27 September 2009

Slimy ships for better speed

Any hard surface immersed in the sea eventually gets covered in marine life. On ships, this can add weight and drag, slowing them down and raising fuel costs. The traditional method to prevent this is to use toxic paints. But this pollutes the environment. Last year, the toxic antifouling paint based on tributyltin (TBT) was banned.

One idea being explored is to create a slimy skin over ship hulls. The idea was inspired by the skin of the long-finned pilot whale which is criss-crossed with a network of nanoscale canals too small for any barnacle larvae to gain any purchase. It was also found that the canals are filled with a gel of enzymes that destroy proteins on the surface of bacteria and algae.

It pays to look more closely at our marine life for solutions to many of our modern predicaments.

Slimy-skinned ships to slip smoothly through the seas
Paul Marks, New Scientist 26 Sep 09;
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' underside.

The idea, which is being tested by Rahul Ganguli of Teledyne Scientific in Thousand Oaks, California, and colleagues is being financed by the US Department of Defense (Smart Materials and Structures, DOI: 10.1088/0964-1726/18/10/104027).

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.

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's 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's team reported that the surface of the whale's skin is criss-crossed with a network of nanoscale canals too small for any barnacle larvae to gain any purchase (Marine Biology, DOI: 10.1007/s00227-001-0710-8). 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's 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.

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. "We think they will be safe for marine life," he says. When the chemicals were squirted through holes under a mesh on a piece of a ship's 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 Peilin Zhou at the University of Strathclyde in Glasgow, UK. "If you do not have to bring the vessel to dry dock it would save a lot of money."

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