The term underwater warfare conjures images of submarines launching torpedoes as destroyers drop deadly depth charges. But there’s another intractable and silent battle being waged beneath the waves. While not as potentially lethal for ships and crews, the attacks are relentless and costly – both for the Navy and for Australia’s marine environment.
The enemy is marine animal and plant life that attaches itself to the hulls and propellers of ships.
This build-up is called fouling. If left unchecked it can double the amount of fuel a ship needs to sustain operational speeds because of the increased friction, which inevitably slows the vessel, increasing its consumption of fuel.
Defence Science and Technology (DST) in collaboration with partners in Australia and overseas are developing new ways to counter the intense fouling that plagues the Navy’s aluminium-hulled patrol vessels.
Research Scientist Dr Andrew Scardino, of DST, is leading the fight against biofouling and says finding a solution is a continuing battle.
“These creatures are desperate to find a home to settle on. If they can’t, they die,” Andrew says.
“But we’re trying just as desperately to repel them. It’s effectively an arms race because as we come up with new ways to deter them they either evolve better strategies to cling on or a new species replaces the ones that can no longer compete.”
The research is being driven to cut fuel costs. Even mild marine growth can quickly increase fuel consumption by 10 per cent, reducing a ship’s top speed and operating range, while raising emission levels and adding to maintenance costs and decreasing vessel efficiency.
Fouling is also a serious biosecurity issue. Navy vessels regularly call at foreign ports and ridding them of their unwanted stowaways is critical to reduce any threat to Australia’s native marine resources, fisheries and aquaculture industries.
Strategies to prevent fouling have included the use of tin (which is now banned), chlorine, ozone, iodine, air, sound, vibrations, and a number of mechanical solutions – most with only short-term or species-specific success.
It has long been accepted that a single solution is unlikely as so many factors impact on bio-fouling rates: The diversity of ship types, the variety of hull forms and materials, variable speeds, the impact of the climates they operate in – even the length of time a ship spends in port.
A copper-oxide coating that was overseen by DST and adopted on Navy’s steel ships in 2016 corrodes aluminium so is unable to be used to protect aluminium-hulled vessels, such as the Armidale-class and Cape-class patrol boats, which operate for extended periods in Australia’s northern waters, where fouling is intense.
The latest hybrid hydrogel coating contains an aluminium-compatible biocide, copper pyrithione and has proven particularly successful in trials on Cape-class boats in the tropics. If the new coating continues to perform well, it is likely to be rolled out for all the Fleet’s aluminium vessels.
One of the more exciting areas of research has been DST’s radical approach to protecting Collins-class submarines.
A biocide has been incorporated into the elastomer or rubber skin that covers the hull, which is then slowly released through the skin’s surface over time to prevent fouling and Andrew says field trials have proven encouraging.
“Prototype elastomers have remained fouling-free for over three years and we hope to completely eliminate the need to apply an extra underwater coating. By altering the thickness of the rubber skin, we expect to be able to increase the reservoir of biocide and extend the period fouling can be kept at bay,” he says.
As often proves the case, the advances that flow from these DST programs will have broader military and civilian applications, with the potential to deliver extensive economic and environmental benefits.
Even a small reduction in fouling would save the Navy several million dollars a year, reduce fossil fuels and exhaust emissions, and give our Fleet greater speed and range to optimise its vital role in maintaining Australia’s border security.
Visit the DST’s website at http://www.dst.defence.gov.au to listen to a podcast on antifouling.