Faster Olympic swimmers paddle like a duck, kick like a dolphin

(This is the fourth in a series of stories about the science behind the Olympics to run daily this week.)

By Sharon Begley

NEW YORK (Reuters) - Olympic swimmers need to ignore their intuition when it comes to the best ways to propel themselves through the water to glory next week.

Scientists have made several discoveries about the mechanics of swimming since the last summer Games four years ago, refuting some seemingly obvious assumptions.

Take freestyle swimming. It seems that forming each hand into a tight cup maximizes the force it exerts on the water, while keeping the fingers apart produces a wimpier stroke.

Not so, explains Adrian Bejan, professor of mechanical engineering at Duke University in Durham, N.C. Inspired by animal locomotion, he and colleagues analyzed the force that fingers and toes generate when pulled through the water.

When the digits are separated by between one-fifth and two-fifths of their diameter, the force exerted is 50 percent greater than when they are pressed together with no spaces in between, Bejan and his team will report in an upcoming issue of the Journal of Theoretical Biology.

The reason involves something called a boundary layer. When fingers move through water, viscosity causes a thin stream of fluid to stick to (and move with) them rather than flow between them. As a result, slightly separating the fingers creates a wider surface than keeping them close together. Just as a wider oar pulls with greater force than a narrower one, so does a wider hand.

The greater force pulls the swimmer both forward and, on the down stroke, upward. The higher the body is above the water, the faster forward it can go, since air resistance is less than water resistance.