Reducing the scale of drag

A coating for the hulls of ships that mimics sharks’ scales could prevent fouling by algae and barnacles without causing pollution, its developers claim.

When organisms attach themselves to ships, drag increases, reducing fuel efficiency. Conventional coatings can prevent this but leach poisonous copper into the ocean. This accumulates in harbours, threatening marine life. Researchers at the University of Florida have created an alternative coating composed of tiny scale-like elements.

The project is sponsored by the US Navy, which estimates that it could save up to £25m per year in fuel by reducing fouling-related drag. Full-scale ocean tests will begin later this month, and researchers at Birmingham University and an Australian team are also providing support.

Project leader Anthony Brennan, professor of materials science and engineering, realised that sharks remain free of barnacles and algae despite spending their lives submerged.

Sharks have placoid scales, consisting of a rectangular base embedded in the skin with tiny spines that poke upwards from the surface, making them feel rough. He mimicked this by creating a plastic and rubber composite coating made from billions of raised diamond-shaped patterns, each measuring 15 microns. Each diamond also contains seven raised ribs.

Placoid scales are found in sharks and rays, and can vary greatly in external appearance. They do not increase in size as the fish grows, instead new scales are added. Placoid scales are often referred to as denticles. Phot courtesy Australian Museum Online.

Tests have so far shown that a common and aggressive algae called ulva, which likes to colonise the inlet ports used to cool the nuclear reactors of submarines and battleships, has trouble attaching its spores to the diamonds. ‘The scales are a tenth of the size of a real shark scale,’ said Brennan. ‘So far we don’t have any settlement of ulva spores.’

By adding less than one volt of electric current, the material flexes as the ribs shrink and swell, preventing accumulation of silt and other debris. This is often a precursor to barnacle and plant growth, and the team hopes the material’s movement should deter organisms other than ulva from attaching themselves.

The research could also lead to the creation of better medical implants such as catheters or heart valves, which may suffer reduced function over time as they are colonised by cell and tissue deposits.