DESIGNING ships to exude slime from their hulls could cut their fuel consumption by up to 20 per cent. The slime would form a gelatinous skin that continually sloughs off, taking with it the barnacles and other marine life forms that cause energy-sapping drag as they accumulate on the ships鈥� underside.
The idea, which is being tested by Rahul Ganguli of in Thousand Oaks, California, and colleagues is being financed by the US Department of Defense ().
Fouling by marine life is a problem for shipowners, as it requires vessels to be brought into dry dock every couple of years to remove plants and animals from the hull. It has been made worse by the banning last year of antifouling paints based on tributyltin, which is toxic to marine life.
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At the root of the fouling problem are micro-organisms such as bacteria and algae, on which larger plants, barnacles and tube worms can grow.
Ganguli鈥檚 solution is inspired by the skin of the long-finned pilot whale, Globicephala melas, which was investigated by Christoph Baum at Hannover School of Veterinary Medicine, Germany. In a paper published in 2002 Baum鈥檚 team reported that the surface of the whale鈥檚 skin is criss-crossed with a network of nanoscale canals too small for any barnacle larvae to gain any purchase (). They also found that the canals are filled with a gel of enzymes that destroy proteins on the surface of bacteria and algae.
Ganguli is now working on a way to make a ship鈥檚 hull perform a similar self-cleaning trick. His idea is to cover the outer layer of a ship in a metal mesh, beneath which is a regular pattern of holes that exude a sticky, biosafe chemical that becomes more viscous on contact with seawater.
As the secretion oozes out of the pores it fills the gaps in the mesh and pools on top to form a viscous skin coating the entire hull. This skin steadily wears away, taking with it any life that has gained a foothold, and is replaced by new slime from below.
鈥淐hemicals ooze through holes in a metal mesh to form a viscous skin coating the entire hull鈥�
Ganguli has tested the idea with two chemicals used on oil rigs. One is used to thicken seawater to force open rock formations, while the other firms up acid used to dissolve rocks. 鈥淲e think they will be safe for marine life,鈥� he says. When the chemicals were squirted through holes under a mesh on a piece of a ship鈥檚 hull, a smooth, slimy skin 700 micrometres thick formed on top.
Tests of the system in tanks of seawater showed that after 11 days there was a 100-fold cut in the number of Pseudomonas carrageenovora colonies that grew on a mock hull plate compared with the plain steel plate used as a control. P. carrageenovora is one of the bacterial species known to form colonies on which larger fouling organisms, such as plants or barnacles, can grow.
The team also showed they could control how fast the skin wears away. They say it may even reduce drag compared with a clean steel hull.
Making the idea work would be lucrative, says naval architect at the University of Strathclyde in Glasgow, UK. 鈥淚f you do not have to bring the vessel to dry dock it would save a lot of money.鈥�
