Kurt Steiner, the world stone- skimming champion, knows when a throw has gone well. First he hears the telltale fluttering sound. Then he sees the first impact. Finally, as his arm recovers from his famed “whip” technique, he counts the bounces. “You’re in control of magic,” he says. “It’s like watching a fire or a sunset.”
For him, the process is instinctive. Beneath the water, though, a mathematical study has suggested, something else is going on, something less magical. For the best throws, the curvature of the underside of the stone should, scientists argue, be precisely tailored to match its weight. And the heavier the stone is, the curvier it needs to be.
“The curve means it can bounce before it floods,” said Ryan Palmer, from Bristol University. It lifts the rock out of the water earlier than it would otherwise be lifted and is more important as it gets heavier. If a rock increases in weight eightfold, his calculations show that its curvature needs to increase fourfold to keep up.
There are implications for the professionals, but perhaps even more for the amateur who wants to make any old stone work. His insight means, he says, that people might be eschewing some rocks at the heavier end that might actually give a perfectly serviceable bounce — provided they are shaped correctly.
“You might be surprised how one of the stones you reject might skim, or the sort of trajectories you might get out of them. Rather than lots of little skips, you might get one almighty one that surprises you,” he said.
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Stone-skimming is a growing sport. Steiner, from the US, holds the record for the greatest number of bounces, exceeding 80, but there are other disciplines too. The distance world championships are held annually in Easdale in Scotland, with the winner being crowned a “top tosser”.
Ideally, a stone should be spun as fast as possible, largely for stability, and should hit the surface at a slight angle. As it hits the water, the pressure on its underside generates the force that pushes it back out. If, though, its leading edge is pushed under, then it will suddenly sink.
It was this dynamic that Palmer investigated, in a theoretical paper in the journal Proceedings of the Royal Society A. He said: “When it’s more curvy, it shortens that interaction with the water, lifting it out sooner — so it doesn’t get to the point of flooding.”
The finding also explains how, sometimes, heavier stones can behave more erratically — leaping unpredictably after a single bounce in what Palmer calls a “super-elastic response”. This is what excites him about the findings. Unlike Steiner, he said that his “skimming game is the bigger jumps”.
He added: “When I’m walking down the beach with my kids, I’m always up for trying to throw the obscure, odder rock and seeing what happens.”
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Steiner, though, has long learnt that scientific analysis is not enough. For him, understanding the rock is about understanding ourselves. “In this world, where everything is so connected and everybody is so focused on some device in their hand, I find it essential to replace that with a piece of the earth, a rock. It’s so primal.”
