WHO, WHAT, WHY?

The Magazine answers...



Most of us have tried stone skimming - a game sometimes known as Ducks and Drakes - on the beach or by a river. We might have been pleased with several consecutive bounces... but 51 times seems impossible.

That's what 43-year-old engineer Russell Byars managed on the Allegheny River, near Pittsburgh, in the US. Luckily, he is happy to share his technique.

Speaking to the BBC, he promises that by following the rules, even the most novice skimmer could achieve an impressive 15-20 bounces or more.

1: Pick flat, oval, palm-sized stones. Use forefinger and thumb to spin 2: Throw the stone fast and flat 3: Stone should strike water at an angle between 10-20 degrees

"I always pick out a spot ahead of me on the water where I want the first bounce to be. Then I sort of push the stone at the water when I throw it down," he says.

But he confesses himself baffled by some aspects of his throwing technique. "I have to say I've watched the video of my throw and I see that I kind of whip the stone and throw it so it looks like I'm throwing it underhand.

"I have to be honest with you though - I don't really know how I do that."

Some of the mystery can be explained by the science - of which there is a surprising amount - into the art of stone skimming.

The size and shape of the stone is key: some people believe it needs to be oval and quite big - about the size of your palm. This is heavy enough to be immune to breezes and turbulence, but light enough to be thrown with accuracy.

Water drag

The texture of the pebble surface is important too but here the experts are divided: 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 Lyon University, say 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.

Professor Bocquet and his fellow researcher Professor Christophe Clanet from the Institute of Research on Non-Equilibrium Phenomenon in Marseille have sought to deconstruct the serious physics underlying a simple game.

WHO, WHAT, WHY? A regular part of the BBC News Magazine, Who, What, Why? aims to answer some of the questions behind the headlines

In an article entitled "The mystery of the skipping stone" they answer questions such as why does a stone skip at all; how many skips can it perform; and how can the number of skips be maximised.

Using a specially designed catapult, the scientists shot aluminium disks across a pool of water and then filmed the splashes with a high-speed video camera. After analysing the results, they discovered the optimum speed and angle a stone-thrower would have to attain in order to achieve the perfect skim.

Just why a stone bounces on water is, according to the professors, "easy to identify". It's all down to the conservation of momentum principle, which dictates that as the stone enters the water and pushes some of it downwards, the stone is, in turn, forced upwards. However, the stone needs to be moving at a certain speed - at least one kilometre an hour - otherwise the stone "surfs" on the water for a short distance, and then sinks.

Spinning the stone is also important - the "gyroscopic effect" ensures stability. To remain stable, the stone must rotate with a certain minimum rotational velocity - at least once during its "collision time".

Optimum angle

As "Rock Bottom" already knows, the angle of the throw is crucial. The professors estimate the optimum angle at which the stone should enter the water to be between 10 and 20 degrees.

Old school: Sting practises the legs-bent technique

He's very aware that for that record-breaking skim, there's a need for speed - the faster the better.

Using his research Professor Bocquet has worked out how fast Russell might be throwing his stone.

"Although I don't know anything about the conditions of this throw, my rough estimate suggests that he has to reach a speed of at least 80 km/h. This is quite a lot, but still below fast balls in baseball, which can reach to 150 km/h," he says.

"So I would say it is still possible for someone else to break the record."

Mr Byars concedes that his strong right arm - he used to be a construction worker - is a big part of his success. Apart from stretching exercises, he doesn't do any special training before an event. "I might take the dog out a few times before an event and throw some stones.

"That's it," he says.

And he's adamant that his 51-skip record was not a one-off fluke. "When I sent the video of the attempt off to the Guinness Book of Records there were three throws with over 40 skips, and two which were over 50," he says.

The type of lake/water appears to increase the number of skips too - at Lake Louise in Canada I got about 21 skips, whereas back home at Ullswater I'm lucky to get 7! I wonder if this is due to the suspended particles from the glacier that feeds Lake Louise?

Sally, Newcastle upon Tyne, England

On holiday recently, I attempted, on a whim, to skim bread which had been compacted into roughly the correct shape. It was a very reasonable substitute for a stone. I didn't have the time to test quite how reasonable, but it certainly performed moderately well.

Philip Walker, York, United Kingdom

I note that the record-breaker threw his stones on a river. My best results (nothing like so good) were achieved on a Scottish loch whose waters were glassy-still. Smaller, lighter stones can be used in these conditions. Is water turbulence generally regarded as helpful or a hindrance? Is it better to throw with, against or across the current?

Jason Mills, Accrington

I can remember, as a young lad, skimming stones on the beach at Penarth. I once skimmed a rather large flat slab and as it went out to sea, a swimmer poked his head up from below the water. (well he could hardly have poked his head up from above the water could he?) Anyway, he ducked down to let the stone continue on its way. A true sportsman!

Brian, Penarth, Glam.

And no mention of the Dambusters...

Alexander Lewis Jones, Nottingham, UK

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