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Shark skin is very rough, in fact so rough that dried shark skin can be used as sanding paper.
The skin is covered by little V-shaped bumps, made from the same material as sharks' teeth.
The rough surface has been shown to reduce friction when the shark glides through water, which is why sharks are surprisingly quick and efficient swimmers.
Fabrics modelled on sharkskin designed to reduce drag by channelling the water along grooves
in the fabric. These grooves allow the water to spiral in microscopic vortices, a hydrodynamic
advantage.
After looking at shark skin, NASA pioneered the use of longitudinal riblets, ridges perpendicular
to surface, to reduce drag on flat surfaces of ships and aircraft. Riblets were used successfully
to reduce drag on the ‘Stars and Stripes’ America’s Cup yacht and were thought to offer
such an advantage that riblets were banned from competition for subsequent events. Shark
skin itself is far more complex than simple longitudinal riblets
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