(This is the third in a series of stories about the science behind the Olympics to run daily this week. The full list will be updated at )
By Sharon Begley
NEW YORK, July 17 (Reuters) - The sharkskin has been retired. The swim bladder, too. For London's Olympic swimmers, the watchword is more "barracuda".
Swimsuit makers have long sought inspiration in the oceans, where evolution spent about 500 million years sculpting bodies shaped to cut through the water with the least drag or studded with features that increase speed. That led some manufacturers earlier this decade to develop full-body, impermeable suits: water could not get in and air could not get out, trapping air inside and increasing buoyancy much as fish's swim bladders do.
Water offers greater resistance than air, so swimmers whose bodies rise higher in the water have an advantage. That led international swimming federation FINA to outlaw full-body, impermeable suits in 2009.
The other innovation of the last decade was Speedo International's sharkskin-inspired suits. Called Fastskin and introduced in 2000 for the Sydney Olympics, the suit was studded with tiny hydrofoils with V-shaped ridges like the "dermal denticles" on sharkskin. It seemed to be a success: 83 percent of swimming medals at Sydney went to athletes wearing Fastskin.
That was not quite slam-dunk evidence of its superiority. Speedo, owned by the Pentland Group, had reached out to the very top competitors to wear its suits in the first place.
In March, researchers at Harvard University published an analysis of Fastskin and real shark skin in the Journal of Experimental Biology. They found that although real shark skin was 12 percent faster with denticles than without, Speedo's fabric was not. It was actually faster inside out than with the denticles exposed to the water.
By the time the study appeared, Speedo was already years into research on Fastskin's successor. Called Fastskin3 and first used in competition this year, it includes caps, goggles and suits that redistribute water flow around a swimmer to decrease drag, partly by compressing the body's silhouette along the lines of the predatory barracuda.
"We operated on the principle of minimal gains," said Joe Santry, research manager for the company's Aqualab. Rather than getting a huge increase in performance from a single design change, Speedo's scientists squeezed a few percentage points of improvement from many individual modifications. The fruit of their efforts will be on display next week at the London Olympic Games.
THE PERFECT BODY
They started at the top. "The cap and goggles are the first things that hit the water, so they can create turbulence downstream in a way that affects the performance of the suit," said Santry.
The scientists scanned athletes to produce a three-dimensional, digital avatar. They then manipulated various parts of it - squeezing here, filling in there - and used computational fluid dynamics to calculate how each change altered drag "until we found a theoretically perfect body form."
Take the cap. The standard model is a plain silicone bowl that wrinkles at the top. Those wrinkles, like any protuberance, create drag. But shaping a cap to the average human head shape - determined by the 3D head-scanning - minimizes wrinkles.
The new cap also has room to pack hair at the nape. That fills what is otherwise a dip between the head and the back. "That dip creates a pressure drag," explained Santry: a region of lower pressure that ever-so-slightly sucks the swimmer backward. Packing the hair into the gap decreases that drag 3.4 percent compared to standard silicone caps.
The most hydrodynamic goggles shape - a full-face mask reminiscent of Batman's - violates FINA rules. But the runner-up takes the water flowing over the head to the eye sockets and basically attaches that flow to the face, creating an ultra-smooth "boundary layer" of water.
Smoothness is key. Boundary layers naturally flow toward regions of low pressure, which can split up the layers and create turbulence, increasing drag. The optimal goggle design is shaped like a water droplet and turned up at the temples. "It's unlike anything on the market," said Santry.
The goggles cut drag 2.2 percent versus other Speedo models.
The suit redesign addressed the fact that 80 percent of the drag on swimmers comes from their shape. That meant compressing fleshy areas like the thighs, rear end and, for women, the chest, all with Lycra panels sewn into the suit.
While other suits absorb water, Fastskin3 repels it. Less weight from the suit means each stroke propels a swimmer farther. So after four years and 55,000 hours of research and testing, the complete outfit reduces drag by 16.6 percent compared to standard gear, Speedo says.
That translates into an approximately .11 percent potential increase in speed. Not huge, but then the difference between winning gold and silver can be thousandths of a second. At Beijing, American Michael Phelps finished 0.01 second ahead of Serbia's Milorad Cavic in the 100 meter butterfly. Phelps will be wearing Fastskin3, as will team mate Tyler Clary and Britain's Rebecca Adlington.
"The recent suits work," said kinesiologist Joel Stager of Indiana University, who has studied the science behind swimming for decades. "The earlier ones (with denticles) didn't."
As it happens, compressing swimmers' bodies has a benefit besides reducing drag: it also increases blood flow. Think of it as a legal version of blood doping. (Editing by Michele Gershberg)