Speed Is Skin Deep, Too.

speed limit sign in the waterDolphins were astounding us with their extraordinary swimming speed (nearly 25 m.p.h.) even as early as 350 B.C. when the Greek philosopher Aristotle marveled at their swiftness. Anyone who has been lucky enough to swim with dolphins can attest to their breathtaking agility. It’s not surprising they have enchanted humans for so long.

English zoologist Sir James Gray concluded in 1936 that, given the density of water and the amount of muscle dolphins have, the energy needed for a dolphin to swim so fast is nearly 10 times that which they can produce — an observation known as “Gray’s Paradox.”

Gray theorized that to achieve this impossible feat the powerful motion of the dolphin’s tail must cause water to attach tightly to the dolphin’s skin due to a concept called “laminar flow,” which eliminates turbulence.   Any moving object splits the medium through which it passes.   This turbulence increases friction, and thus drag.

Scientists searched for years for an answer to Gray’s Paradox — what mechanism stops the turbulence and allows for the dolphin’s seemingly impossible energy output?

Although it took several decades, his calculations were ultimately proven flawed because Sir Gray’s conclusions about the amount of energy output needed were based on sprinting speeds that dolphins can’t sustain.   But despite those flaws, scientist Frank Fish of West Chester University credits Gray’s Paradox for having brought about tremendous research focus on marine propulsion.   A number of mechanical factors have been discovered that all contribute to the ‘secret’ of the dolphin’s speed.

Most notably, this speed is made possible by the tapered body shape with ears, eyes, and blowhole that are flush with the skin.   Dolphin’s eyes, researchers say, secrete a mucus that lubricates skin surfaces for increased swimming speed.   This streamlined shape combined with large, powerful muscles that are mechanically linked to an oscillatory pair of crescent-shaped tail flukes enables dolphins to produce thrust with far greater efficiency than any man-made propulsion system.

Also part of the secret is the dolphin’s blubber, which is far more than simple fat.   Instead, dolphin blubber consists of a complex of fat cells and collagen fibers in a elastic criss-cross pattern that acts as a spring.

Other recent research highlighted in Discover magazine has revealed that dolphins constantly shed, replacing their entire outer layer of skin every two hours.   And while water flowing across the skin forms tiny vortices, the flakes of constantly shedding skin disrupt the vortices and dampen turbulence.

Yet another article at ThinkQuest offers insights about small folds in the dolphin’s skin, called dermal ridges.   These ridges run parallel to the length of the dolphin’s body and are constantly moving which goes to further allow the dolphin to propel effortlessly through the water.