Flapping fin propellor - University of Oxford

From the Economist Magazine - Sept 5th edition:

Flapping about

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.

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.

*** end of article ****

I can see all kinds of apples being thrown at this article. But if might be interesting to revisit this attempt in few months and see if it worked in any manner. I am not a physics expert, but I aside from mechanical complexity, I am not sure it would be more efficient, even at low speeds. Anyway, an interesting concept.

 

Amazing that such a reputable mag would make such a complete hash of describing a boat. Dr Thomas must have wanted to eat his hat when he read that. Good luck to you, Dr Thomas, whatever it is you are doing.

PS. If you put the flapper in front of the foil you can try to exploit the Katzmayr Effect.

https://ia800303.us.archive.org/1/items/nasa_techdoc_19930081006/19930081006.pdf

 

I hear the same thing about The Economist whenever anyone 1/2 knowledgeable about the subject at hand reads an article. Hearst's Law: "When reading news story about which you have 1st hand knowledge, you will think it is a completely different event which coincidentally happened about the same time or place".


I want to make a flapper Mermaid suit. It would be driven by the body's most powerful muscle groups in their natural range of motion (straight up and down of feet about 1ft range of motion. Keeping streamlined would be a challenge. I'm thinking a spandex cover over the legs and mechanism, with air bladders so that less mass and viscosity would allow it to change shape without too much energy loss, and the air bladders would allow the mechanism to be heavier while maintaining neutral buoyancy.

I guess the air bladders could also be used to extend the underwater time of the Mermaid.

 

Mermaid tails are available: https://www.swimtails.com/ http://www.finfunmermaid.com/
Mermaid swimming lessons are also available: https://www.swimtails.com/swim-like-a-mermaid/ http://www.worldofswimming.org/mermaid-school

 

Not just tails!

 

Nice, but like I said I'd want to use a diff set of muscles, and run it through gearing.

At $49 + $19 for custom size those are a bargain, and would be worth it just as a starting point.

 

Folks here at the marina toss out top dollar swim fins with broken heel straps. I've collected 5 pair from the trash. Plan a fin propelled dinghy/tender for my main boat project once completed. Arm muscles powered.

 

This is an electro polymer plastic robot ray swimming.
http://wn.com/festo_aqua_ray

 

That is quite cool.

 

I was reading an article yesterday or the day before in a marine magazine - it briefly described a "future" highly efficient cargo ship from a study that used a fin instead of a propeller. It didn't go into much detail though, but I suppose there must actually be some basis to it all (perhaps with many caveats and problems to overcome) for separate studies to be coming to similar conclusions...

If I can find the magazine tomorrow I'll post the name and issue.

 

I don't know why, but flow dynamic studies seems to fall into the biometric, biomechanical, let's see if we can mimic nature routine, on a surprisingly steady basis. It rarely pans out, but some of the stuff shows interesting data, if viewed through a narrow focal range. Hell, I even played with HumpBack whale fin, semi rigid lead edge sails, a few summers ago.

I remember all the rage several decades ago, when sailboat appendages took on the "shark fin" plan forms, then of course is the elliptical plan form for wings that the SpitFire was so noted for, supposedly mimicking bird wings, blaa, blaa, blaa . . .

In reality, we've taken cues from nature, possibly saving some research time and after testing, found we need to change things, to further improve (whatever). The elliptical plan form is a classic example. Show me one modern piston driven aircraft that employs them now. Why not, well we tested it and yeah the elliptical plan form does produce less tip vortice turbulence, but at a loss of a significant lift area. So, we accept the increased tip turbulence, employ a squared tip and gain more lift, for the same span, in spite of the extra drag.

Another example is the HumpBack fin leading edge. Yep, it works, but only at very specific and a fairly narrow Fr range, so it's application is limited. This said, wind power generators are looking into this approuch, because of their relatively narrow operation range, but in a sailboat, particularly performance oriented, not so much.

As to the flapper or fin propulsion, well I think this will see the same accelerated evolution, that human testing can offer over mother nature. Maybe in another million years, bird's wings will be more squared off and shark fins will have a higher aspect ratio, but we can speed up the process, which is why we start with hints from nature and it morphs into something more efficient, looking nothing like the original concept or beast we blatantly stole it from. So, don't hold your breath on the flapper thingie driving cargo ships. It'll like change to a underwater pulse jet, maybe stealing ideas from jellyfish.

 

I'm not.
There was nothing more than the briefest mention of it (perhaps less than a sentence - I don't remember).

I simply find it fascinating that, even if it's only within a narrow set of parameters, a mechanical fin could be more efficient than a propeller - it's not something I'd ever have guessed, despite what we see in nature.

 

Bird wings and fins aren't the only design from nature worth studying.


http://china-girl.org/William Thomas/Dragon Quest Pt1.htm

"A Chinese selling powerboats in Aberdeen told me that many of these trading junks are 80 and 100 years old. I can well believe it. Gaping, paint-starved planking and sails that look like a kicked-in picket fence are signs of significant entropy. Just add teredos - for which this coast is justifiably proud - and the whole rickety assemblage of an authentic China trader looks more precarious than promising.

Until you try to catch one.

Now, Celerity is no sluggard. Even loaded with cruising gear, the gives us eight knows to windward in a fair breeze. We usually needed that and more to overhaul junks setting out for home on the wings of the winter monsoon.

We could outpoint them - just. Our trimaran tacks through 90 degrees; the junks we pursued managed to get around within 100 degrees, This is a weatherly as many cruising ketches, and far superior to the 130-degree tacking angle of square-riggers at the height of their development.

Leeway was noticeable close alongshore - where we often found ourselves trying to coax capricious winds around an obstructing headland. Celerity's shoal-draft provided an identical handicap. In neither case would I call this leeway excessive - certainly no more than that of the old gaffer I once took through Canada's Gulf Islands.

Though somewhat slow in stays, an 80-foot junk tacks at least as quickly as the brigantine that later carried me to the Philippines"

"The engineless sailing junks we later ranged alongside carried no structure above their raised poops. All showed an exceptionally clean wake. Here again, the Chinese found the key to reducing water resistance millennia before the West got over its fish fixation. Up until the late 16th century, European naval architects placed the master-couple - the cross-section of maximum beam - forward of amidships. They thought that ships could swim as well as fish if their hulls were shaped like a cod's head, with a mackerel tail.

But a ship lives on top of the water, in a turbulent interface of waves and wind. So the Chinese placed the master-couple aft of amidships, like a duck. Not only is this a more effective shape for moving across the water at all but very high speeds, but in foul weather the broad stern sections of the duck and the junk help keep both creatures weathercocked into advancing seas. Tailfeathers - and high poops - help, too. "


Let me think. Aha! Duck Feet propulsion!
Two tracked paddle wheels might be close to a duck's swimming. Like side by side pair of snowmobile tracks with cupped blades protruding.

 

Really, caterpillar drive?

 

I wondered if there were any examples of this stuff commercially available, and found this Hobie kayak, propelled by pedal driven fins. Supposedly more speed for less effort than paddling.

For anyone curious: https://www.youtube.com/watch?v=lD6OQhCeXqs

Very interesting to watch, especially the control the user has from around 0:48.