The sight of a shark brings out fear in most people. But there's something to be admired in these elegant fish, which also have the reputation of being vicious predators. Engineers believe that sharkskin holds a secret that could help shipwrights create vessels that can save a lot of fuel.
"Resistance to the current is reduced through this groove structure on the shark's skin, which means sharks can swim faster while expending the same amount of energy," said Christian Johannsen of the Hamburg Ship Model Basin (HSVA).
For people involved in making and operating ships, this is a good reason to imitate the sea predators. The formula is simple: lower resistance means that a ship needs less fuel and engine power. And that would lead to fewer emissions - meaning that both the shipping industry and the environment benefit.
The secret is in the grooved structure of the sharkskin. "It is a fallacy that a smooth surface has the lowest resistance," said Johannsen, who heads the HSVA propellers and cavitation section. A grooved structure, like what a shark has, is more suitable.
So-called riblets, or movable scales, extending from a shark's head to its tail fin reduce friction because they run parallel to the direction of swimming. They prevent the formation of unwanted, energy-consuming crosscurrents in the layer between the ship's surface and the undisturbed current.
But the fact that riblets make sharks high-speed hunters has been known for a long time. What is yet to be tested extensively is whether a similar structure could also have such an effect with large container ships. A project led by the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research is testing the use of a "riblet coating" for large ships.
The collaboration, which also involves the HSVA, is called "Hai-Tech" in German - a play on the German word for shark: Hai. The HSVA facilities in Hamburg are being used to test whether the energy-saving riblet effect of sharks also affects very large ship types.
Test in Hamburg
Since 1913, models are tested and improved in large pools at the Hamburg-based test facilities, which include a large towing pool and an ice pool, where ship makers for example run tests on experimental fuel-saving ship designs.
Fraunhofer Institute workers coated an eight-meter-long (26-foot) torpedo with a riblet-covered film, using the hydrodynamics and cavitation tunnel to test run the sharkskin-like riblet coating.
"You need a large body and a high speed," Christian Johansen explained of the experiment. The water rushed past the immobile sharkskin torpedo at 36 kilometers per hour (22 miles per hour). The result: the higher the speed, the more the the riblets reduced the resistance. That means that ships could save fuel with riblet coatings by going faster - which could save a ship owner up to 234,000 euros ($300,000 dollars) per year.
However, since large container ships are at sea for many years, the special coating would also need to hold for a long time. To find out about the wear of resistance reducing the "riblet effect," the researchers went at the coating with sandpaper.
"We found that by the time the structure is worn down by approximately 50 percent, the benefit is almost gone," Johannsen said.
The researchers also considered another problem with the ship coating - mussels that grow on the ship's hull. Mussel growth is an existing problem that increases the ship's resistance. The researchers employ poisonous varnish to protect the ship hull from becoming infested with mussels.
But whether and how sharkskin coatings, which are particularly dependent on an intact groove-like structure, could be effectively combined with anti-mussel measures remains to be explored.