that around 50% of the ships had a level of polish through of the antifouling on arrival at dry dock including around 30% of the ships with up to 20% polish through and around 18% of the ships with more than 20% polish through (see Figure 2).
Observed Polish Through Levels
Polish Through
More than 20% Polish Through
Up to 20% Polish Through
No Polish Through
The data shows that premature polish through of the antifouling coating is clearly a significant problem, and the accumulation of biofouling on areas of polish through will lead to the following key industry issues:
- An increase in underwater hull roughness (from fouling species), which has a direct impact on fuel consumption and consequently the emission of air pollutants (greenhouse gasses) which the IMO has adopted regulations to address.
- An increased risk of translocating non-native, potentially invasive aquatic species.
However, the most significant financial penalty for the shipping industry is the increase in fuel consumption (whilst maintaining a constant speed) due to the adverse effects of hull fouling on the hydrodynamic performance of the vessel.
As such, the expected parameters for speed, activity and seawater temperature need to be carefully considered when designing the antifouling scheme. Typically, this is done using the vessels historical Automatic Identification System (AIS) data for the in-service period since the last dry dock, which is then analysed to obtain the required parameters. However, this historical data is only useful if the vessel is going to continue the same trade after the upcoming dry dock and any predicted changes need to be carefully considered and taken into account.
Whilst the vessel speed can be relatively easy to predict, for example charterparty agreements typically state expected speeds and a ships activity will generally remain within an expected range, predicting the expected seawater temperature over a three or five year scheme life is a significant challenge.
The polishing rate of all antifouling technologies is affected by seawater temperature and whilst a faster polishing rate, and hence an increased rate of biocide delivery in high fouling challenge warmer waters, can be a positive, it makes calculating the correct scheme DFT more critical.
Based on Safinah’s knowledge of antifouling schemes, a significant change in scheme thickness (DFT applied) can be seen for what appears as relatively minor changes to the seawater temperature. For example, moving from 25°C to 28°C, a relatively small increase of 3°C, the scheme DFT required significantly increased by around 70µm (an approximate 30% increase in the total DFT required).
Therefore, assuming a linear polishing rate for simplicity, a vessel applied with a five year scheme (for a seawater temperature of 25°C) which spends a high proportion of actual in-service operations at a temperature of 28°C, could expect to see significant areas of polish through of the antifouling a year before the next scheduled dry dock. These areas of polish through would quickly foul with a subsequent increase in fuel consumption (when maintaining speed) and greenhouse gas emissions as well as significantly higher future dry dock costs, such as increased cleaning and blasting costs and/or increased paint costs as exposed tie coats cannot be directly overcoated with new antifouling.
To further add to the complexity, from analysis of antifouling schemes and in-service performance results from the dry dock database, Safinah has found that simply using average seawater temperatures is not accurate enough and a more in-depth analysis of the raw temperature data is required.
By analysing the condition of the hulls at dry dock and the scheme applied, Safinah has developed a method to incorporate other factors into the seawater temperature calculation which provides a ‘functional seawater temperature’ more fitting of the conditions expected to be encountered by the vessel.
If you would like to find out more about our services and how we can help you with antifouling product specification and selection, please get in touch with our team at enquiries@safinah.com
