Three swimmers in a race

At peak racing speeds, swimmers should position themselves just under four body lengths behind the leader for maximum drag reduction, modelling work suggests. Credit: Robert Daley/Getty

Fluid dynamics

How lagging swimmers can use physics to get ahead

Racers can take advantage of a competitor ahead of them to save energy.

Swimmers can sharply reduce drag when moving through the water by trailing in a competitor’s wake.

During races in both open water and pools, swimmers often ‘slipstream’ by lagging behind and to the side of competitors. The trailing swimmers conserve energy by riding on the leader’s bow wave, the V-shaped wave created as an athlete cuts through the water.

Mingxin Li at the University of Strathclyde in Glasgow and his colleagues used hydrodynamic models of simplified human-like figures to calculate the benefits of slipstreaming. The researchers found that at racing speeds of two metres per second, the optimal drafting zone is just under four body lengths behind the leader and 1.7 metres off to the leader’s side; in this sweet spot, total drag was slashed by roughly 30%.

A properly positioned trailing swimmer generates a bow wave that interferes destructively with the lead swimmer’s bow wave. This interference partially cancels out the leader’s bow wave, leaving calmer water in its wake.

The benefit is reduced by the lane ropes in competition pools but remains significant, the authors write.