As the whale turns: the shape of the humpback's flippers might hold the secret to more maneuverable submarines

Natural History, June, 2004 by Adam Summers

The body of a whale is relatively stiff, and so the animal cannot curve sinuously into a turn the way a swimming seal can. To propel itself around a corner, a whale instead relies on the lift generated by its flippers. Fish and his colleagues found that the tubercles enable a flipper to continue generating lift at angles of attack 40 percent steeper than are possible with a smooth wing [see illustration above]. In other words, when a humpback turns, it can roll farther onto its side without losing its "grip" on the water, and so make a sharper turn, because of the tubercles.

But the tubercles would seem to solve one problem only to introduce another. Protruding into the flow as they do, they would appear to increase the drag of the flipper as the whale swings into a turn. Appearances can be deceiving, though--particularly where drag is concerned. When the scalloped flipper is held nearly horizontal in the flow, the drag is no different from that of a smooth flipper, and at high angles of attack the drag is actually lower. So, just when the whale is using its flipper to turn the tightest, the flipper slides through water more easily than it would if it had a smooth leading edge, even as it generates far more lift than a smooth flipper could.

With their stiff bodies, whales are far better analogues for man-made submersibles than more sinuous aquatic animals are. Fish and his colleagues hope to design guidance fins for underwater vehicles that can give them some of the agility that enables a thirty-ton animal to turn on a dime--or at least a large doormat.

ADAM SUMMERS (asummers@uci.edu) is an assistant professor of ecology and evolutionary biology at the University of California, Irvine.