Actually, most swimmers at this level of competition wear two caps.
Dana Vollmer, who won gold with a new world-record time in the 100m butterfly, actually lost her outer cap in the pool on the return leg of that event.
Thus, the first reason; redundancy.
These caps are latex rubber, which doesn't mix well with chlorine and will eventually degrade, and as Dana's swim shows, even a brand-new cap can slip right off.
If the swimmer, especially a female swimmer, had only one cap, losing it and releasing their hair would drastically increase the drag they have to swim against in the water.
Second, as Rory states, a common (though not universal) way to put all your headgear on is to put one swim cap on, then put your goggles on, then cover the goggle strap with another swim cap.
Not only does it reduce drag on the goggle strap (which is minor), it helps keep your goggles in one place (MUCH more important).
I was on a swim team when I was much younger, and I lost count of how many times the shock of entering the water ripped my goggles right off my eyes.
You still have to swim if it happens, but it becomes a HUGE distraction that can take you right out of the race.
At your average junior swim meet, losing a race to your goggles is embarassing but not devastating.
At the Olympics, it would be a heartbreaker.
sports.stackexchange.com/users/528/KeithS
The two caps are also made of different materials.
The inner cap is latex, which stays on the head better, but wrinkles.
The outer cap is silicone, better for reducing drag.
"The outer silicone cap better maintains the shape and does not wrinkle as much, thereby causing less drag," Salo told Yahoo!
Sports.
- Why do Olympic-level swimmers wear two caps?
sports.stackexchange.com/users/648/John Roberts
It is for speed- if you have your goggles on over your cap
it upsets water flow, so the second cap over that holds the goggles safely on and streamlines the head.
Sure, it's an infinitesimally small benefit, but at that level it is important.
Additionally, some sports manufacturers make some specifically to be worn like this to reduce drag further.
sports.stackexchange.com/users/103/Rory Alsop
It helps them swim faster.
it not for hair.
Its help b/c
the water will just flow over the cap.
I know because we are talking about it in science
sports.stackexchange.com/users/1798/caitlin
he wore two caps only because if one comes off he has a back up already on and it supports the goggles making them stay in their lace under the second cap as the goggles may come off from diving.
sports.stackexchange.com/users/3717/madswimmer
Just now I saw Michael Phelps competing in the 400m Medley heats (London Olympics 2012). After he finished the race, I saw him take off two caps. I can imagine that a swimmer uses one, but what is the reason for wearing two caps?   Is this something new? Does it help him swim faster?
Why do Michael Phelps and some other swimmers wear two caps?
sports.stackexchange.com
<equipment><swimming><olympics>
sports.stackexchange.com/questions/1236
sports.stackexchange.com/users/392/Bernhard
Swimmers wear two caps to reduce drag, hold their goggles in place and ensure their hair is not released if a cap is lost. The second cap is made of a more aerodynamic material. The second cap also holds the goggles in place and the two caps ensure the hair is not released as loose hair, especially if long, creates a lot of drag. A reason to wear two caps is to have a back-up in case the first one comes off. In competitive swimming, released hair is a serious disadvantage. A second cap is also a means of supporting and securing your goggles in place. Some argue that the caps are designed specifically to reduce drag and increase swimming speed. To this end, one is made out of latex and one out of silicone.
A reason to wear two caps is to have a back-up in case the first one comes off. In competitive swimming, released hair is a serious disadvantage. A second cap is a means of supporting and securing your goggles in place. The caps are designed specifically to reduce drag and increase swimming speed. To this end, one is made out of latex and one out of silicone.