Having a break from the water to bounce stones mindlessly across it can be extremely therapeutic. It can also be a way to work up the courage to enter a cold river, an excuse to linger longer beside a beautiful one or something to do while waiting for a slower colleague. More pragmatically, many of the hours I’ve invested in skipping rocks have also been spent thinking, solving problems or just shifting onto a more manageable emotional plane.
Rock skipping starts with a stone, preferably a flat one, that has been crafted and shaped by water and friction into a rounded shape. Fortuitously, the final polishing of rocks into their skippable form most often happens at a river or beach, which just happen to be the perfect locations for skipping.
I’ve got a rock on my desk that I found on a beach near Wollongong. It’s a piece of smooth sandstone two centimetres thick and eight centimetres on the longest axis. It has a roughly rhomboid shape with rounded corners. When I found it there was a storm and the sea was too rough for skipping. I don’t know why, but I subconsciously slipped the rock into my pocket and it travelled the 1000 km home with me. Unthrown and untested, I pick it up every now and then to examine different ways of holding it. Sometimes I even swing my arm back in a slow-motion practice throw, training my arm for the perfect throw.
But what is perfection in rock skipping, and how is it achieved? Certainly, the rock shape and weight are important factors, as is the surface it’s skipped over, but these are both determined by nature and are aspects we have very little control over. To achieve a satisfying skip, the athlete must work on their throw. This is the only thing we have control over. It’s where practice, skill and persistence pay off, and where art, style and flair come into play.
According to a paper published in the American Journal of Physics, the number of times a rock will skip is determined by the forward speed of the stone, the velocity of its spin, and the angle that it strikes the water1.
A rapidly spinning stone thrown at high speed will bounce more times than a slow-spinning stone thrown with less vigour. And hitting the water at an angle of about 20° will further increase the number of skips2.
In September 2013, Kurt Steiner got his throw just right. He skipped a stone 88 times over the slightly ruffled surface of Allegheny Reservoir in Pennsylvania3. While this is the world record for the number of skips, the World Stone Skimming Championships value a different metric. Held each year in a disused quarry on Easdale Island, Scotland, WSSC competitors get three throws each of a sea-worn piece of Easdale slate (no more than 3” diameter). The competitor with the greatest cumulative distance wins4.
However, for many practitioners, an analysis of rock skipping is outside of the spirit of the pastime. This is because the main attraction of rock skipping is that it has no real purpose at all. It’s simply a way to idle away time, elegantly propelling a rock over a nice piece of water.
For me, I usually have no real goal in mind when I pick up a rock. I’ll just find a one that looks nice, take a few steps, crouch fluidly like a tiger and send it flying over the water, low and straight towards its first bounce. There’s no real thought that goes into the process, just a random assemblage of physics, geology, hydrodynamics, biomechanics, meteorology and philosophy that are way beyond my ken and usually result in something graceful and calming.
1 Bocquet, L. 2003. The physics of stone skipping. Vol 71, pp.150-155.
2 Clanet, C., Hersen, F. and Bocquet, L. 2004. Secrets of successful stone-skipping. Nature. Vol 427, p.29.
3 Kurt Steiner’s world record stone skip
4 Website for the World Stone Skimming Championship http://www.stoneskimming.com/