Bionics in freediving? (curious!)

[e.m.p!]

Hi Guys,

I've been reading through the other thread dealing with the folding of Monofins... Well, I'm far from being an expert (just started freediving recently and happy with my Bifins), but one thing always crossed my mind: As a student I visited quite a bit of lectures and exhibitions about bionics and biomechanics, and what I found out is that i.e. a Penguin has the best hydrodynamics (yes, better than a shark), a shark has the best friction values (due to it's rough skin: waterdroplets are catching in it and result in less friction to the surrounding water) and the tuna is one of the fastest fish around (up to 70km/h) due to its (guess what) fin design.
I'm sure a lot of people already experimented with it and there are more factors to this fish then just the shape of the fin, but why isn't it a common design in freediving? Nature isn't a bad inventor and the tuna is not the only species in the sea reaching top speeds, other fast fish have a similar design.
I guess the folding problem wouldn't occur anymore, thou the fin may need to be a bit bigger than the usual Monofins. Any thoughts on that?

BTW: if someone needs a test pilot for this new fin, give me a call!

[gingko]

Long time ago, I've read something about the speed motion of the tuna : it seems to depend of the Reverse Karman Vortex Street. The RKVS produces something like a jet-stream, but it needs a quick oscillation of the tuna fin (something like 5 to 10Hz, and that's impossible to do it with a fin by a human).

I've found a sort of abstract from the MIT web site :
http://web.mit.edu/ahtechet/www/CMI/CMIpt1by2.pdf

.../...

[trux]

Well, the rigid caudal fin of tuna is in fact a hydrofoil. It is a low profile, very thin, high width-to-lenght ratio hydrofoil - these work best at high speeds becaue they do not need to flex - changing the angle of attack at the end of the tail movement, hence having practically no loss of energy at the dead points of the "kick" amplitude. The low profile of the hydrofoil (its small thickness) reduces the forward drag, and the high width to length ratio reduces parasitic induced drag (inavoidable at hydrofoils and wings) - the drag reduction is especially important at high speeds.

Low profile hydrofoil (or tuna's tail) when working at high speed, have extremely low amplitude of "kick". Very small movememnts are sufficient for effective propulison without changing the angle of attack. The problem is, such fin is not very efficient at low speed for acceleration. You need to flex the tail at low speed to change the angle of attack. Tuna uses also the entire body for the propulsion at low speeds. And since human, thanks to the body anatomy cannot reach the speeds of tuna, where the rigid fix-angle hydrofoil is the most effective; does not have tail that can change the angle of attack as needed depending on the speed; and does not have body helping with the propulsion, simply attaching a tuna fin to a human won't really work.

However, there are attempts to address both problems - using high ratio hydrofoil, and coping with the necessity of variable angle of attack. The best examples are probably the Aqueon and the Lunocet. There were also other attempts to use tuna shaped fin - for example the Mor-Fins. You can find all of these inventions and many more in my Collection of Weird Fins, and there are several very interesting threads about them here on DB too. For example these ones (use the search function to get many more):

http://forums.deeperblue.net/monofin...st-reason.html
http://forums.deeperblue.net/monofin...odynamics.html
http://forums.deeperblue.net/bi-fins...eird-fins.html
http://forums.deeperblue.net/freediv...last-past.html

2_G.jpg lunotec.png lunocet-new.gif morfin.png

[Waterenthusiast]

I just wanted to add the remark that Bionic is not about "copying" nature, in the way of attaching a Tuna fin to a human body, but the analysis of function and mechanisms that natural models are displaying and trying to transpose them into a model adapted to our own characteristics.
Greetings from the lab,
Christophe

[e.m.p!]

Uh yes, thanks trux, I checked out your collection before but didn't think about it before. Thanks guys that you've shed some light on that and for the link collection. That was a really interesting read, but still think we haven't reached the end of the evolution yet. Just have a look into the development in thermoplastics and such, I wonder where all this ends (if). I remember seeing an experiment simulating a Tuna in motion, the whole setup looked rather complex and way too complicated to be mimicked by humans. Think it was conducted in the flow channel of the TU Berlin.... (not sure thou). Bottomline: As you guys mentioned before there is more to it than just the findesign.
Anyway, this thread is about bionics in freediving, so I hope to have started a discussion not only dealing with fin designs but with other nature inspired gear as well?! Keep it coming!
P.S:
Christophe, are you working for this lab that tests all the new sport gadgets people are trying to get a TÜV/GS certificate for? Or is this one in Berlin? Schöne Grüsse aus Neuseeland!

[Waterenthusiast]

Originally Posted by e.m.p!

Christophe, are you working for this lab that tests all the new sport gadgets people are trying to get a TÜV/GS certificate for? Or is this one in Berlin? Schöne Grüsse aus Neuseeland!

I wish i would , no actually i'm working in a completly different field (Plant phytochrom systems), but during my biology studies i did several practical courses at the plant biomechanical and biomimetics group lead by Professor Speck at our university in Freiburg.
Off to training,
Christophe