Every competitive swimmer has the same nemesis, and it’s not Michael Phelps or Katie Ledecky. It’s the water itself.
Combating drag from the pool motivates just about every decision a swimmer makes, from swimsuits to shaving to swim caps. Some swimmers even double up on their rubbery caps, something you may have noticed watching the Rio Olympics, as athletes adjust their headgear before leaping into the pool or peel off layers post-race.
But what can two caps do, that one cap won’t?
Each swimming cap actually serves a specific purpose: The inner cap is typically latex, a material that many athletes believe sticks to their heads better. But the material can wrinkle in the pool when a swimmer pushes through the water, which causes drag. (Latex isn’t the only material Olympians use for inner caps. US champion Michael Phelps wears a thin fabric inner cap to compress his hair.)
To fight the resistance from the water, many add a smooth second silicone cap. “The outer silicone cap better maintains the shape and does not wrinkle as much, thereby causing less drag,” Dave Salo, the head coach of men’s and women’s swimming at University of Southern California, told Yahoo Sports in 2012. The second cap also helps keep goggles in place, sandwiching the straps between tight layers.
Whether a swimmer double caps or not, the goal is always to have as smooth a head as possible. Just like a car’s curves help it cut through air, a swimmer’s smoothed-over shape helps streamline a path through the water.