Sharks are known for their effortless swimming, but past studies have focused only on how their streamlined bodies contribute to the ease with which they swim. George Lauder from Harvard University focused his efforts on how the shark’s skin boosts its swimming capabilities.
The skin of a shark is coated in razor sharp tooth-like scales, called denticles, and is thought to behave like the dimples on a golf ball: the denticles disturb the flow of water over the surface to reduce drag.
Lauder didn’t completely agree with this. He explains, “all of the shark skin studies were done on flat shark skin mimics that were held straight and immovable. But shark skin moves.”
To complete the study, Lauder and graduate student Johannes Oeffner went shopping in a market in Boston where they found several large makos. They removed the skin, attached pieces to rigid aluminum foil, and submerged it in a flow tank. They were able to replicate the movement of sharks in water and measured the ‘swimming’ speed of the foil by matching it with the flow of water moving in the opposite direction.
Next, they sanded off the denticles and repeated the experiment. Instead of slowing down, the aluminum shark actually sped up. “But then we remembered our premise that the sharks aren’t rigid,” said Lauder.
They repeated the experiment with two pieces of shark skin glued together and found dramatically different results. The intact skin-foil swam 12.3 percent faster than the sanded skin, proving that the shark’s rough surface greatly improved swimming performance.
By continuing their work, Oeffner and Lauder discovered that in addition to reducing drag, the skin was actively generating thrust.
“That’s the number one surprise. It’s not just the drag-reducing properties, but the denticles alter the structure of flow near the shark skin in a way that enhances thrust,” explains Lauder.
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Copyright © 2012 by Marine Science Today, a publication of OceanLines LLC.