Longtime readers may recall my older article Speed Is Skin Deep, Too in which I mentioned "Gray’s Paradox," a theory published in 1936 that contended that dolphins were physiologically incapable of producing sufficient power to achieve the speeds mariners had observed. Researchers have long since disproved this, discovering key flaws in Gray’s calculations. Then a couple of years ago, Japanese scientists discovered that the way in which water flows over the dolphin’s rubbery & continually shedding skin significantly reduces the vortices that would otherwise create too strong a drag on the tail.
But now, as reported in the March 2008 New Scientist article "Dolphins swim so fast it hurts," findings from a pair of researchers at the Israel Institute of Technology in Haifa have discovered that the main limiting factor of a dolphin’s maximum speed is neither physical ability nor friction-causing turbulence. What’s holding dolphins back from even more impressive speeds?
As dolphins propel themselves through the water, there’s a continual formation of microscopic bubbles around the tail, a phenomenon known as "cavitation." These bubbles form as a result of the pressure difference created by the movement of the tail fins or flukes. As the bubbles collapse, they produce a shockwave. This same condition produces the foamy wash that streams behind boats & ships and is known to deteriorate the metal on propellers.
The Israeli scientists revealed that, to dolphins and other fast-moving aquatic mammals, cavitation is painful since they have nerve endings in their flukes. According to their calculations, this begins to happen when dolphins swim at speeds greater than 22 miles per hour. So shorter bursts are tolerable, but to sustain that speed becomes too painful for the animal to endure.
Dolphins can cheat around this problem by swimming deeper in the water rather than at the surface because cavitation decreases as pressure increases. With measurements being so much more difficult in deeper waters, the theoretical top speed of dolphins is once again anybody’s guess.