Due in part to the difficulties inherent in working with predatory animals and in part to problems simulating their habitat there is little accurate data available on velocities of pelagic sharks (Bechert et al. 1986).  One estimate (Gupta et al. 1996) puts the top speed for the swift hammerhead shark at 72 kilometers per hour (about 45 miles per hour). This incredible speed is achieved in part due to the scales of fast-swimming sharks. These scales have fine grooves in them, about 0.1mm wide, running in a streamline pattern (Nitschke 1984).  These grooves function by reducing the momentum exchange and thus the drag of the surface, relative to a smooth surface. A more detailed look at these grooves, or riblets shows that they affect the flow of streamwise vortices. If a normal (smooth) surface is moved through water (or has water moved over it) these vortices produce upwash of fluid away from the surface. This upwash slows the fluid moving over the smooth surface, and also causes high fluctuation activity, further slowing the flow. Thus, a mechanism which hampers these streamwise vortices would serve to reduce drag (Bechert et al. 2000).

In the above discussion, there are actually three types of drag:

• "Frictional drag is caused by the friction created between the skin and the boundary layer. This type of drag can be reduced if the boundary layer maintains a turbulent flow" (Bargar and Thorson 1995).
• Pressure drag is caused by water deflecting off the moving body of a shark. This type of drag may be minimized if the boundary layer remains stable and in contact with the body along its entire length (Bargar and Thorson 1995).
• "Induced drag results from the turbulence of the vortices formed along and behind the posterior edges of the fins, causing a wake. The wake is formed from the pressure difference between the pressure drag and frictional drag as the boundary layer separates from the body of the fish and interacts with the outer water layer" (Bargar and Thorson 1995).

Of the three types above, frictional drag is the greatest element of drag.

Important factors affecting the drag-reducing properties of riblets:

• Sharp edges--all fast swimming sharks have sharp-edged riblets on their skin (Bechert et al. 1986).
• Protrusion height--there is an optimal height for riblets to protrude into the boundary layer (Bechert et al. 1986).
• Lateral Spacing--riblets should have finer lateral spacing than the spacing of the streamwise vortices (Bechert et al. 1986).

In the experimental work that has focused on reproducing the condition of shark skin in a laboratory, there has been no breakthroughs (Bechert et al. 1986, Bechert et al. 2000). Attempts to optimize the drag reducing effects of riblets have failed to show a large (>8%) improvement over smooth surfaces. To date, no "advanced" riblet construction has shown better drag reduction than the skin of fast-swimming sharks.