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The perfect way to skim a stone: Throwing at an angle of 20 degrees boosts number of bounces when water skipping

A Utah researcher has investigated how objects skip on water
He found that the optimal angle to skim stones was 20 degrees
However the heavier the object was, the flatter this angle needed to be
Water skipping is a popular past time with stones and toys
But it is also a useful military tactic, used by bouncing bombs and cannons

Published: 11:41 EST, 5 December 2014 | Updated: 04:05 EST, 22 September 2015

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A disc-shaped stone has to be tossed at an angle of 20 degrees to achieve the maximum number of bounces across water.

Inspired by English scientist Barnes Wallis, who invented the bouncing bomb, researchers have described the complicated physics behind the water-skipping behaviour in stones, toys and even cannonballs.

Naval gunners as far back as the 18th century bounced cannonballs as a military tactic - removing the launch angle as a variable in sinking enemy ships.

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What's the best way to skim a stone? A Utah researcher has worked out that you should throw it towards the water at an angle of about 20 degrees (illustration shown) to get the best number of bounces. For heavier objects, though, this angle needs to be lessened

Professor Tadd Truscott, Brigham Young University, Utah, and his colleagues said: 'Barnes Wallis recaptured the variable-reducing military advantage during World War II with his bouncing bomb.

'Designed to breach German dams, the weapon skipped over the water surface to avoid undersea torpedo nets.'

THE SCIENCE OF... PILLOW FORTS?

Writing on the blog Fatherly, New York architect Ben Pell explained there are three different types of fort - tunnel forts, buttress forts and compound forts.

He believes that many children manage to trap themselves inside forts and should instead build the exterior first.

'Kids, left to their own devices, pile up pillows and then figure out how to get inside,' he said.

'Or, they’ll build it around themselves and then they can’t leave without destroying it.'

Mr Pell, who in his day job works for Pell Overton, urged children to think like an architect.

He warned prospective builders should first sort the available pillows based on which ones are best for walls and which ones are a good for laying on inside the finished fort.

Writing in Physics Today, they say water skipping has returned to more docile roots of late with toy balls that make it much easier to achieve multiple hops.

The researchers said: 'Those spheres are made of an elastic material whose high compliance - or ability to readily deform - introduces some interesting changes to the skipping phenomenon.'

But, with a side throw and flick of the wrist, people young and old have been skipping stones across water for thousands of years with the object of getting as many bounces as possible.

The world record, according to the Guinness Book of Records, is 88 skips set this year by an American called Kurt Steiner.

Professor Truscott said the skill depends on what angle the thrower chooses to hurl an object at, with the optimal for disc-shaped stones close to 20°. But the heavier the object, the flatter this should be.

He said: 'Spheres are generally harder to skip than discs. In fact, previous research suggests an upper bound on the initial course angle 18°.

'Thus for steel cannonballs, skipping can only occur for angles shallower than seven degrees.'

Naval gunners as far back as the 18th century bounced cannonballs as a military tactic (stock image shown) - removing the launch angle as a variable in sinking enemy ships. To get the best bounce on the surface of the water in this instance, Proffesor Truscott said the angle should be less than seven degrees

The world record for skimming stones, according to the Guinness Book of Records, is 88 skips set this year by an American called Kurt Steiner. Professor Truscott said the skill depends on what angle the thrower chooses to hurl an object at (stock image shown)

An alternative method involves imparting a great deal of backspin to a rounded stone so it dives under the surface and then pops up and out of the water.

Proffesor Truscott said: 'The technique does not yield many skips but it's impressive nonetheless.

'To pull it off you need to throw the stone at the water surface at a much higher course angle than for the usual disc skipping.'