Video: A toy boat demonstrates how waves benath the sea’s surface can invisibly slow a ship’s progress. Does the same effect cause trouble for swimmers?

It may sound like a superstitious excuse for a poor day’s swimming, but it is not uncommon for triathletes to complain that the water is behaving badly – even that it is “evil”. Now a study suggests what they are feeling is real.

Leo Maas, a fluid dynamicist at the Royal Netherlands Institute for Sea Research, and colleagues found that “dead water” – an obstructive effect encountered by ships at sea – can strike swimmers too.

As ships sail over a layer of warm water sitting over saltier, or colder, layers, waves form in the boundary between the two layers. As these waves grow, they form a gulf beneath the ship, sucking away its speed. This effect can stall boats at sea, reducing their speed by up to 80%.

Maas and his colleagues ran two experiments see if dead water could strike swimmers too.


Deadly layers?

First, to measure any effect on arm power, they asked two subjects to use the front crawl stroke while lying on a carriage supported just above the water’s surface. They measured the resulting gain in kinetic energy of the carriage and found that it was far less in water where an upper layer of different density (and of a depth close to arm length) was present than in homogeneous water.

In the second experiment, four subjects were asked to swim a short distance in both homogeneous water and in stratified water. The team found that with the same stroke frequency being used in both conditions, swimming speed in stratified water dropped by 15%, with a 40% loss of propulsive power.

“We’ve been considering the possibility that the drownings of strong swimmers in fair-weather conditions might be the result of dead water, but until now, we weren’t really sure if this phenomenon could strike something as small as a single human,” says Maas. “Now it seems that it can.”

While the discussion in terms of dead water is compelling, stratification would also affect how fluid moves around a swimmer’s hands, explains Ted Johnson at University College London. “It would be great if future work could visualise these three-dimensional flow patterns,” he says.

Journal reference: Naturwissenschaften (DOI: 10.1007/s00114-008-0493-6)