'RoboClam' could anchor submarines

A new burrowing robot for anchoring miniature submarines has been developed - inspired by the humble razor clam.

"RoboClam" could be used to lay undersea cables, and potentially even destroy mines, its inventors say.

The device mimics the digging action used by razor clams to turn solid soil into liquid "quicksand", helping them slide through.

A prototype is described in the journal Bioinspiration and Biomimetics by engineers from MIT in Boston, US.

They set out to design a new low-power, light-weight anchor for autonomous underwater vehicles.

"Luckily, nature had already done the work for us," said Dr Kerstin Nordstrom, of the University of Maryland, who collaborated on the research.

The answer was poking out of mudflats off the coast at nearby Gloucester, MA.

The Atlantic razor clam, Ensis directus, has been dubbed "the Ferrari of underwater diggers".

An animal of its modest frame (10-20cm) should only be strong enough to penetrate 2cm into packed sand. But it can burrow up to 70cm in just over a minute.

Compared to existing anchor technology "the razor clam is about 10 times more efficient," Dr Nordstrom told the BBC's Science in Action.

To dig for half a kilometre, it would only use the energy in an AA battery.

"But when you try plunging the shell into the sand, it doesn't actually penetrate very far," said Dr Nordstrom.

"What this shows is the clam must be actively doing something to the ground when it digs."

To find out the razor clam's secret, they studied its digging action and modelled it mechanically.

The repeated open-shut of the clam's valves turned the hard-packed soil around it into quicksand.

"The clam's trick is to move its shells in such a way as to liquefy the soil around its body, reducing the drag acting upon it," said Amos Winter, of MIT's Department of Mechanical Engineering.

"Pushing through sand costs a lot of energy. But if the sand is excited, it's actually very easy. That's the trick," added Dr Nordstrom.

By mimicking the action of the razor clam, they built their own robotic prototype - which has achieved the same digging speed - about 1cm per second.

The first "RoboClam" can only reach 20cm, and requires a significant rig of machinery to propel it.

But having demonstrated the principle, the team now aims to develop a larger, self-contained unit, that can burrow more than 10 metres.

This could be used to anchor larger vessels, and may have military applications - such as detonating mines, the researchers suggest.

"The cool thing is this technology is already 10 times more efficient than any anchor. If we can keep scaling things up, some day it will affect big boats," said Dr Nordstrom.

"Also - undersea cable installation is happening more and more frequently. If we can do it more efficiently we can save costs and cause less disturbance to the environment," she said.

Amos Winter agrees: "Having a system that could just latch onto the cable, work its way along, and automatically dig it into the soil would be great," he said.