Thursday, 17 April 2014
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Turtling along

Julia Pierce

A highly manoeuvrable unmanned underwater vehicle that is easy to control in water too turbulent for other craft could soon be used for mine clearance by the US Navy.

The TransPhibian, from Nekton Research of Durham LLC, North Carolina, is shaped like a turtle and uses flippers that mimic marine creatures' locomotion. It is designed to operate in the choppy water surrounding docks, bridges and in shoreline surf, where other submarines have difficulty maintaining control.

If TransPhibian is washed up on shore by a wave, its four flippers can be used to make it crawl back into the sea.

In water, in less than a second it can go from a standstill to making two rotations per second around its long axis. By slamming its finsforward, it can come to an immediate standstill, unlike any other watercraft. This means that if it is rolled over by a wave it can roll back into position before the next wave arrives.

TransPhibian's ability to come to a standstill allows it to avoid hitting unexpected objects, making it ideal for mine clearance or inspecting ships' hulls for terrorists' mines before they are allowed into commercial ports.

'The fin configuration means we can set up antagonistic thrust vortices to dig into the bottom of the ocean while the device hovers above it,' said Brett Hobson, director of ocean engineering, who designed TransPhibian with director of physics Dr Mathieu Kemp. 'It can then uncover mines and ordnance, or can be used by archaeologists to study shipwrecks.'

The US Navy has sponsored design work leading up to the development of TransPhibian, and has shown interest in the device. At around 78cm long, 20cm high and weighing 18kg, TransPhibian is small enough to be launched by hand from the side of a boat,making it easy to deploy.

Each of its four fins has its own single motor which can either twist the fin, making it undulate like a fish to move the craft forward, or can jerk the fin in a new direction. This moves a huge mass of water, generating the great force that allows TransPhibian to stop or change direction violently in a fraction of a second.

Each polyelastomer fin consists of a single piece, making it cheaper and more robust than mechanical fins with many parts.

Compared to marine creatures such as dolphins and penguins, vessels using rigid propellers and rudders are clumsy and inefficient, leaving a choppy wake behind them as they steer through water.

When they are flexed, supple materials store potential energy that is then released when they snap back to their original form. By applying specific forces to flexible materials they can be made to oscillate, alternately storing and releasing energy as they move.

The TransPhibian includes an array of sensors, a sonar system, and is powered by a nickel hydride battery.

It is controlled remotely, though by the end of 2004 its developers said it will be able to process sensor and sonar data to maintain its position autonomously.Julia Pierce

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