Hull Fouling Control
Updated 17 Oct 2019
Although the IMO may only recently have issued guidelines on hull fouling and established the GloFouling project that may in time lead to global regulation on biofouling, it remains a fact that the issue has concerned shipowners for millennia.
There is evidence to show that in Roman times, lead sheathing was used to protect from worms and fouling. The practice was revived in the 17th century and later copper sheathing was found to be more effective. All manner of other substances have been used as coating over time.
Operational efficiency in all vessels is affected by a number of variables but the condition of the underwater hull and more specifically the lack of fouling is by far the most important one to control. Preventing weed and animal growth on the submerged parts of a ship’s hull is one of the most effective means of reducing fuel consumption and it also reduces the risk of transporting species around the globe to the detriment of local ecology systems.
Even in the days of wooden vessels and wind power when fuel consumption was not a factor to consider, there was a need to protect the hull from fouling and attack by animal such as the Teredo worm which would literally eat the timbers of the vessel. Copper sheeting was considered the most effective and although ships are no longer ‘copper bottomed’, copper is still used as a biocide in fouling protection and, as a result of the ban on coatings containing tin, is once again in common usage.
There are products available that are developed with almost all operating strategies, budgets and environmental conditions available. Costs are extremely diverse recognising that while some owners will need products for just a few years in relatively benign conditions, others have much more demanding conditions to contend with.
Following the banning of TBT products, development of systems has advanced rapidly. Although many will admit that as far as keeping the hull clear is concerned, the ban on TBT was a major step backward, the efforts of manufacturers to develop alternatives and in many cases to make use of new technologies is to be applauded. The possibility of mandatory controls on biofouling is now a very real one following the establishment of an IMO body to promote the issue.
While the likelihood of international regulation in the immediate future is considered remote, as with ballast water and emission controls there are already actions being taken on a regional basis and New Zealand has already prevented at least two loaded vessels from discharging because of the level of fouling on their hulls.
Bio-fouling reduction technology
It is accepted wisdom that the roughness of the hull on a microscopic scale also increases friction and reduces fuel efficiency. It also gives fouling organisms an easier surface to adhere to. The improved smoothness that any underwater coating can allow is considered a benefit and is used in marketing coatings products. Conversely, there is now some evidence that microscopic roughness that is scientifically designed can mimic naturally-occurring substances such as shark skin, which is believed to reduce friction on the surface of some animals such as sea urchins, which will deter organisms from settling and adhering. This is an area of innovation that so far has produced only a small number of products in the market but which may yet attract more research.
Aside from the innovations mentioned, at least three different technologies are used to combat or prevent fouling; all have pros and cons. Anti-foulings contain a biocide that releases gradually, killing weed and organisms that attach to the hull.
Foul-release systems contain no biocide and work by preventing weed and organisms from attaching to the hull. These break down into two types: hard coatings and silicone-based products. The latter type have been further developed and there are now several products with nano properties that not only present a smooth surface that fouling finds difficult to adhere to but are claimed to actively repel any organism that may attempt to settle on the hull.
Several grades of products are produced by leading manufacturers with performance generally increasing in line with price. Anti-foulings contain a biocide component designed to kill any organism that attaches itself to the ship.
The biocide in the current crop of anti-foulings is invariably a copper-based substance. There are plans by some US states to ban the sale of copper-based anti-foulings at some future point but a similar move in the Netherlands for leisure craft was overturned by courts there. With no obvious replacement for copper biocides yet available, a ban for commercial shipping is not considered imminent and the EU has recently approved copper-based coatings, so its use is secure for the time being.
At the lower end of the scale of anti-fouling products are what are known as contact-leaching coatings that release their biocides by diffusion into water that penetrates them. Typically, these contain low levels of rosin and have a very short life span sufficient only to last between interim and special survey drydockings. Improved versions have higher levels of rosin and the depletion rate of the biocide is more controlled and can last up to 60 months according to claims made by producers. Both types do little to improve hull roughness beyond the quality of the application.
A further step up the scale are self-polishing coatings which, as their name suggests, are polished smooth all the while the ship is underway. Two basic types are available but not necessarily from all manufacturers. Silyl acrylate-based products initially polish slowly before the rate of polishing steadily increases, whereas metal acrylate systems initially polish rapidly before reaching a steady state.
The now-banned TBT coatings were considered to have an ideal linear polishing performance with the rate of polishing and biocide release being constant throughout the life of the product. Manufacturers strive to develop products that match this performance with greater or lesser degrees of success. Products that come closest are considered the top tier of anti-foulings and command a premium price.
Since these products contain no biocides of any description they are considered more environmentally friendly and are unlikely to be subject to any restrictions in the same way that anti-foulings may. Some foul-release coats are passive and some have an active repellent property as an added feature. Some products in this category have a long track record having been used extensively by operators in Japan where the use of TBT was ended well before the global ban came into effect.
Most foul-release coatings are silicone-based and have a relatively soft surface. Most of the species that attach to ships’ hulls do so using some type of natural adhesive. In order to attach strongly this adhesive must cover a relatively large area of the hull and bond to it. The very smooth non-stick surface that is made possible by foul-release coatings coupled with the vessel’s movement through the water mean that weed and organisms do not easily find a footing on the hull.
Those that do manage to attach during periods of inactivity when the vessel is in port or at anchor for prolonged periods may cause some reduction in performance but to a far less extent than on an untreated hull or a depleted anti-fouling. The very smooth surface of foul-release coats also means less drag and so allows for a higher fuel efficiency. It is for this type of product that most of the claims of significant fuel savings are made. There certainly seems to be no shortage of ship operators willing to endorse foul-release coatings.
Standard silicone foul-release coatings are not immune from some degree of fouling and to improve on their performance manufacturers have developed products with an added feature. Nano technology has allowed coatings to be developed with what are described as ‘springs’ that physically repel organisms or in other cases to have a hydrogel layer that supposedly fools organisms by making the coating appear to be nothing more than seawater.
As premium products, it is not surprising that this family of coatings is under constant development. One of the most recent products developed by International Paint claims to prevent even the bacterial slime layer that is found on all ships. Another recent addition to the armoury of coatings manufacturers is the synthetic drug medetomidine, which is also marketed under the name Selektope.
Initially developed as an anaesthetic and analgesic for dogs, it can be used as an anti-fouling substance in marine paint. It is mainly effective against barnacles but has also shown effect on other hard fouling like tube worms. When the barnacle cyprid larva encounters a surface containing medetomidine, the molecule enters the octopamine receptor in the larva. This makes the larva’s legs start kicking and it cannot settle to the painted surface. When the larva swims away from the surface, the effect disappears. The larva regains its function and can settle somewhere else. Selektope is being used by several manufactures and incorporated into numerous products.
Another group of foul-release products is commonly known as hard coatings and these rely on having a very smooth and very hard non-stick surface to be effective. The hard surface is achieved using various means including the inclusion of glass platelets in the product that form an impenetrable barrier. These products make use of the same method as silicone versions to prevent fouling.
In general, these products carry a significant cost premium over other coatings. Against this must be weighed the fact that some makers are prepared to offer ten-year guarantees against coating defects and claim a life span of around 25 years for their product, except for mechanical damage such as may be caused by collisions with other vessels and fixed and floating objects.
For all types of foul-release coatings, the benefits they confer can be maintained longer by regular cleaning. Cleaning can be done in drydock or underwater by divers. The type of equipment will vary from water jetting to hard brushing depending upon the type of coating involved. Unfortunately, some port authorities refuse to permit any form of cleaning or recoating being done in ports and this can make life difficult for operators who serve ports where such rules are in place because cleaning and repair will require a diversion to a more amenable port.