Biomechanics: Replacing a propeller with a flapping fin could help a team of zoologists set a new speed record on the water

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LATER this year a team led by Adrian Thomas, a professor of biomechanics at the University of Oxford’s Department of Zoology, will try to break the human-powered water speed record. The aim is to reach 20 knots (37kph, 23mph) over 100 metres, and beat the existing record of 18.5 knots, set in 1993 by a group from the Massachusetts Institute of Technology. They used a catamaran-style hydrofoil pushed along by a pedal-powered rear-mounted propeller. Dr Thomas’s craft looks more conventional, much like a canoe. But it has a hydrofoil under its bow and instead of being paddled is propelled by an underwater fin which is flapped up and down, in much the same way as a dolphin or whale swims.

The occupant (Dr Thomas will try for the record himself if he is fit enough) uses a pedalling mechanism to drive the fin. As the leg goes through the bottom third of the downward pedal cycle, the fin is driven down, generating the most thrust through the water. The fin goes up in the recovery phase as the leg moves back to the top of the cycle. The action is aided by rubber bands, which smooth out the flapping cycle and act as a suspension system. Getting the craft to ride up onto the hydrofoil in order to pass more efficiently over the water is the hard part. It requires some hefty pedalling.

The fin itself is also being finely tuned into a highly efficient aerofoil shape. The idea, says Dr Thomas, is for the shape of the fin to match the natural flow pattern of the water passing over it, rather than disrupt the flow, as a propeller does. This is where the zoology comes in. Dr Thomas and his colleague Graham Taylor have found that oscillation patterns caused by the fins, wings and tails of flapping animals, such as birds, bats, insects, fish and whales, are similar to one another. They believe this is an example of “convergent evolution”, a process whereby organisms not closely related to one another evolve similar traits independently.

The team reckon that a flapping fin could be better than a propeller—possibly up to 20% more efficient over a broad range of speeds, according to some research. Whereas the whole surface of the fin can be used to produce thrust, not all of the surface of a spinning propeller is available to do that: the hub adds nothing and the tips of the blades are probably turning too fast to do much good.

Animal lessons

A prototype craft has been built for testing and, if all goes well, a series of other versions will be made out of lightweight composite materials for the record attempt. Dr Thomas thinks there might be a market for the flapping craft. Along with Alex Caccia, a business partner, he has set up a company called Animal Dynamics, with the help of Isis Innovation, the university’s commercialisation arm. The idea is to use the understanding of animal movement gained from studies to develop more efficient machines.

The flapping technology, for instance, might be scaled up to propel ships. And if fins were put on the side of a ship they could extract energy from its rolling motion and use that to help drive a propulsion fin at the stern. As the aerofoil shape of the fin is designed to move slowly and efficiently it might also be employed for power generation in slow-moving and shallow water. Such a set-up would not require a huge amount of expensive construction to keep it anchored to a river bed, and being quiet and slow, would allow fish to keep out of its way.

Dr Thomas and his colleagues have other biologically inspired ideas that might be put to work on the land and in the air, too. These range from building a drone that uses flapping wings instead of rapidly spinning rotors to hover. It would, they think, be better able to cope with strong and gusty winds, which can keep some drones grounded. Another idea is to build a multi-legged machine that walks a bit like an ant. It could be used to transport cargo across rough terrain. So, regardless of who has the strongest legs to flap along in the record attempt, there will be a lot potentially riding on this technology.