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Propeller-free robots may soon be swimming with style. A new model of how fish and other aquatic species are able to propel themselves forward without expending much energy may help create energy-efficient underwater robots that swim just like the real thing.

By closely studying and monitoring how fish, dolphins and other sea creatures swim, Mehdi Saadat at Harvard University and his colleagues found that many of them have remarkably similar styles that can be described with a simple model, depending on how fast and far the tail whips back and forth and the length of the animal.

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Scientists previously homed in on just one parameter that relates the beat of an oscillating fish’s tail and how far it oscillates to the animal’s forward motion. But it turned out to be slightly more complicated.


Saadat and his colleagues identified a second important factor: how far the tail goes to and fro relative to the fish’s length. They found that almost all fish – and many other sea creatures – swim in a narrow optimum range of this parameter to generate thrust, with the length of each tailbeat between one and three-tenths the length of the animal.

One for all

“This gives you the right ballpark. Even for different fish shapes and gaits, if you’re in there, you are cruising at a minimum power input,” says Mattia Gazzola at the University of Illinois at Urbana-Champaign.

The researchers developed the model’s parameters by carefully monitoring trout cruising in a tank in their lab. In the tank, water flows in a circle to allow the animal to remain in one place while swimming upstream, like a fish treadmill. The team also tested a robotic system emulating a fish tail to make measurements of how much power it used as it swam.

In addition, the researchers used previously published data on a variety of other sea life, including mackerel, blacktip sharks, dolphins and killer whales, observing that they consistently swim in line with the parameters they found in trout in the lab.

“It’s amazing that so many differently shaped and different kinds of aquatic organisms fit the same general rules,” says George Lauder at Harvard University.

Saadat believes these rules could be used to aid the design of power-efficient aquatic robots. Such robots could, for example, explore the sea floor or map coral reefs without disturbing wildlife, and the US Navy is interested in stealthy robots that move without a propeller.

Journal reference: Physical Review Fluids, DOI: 10.1103/PhysRevFluids.2.083102

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