Wednesday 18 January 2017

How to perform the perfect stone skim

Richard Alleyne

Published 16/08/2010 | 16:32

It is news to gladden the hearts of children everywhere – academics have come up with a formula for skimming stones.

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Mathematicians at University College London have drawn up an equation that works out how to play the childhood pastime of stone skimming or "ducks and drakes".

The model compares the weight and speed of the stone with air and water resistance and gravity to come up the perfect throw.

While the research was carried for fun it also has a serious side as it can be used to calculate the behaviour of ships in rough seas and ice skipping over aircraft wings and fuselages.

Professor Frank Smith, an applied mathematician carried out the work with his colleague Dr Peter Hicks.

He said that there was no reason why up to 50 "bounces" was impossible, according to the formula.

"Theoretically it is possible to reach around 50 but I admit the most I have achieved is eight," he said.

He said that the key to a good throw is to spin the stone, to maintain stability.

"Pick a stone which is as thin and light as possible," he said. "Throw it with as much force as possible and as horizontal as possible and from as low to the ground as you can.

"Spinning it in the air helps it fly and reduces the air resistance. That should get you the maximum amount of bounces."

The record number of skims is 51 held by Russell Byars, an American engineer, who achieved the feat on the Alleghany River in Pittsburgh.

He recommends that you pick flat palm sized stones and use your thumb and forefinger to spin it. Aim to hit the water at between 10 to 20 degrees, he adds.

The texture of the pebble surface is important too but experts are divided over what is best. Some say that the stone needs to be as smooth as possible in order to skip unencumbered across the water.

Others, like French physicist Lyderic Bocquet of Lyons University, says lots of small pits on the stone surface will reduce water drag in the same way that the dimpling on a golf ball reduces air drag.

Telegraph.co.uk

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