a pre-Olympics minipost
To swim faster you want to decrease the amount of drag caused by moving through the water and increase the amount of propulsive force you can deliver. Focusing on propulsion it would be nice if you had more hand area - perhaps webbing between your fingers. It turns out fluid dynamics offers an easy to implement trick.
I've done a fair amount of thinking about the motion of balls through the air. Balls can be made to do something interesting: the dimples in a golf ball letting it travel much farther, the sudden drop or rise in the trajectory of a spinning ball, the wobble of a perfectly executed knuckleball, and so on. These are all artifacts of a layer of air that is dragged along the surface of the ball - the boundary layer.
Boundary layers of one form or another are present whenever an object moves through a fluid: a container ship through the ocean, a mosquito wing beating in the air, a meteor entering the atmosphere, a hand moving through the water. The study of boundary layers can be mathematically challenging so wind tunnels and towing tanks found heavy use until the last few decades. These days good approximations to many interesting problems can be found using computers. Competitive swimming - particularly the Olympics - is an application area for computational fluid dynamics.
It turns out a finger-sized cylinder moving through the water at swimming speeds is accompanied by a large boundary layer. Spread your fingers a bit and this layer effective fills in the gap between your fingers with an effective web made of water rather than skin. For most people the average distance is a bit less than half a finger width. Competitive swimmers will train with spacers that let them feel the proper distance until it's natural. It turns out to be good for as much as a six percent increase in force and a two percent increase in speed, so it's absolutely worth it. An amateur swimmer can get a good effect without trying for perfection.
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