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Up a creek without paddle? Researchers suggest ‘gunwale bobbing’

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Miles, the son of one of the researchers involved in an international study, demonstrates the concept of bobbing gunwale on a stand-up paddleboard. Credit: G.P. Benham et al, Physical Assessment Liquids (2022). DOI: 10.1103/PhysRevFluids.7.074804

Stand in a canoe and you will likely end up in the water soon. Jump up and down the top edges of the sides of the canoe, and you’ll likely end up in the booze, too. But get the right balance and you can move yourself as much as a meter per second, according to a study published in Physical Assessment Liquids examining the drifting of the gunwale.

In gunwale (pronounced “cannon”), a canoeist bobs on the sides of the canoe — the gunwales — near the rear of the craft and jumps up and down, propelling it forward by surfing the waves produced.

After an initial period of instability before finding its rhythm, the canoe reaches a cruising speed that satisfies a balance between the thrust generated by pushing down into the surface of the waves and the drag caused by drag. The technique also works on stand-up paddleboards.

“People have long enjoyed floating on the gunwale as a fun thing to do on the water, but no one has ever really explained how it works,” said Stephen Morris, professor emeritus in the Department of Physics at the Faculty of Arts and Sciences and co. author of the study describing the phenomenon. “By drawing on long-standing linear wave and oscillation theories, we have discovered how to maintain such a balance.”

Looking for the perfect combination of forward, back, up and down motions, Morris and colleagues from the University of Cambridge, the University of Oxford and the University of Paris created a theoretical model to describe the wavefield produced. They then compared it to data from an enthusiastic gunner float crossing a body of water in a canoe using waves created when the two ends bounce up and down.

In a symmetrical system where a canoeist’s body leans forward and back equally — think of the rhythmic motion of a rower’s body in Olympic competition — the waves generated cause the vessel to oscillate back and forth with no net motion . But in an asymmetrical system, such as that created when a person makes a vertical displacement, such as by jumping up and down out of phase with the slope of one end of the craft, the vehicle gets a net thrust.

The researchers calculated the optimal values ​​of the four movements involved to achieve maximum speed, and using their model determined that the technique could propel a typical canoe at constant speeds of up to about a meter per second — comparable to speeds observed for bobbing gunwale. with real canoes.

“Understanding gunwale bobbing can be helpful in competitive water sports like rowing,” Morris says. “Because rowers generate periodic downward forces, such as gunshot jumps, it may be possible to optimize rowing strokes so that the boat responds to the waves it generates in a way that propels it forward.”

For an energetic workout, take it outside

More information:
Graham P. Benham et al, Bobbing Gunwale, Physical Assessment Liquids (2022). DOI: 10.1103/PhysRevFluids.7.074804

Provided by the University of Toronto

Quote: Up a stream without a paddle? Researchers suggest ‘gunwale bobbing’ (2022, Aug 4), retrieved Aug 4, 2022 from https://phys.org/news/2022-08-creek-gunwale-bobbing.html

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