Humans, and the machines we make, are particularly unwieldy swimmers. Many types of fish can travel at speeds of up to 10 body lengths per second. By contrast, an Olympic swimmer manages no more than about one body length per second. And while nuclear submarines can travel at about 80 kilometres per hour, that translates into much less than half a body length per second.
This kind of performance has long fascinated marine engineers who would dearly love to match it. In particular, they have puzzled over the way certain sea creatures can accelerate from a standing start at rates of up to 120 metres per second squared. That’s 10g!
Among the creatures that demonstrate this extraordinary fast-start acceleration are the octopus, squid and certain jellyfish. Marine biologists have long observed these creatures’ ability to avoid prey by filling their bodies with water and squirting it out to generate propulsion. In this way, the octopus, for example, can accelerate at more than 10 body lengths per second squared.
hat gave Gabriel Weymouth at the University of Southampton and few pals an idea. Why not build their own robotic octopus capable of squirting in the same way and seeing whether it can match its natural cousins’ performance. The results are something of a surprise.
Their underwater robot is essentially a copy of an octopus’s head, which expands when it is filled with water and squirts it out through a nozzle at the rear. This robot has no tentacles but instead, the team fitted small fins to the rear to stabilise its movement through the water.
So the robot is roughly ellipsoidal in shape, measuring 27 centimetres in length and is 5 times longer than it is wide. It is created by stretching a rubber membrane over a set of polycarbonate ribs. Weymouth and co chose this shape because of its ability to glide many body lengths through water.
The team then filled the robot with water under pressure, causing the rubber membrane to expand into a bluff body shape that is particularly unsuited to gliding. Finally, they released the robot in a pool and filmed its progress through the water at a rate of 150 frames per second as water squirting through a rear-facing nozzle propelled it along.
The results are extraordinary. As the water starts to squirt out of the nozzle, the robot begins to accelerate. However, it moves little during the first half second or so since its bluff body shape prevents efficient progress.
But then, as the robot shrinks, it accelerates rapidly. And here’s the thing: Weymouth and co say that its maximum acceleration is 14 body lengths per second squared and that it reaches speeds of 10 body lengths per second.
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