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Fin Design

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Deeper Blue Hypoxyphiliac
Oct 24, 2002
Has anyone read, or does anyone know of, any resources related to fin design? By this, I mean things like different design bends, different shapes and what they do, etc etc. Cheers in advance :)
yes, kind of

When I was researching yatch design i stumbled across a hydrodynamics book that explaned at great length fins shapes, designs comp. It was awild mix of biologic studies with quandaries as to weather or not a pair of massive fins would be better than a prop for ocean liners.
In short, if it exists in Italian you can rest assured it will have been translated. The author was Doctor Matteo Infelice. Good luck finding it. If you can't find a trans. of his publication look into naval theory or hydrodynamic studies/theory at you local unversity. This book helped me with all of my designs for fins
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Reactions: loopy
yes my own

Yes they are super ugly but work very well, but if you don't kick and stomp hard they start to make a small vibration. I don't use them often because they are assymetrical and that is just too hard on all leg joints. I use them when I want to move super fast.

Hey do yo know if anyone makes a earth tone(read brown, tan or natural rubber) foot pocket. My next project is to take some vertical grain clear veneers and laminate them in fibreglass. I want blades for converstional piece mounted on my wall with my collection of antique Italian, French, and Spanish guns. I have a wild Pirelli mask too that is ancient.
Sorry, no idea... maybe just paint some other ones?

Do you have any pictures of your fins? What material did you use to make them? How efficient are they compared to normal fins, ie can you do longer distances easier, or are they just for speed? Cheers :D

I made these fins to be stiff. I'm lousy with computers, don't have a scanner or digital camera. I live on an island that doesn't have a Kinkos(I don't know if you have them in Oz but theyare a copier media franchise). Sorry no photo.
I can over power a Cressi HF, Beuchat Elite, and can really make C4 80s work hard for me, so I thought that I needed a harder fin. Wrong man was I wrong. I learned that no matter how strong your legs are they will just start to comsume mad ammounts of O2 if you strap planks on.
I use a vacuum mould and manifold to make my resined bits and pieces. I scored the manifold from some Boeing surplus. Kevlar is my favourite material for now. I suspect that the Waterway is kevlar, kevlar is nearly always yellowish and I think that it would be an odd coincidence for the fibre to be yellow and not be kevlar. My next trick will be to use carbonfibre strand and triaxial kevlar or fibreglass. Epoxy is my preferance polyester is harder to manipulate for elasticity. With epoxy you can play with the hardener, cure temp and time to achieve the thicsaltropic, amorphis solid state that you desire. ie you can play with it to make it more flexy or springy.
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Nice - over here in Aus there's a uni (Aus National Uni - www.anu.edu.au) that creates carbon nanotube fibres, which are much stiffer/stronger than carbon fibre. I think because of this, they may be more 'springier' than carbons, but due to the cost (around $US1000/gram) I don't see any fins coming out of them soon :)

The other amterial I was thinking about, which guys at the uni are going crazy at the moment, is epoxy reinforced glass microspheres... it's fairly cheap, has good compression resistance, and when it's in tension it relies on the epoxy to do the work... might be useful for fins?
I think that the best stiffness (bending line) of a monofin can be found after invention of a new kind of footpocket where the foot doesn't move in a rubbershoe.
All the power will then go straight from the body into the blade.

Very important, I think, is also the angle between the feet and the blade, it has to be made so that the blade is "in line" with the body.

Now I think you have to compensate the blade to beeing stiffer because of that the feet moves 0,5-2 cm up and down when you swim.

I have a partner that is a specialist in laminating carbonfibre, glassfibre etc. (he also works on the Porsche Koenigsegges carbondetails) and we are trying different solutions in laminations, different foot attachments etc.

Maybe I can learn something from this.:)
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Pete, in thermodynamics, you can plot out a process (say the 4 step cycle of a 4 stroke engine) on a graph. The area under that graph is known as the hysterisis loop, and shows you how much energy you've lost performing the process. The same idea can be taken into anything that involves energy (hysterisis loops are pretty popular in electrical engineering) - say the transmission of power from foot to fin. The main aim of efficient fin design would be to minimise this loop.

Your idea of foot pockets would do exactly this - aim to achieve a minimum energy loss. There's other techniques as well - a pretty popular one is the use of rails to prevent vortex transmission and reduce drag. I've never seen a mono with a big rail, but this is something you might want to consider in your design.

As for placing the blade inline with the body, I totally agree - I've drawn a few mono/bi fin sketches in the last few days and every one is inline. The only blades I can think I've seen do this off the top of my head are C4's.

Ideally, you want to get maximum thrust with minimum energy expenditure. I crunched a few ideas through the equations, and the way I see it, the ideal fin shape is entirely dependant on the swimmers stroke, and their fitness level; The fitness of the swimmer will give the allowable resistance, and their stroke will give the ideal bend to minimise the hysterisis loop.

Anyways, that's a bit of food for thought - let us know how your fin making goes :)
Thanks for the input, Loopy!

If you look at SEBAK'S freediving monos, the blade has the same angle as the C4!

A guy from Finland and me have both helped SEBAK in their development of this fins and they perform well I think.
(see, www.sebakfins.com model "Teppo" and "Peter")

I have also put rails on one of my fins (WW model 1 medium) with good results so I agree with you here!

Here is what I wan't to do (in order as follows).

1. Foot attachment
2. Blade optimization
3. Hydrodynamic improvements

It really fun anyway, we will see what comes out of it.;)
If you're really interested in hydrodynamic improvements, check out some of the new aerodynamics add ons to modern planes, like winglets and wing fences. (See http://members.tripod.com/~aravm98/reference/727wing05.htm for a picture of a wing fence - they stop vortex flow across the wing).

Not sure about how that would go (that is wing fences) with regards to skin friction drag - it would reduce it byy inducing turbulence, but lemme think a while to see if that's what you want to do.
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I think my partner mentioned a software he had to do calculations on hydrodynamics.

Thanks again for input, Loopy. :)

Any thougts are welcome!
Just thinking about that skin friction drag thing... two points. First up, aerodynamically (and consequently hydrodynamically), carbon fibre is considered 'slippery', that is, it has a very low coefficient of skin friction drag. I see this as being a good thing in the water, because it allows the water to flow over the fin much faster and with less resistance.

If we extend this analogy, the addition of small jagged fences on either side of the fin (like on the top of the shark skin suit) will dramatically reduce the skin friction drag by inducing a small ammount of turbulence. Now I *think* this would be a good thing, but when it comes to turbulence, the only real way to do it is build it and trial it. Computational models are fairly accurate, but turbulence is unique - look at something as simple as dragonfly wings... according to physics, the dragonfly can't fly, but no one ever told it that :) Another trick to induce a small ammount of turbulence is to make your fin blade bumpy, like the dimples on a golf ball.

Anyways, let me know how it goes :)
I have been using scuba pro twin jets...read patent info and found 3 major areas of foil design violated by tj's and all other fins, to boot...first, when a fin flexes under power, the fin shape is the opposite of what hydrofoil design demands...I have designed a unique set of symetrical pivoting rigid foils with flexible foil surfaces which I call sails...my goal is twice the speed with half the effort of standard fins and so far the results look promising...

ALSO...while trying different ways to make the twinjet fins work better, I discovered a way to increase my speed and decrease my effort with ANY fins...MAKE A BOW...I simply extended my arms fully in the direction of travel, palms flat and facing each other, and touched my fingertips together to make a ship's bow in front of my body...WOW...immediate significant increase in speed !!! :) :) :)
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>I have designed a unique set of symetrical pivoting rigid foils

Repeating myself here but, I'm very interested in your design. An engineer in S California marketed a device about thirty years ago that employed a symetrical rigid foil. It was called 'Aqueon' or something close. It looked strange but any sort of dolphin kick worked and the speed increase was remarkable compared to the old jet fins. He did some formal efficiency tests that showed a major increase too.
Bill..what do you want to know? the fins have pivoting rigid frames with fabric foils, so that with each up or down stroke, foils form in the other direction in the correct shape to develop greater lift and in a more forward direction...the foil frames pivot approx 40% aft of leading edge of foil, and movement is limited by a length of line attatched to the toe section of the footbed and the leading edge of the foil frame...pivoting also allows for free and rapid development of lift by allowing lift vortex to develop, uninhibited by footbed proximity, at the leading edge of the foil assy (I am up on the latest Boeing info on lift development in foils)...foil frames provide air dams (water dams?) at lateral edges of flexible foils to decrease loss of fluid off of foil with resultant loss of lift/forward driving force...each foil has 96 square inches of surface, about the same as my scubapro twinjets...fins can be made of any rigid quiet material (wood, frp/Zytel, rubber-covered aluminum...)...foil/sail can be made of any tightly-woven fabric such as kite material (nylon), or plastic sheet such as visqueen, duct taped ...sail has sleeve at leading and trailing edges to accept foil frame members...
Thanks for the info. I have read very little about hydrofoils and boat design but it is close enough to aerodynamics to understand what you are doing. For a long time I've felt that there was lots of room for improvement in fins. Keep us informed on your progress, please.
best wishes
now make 'em in a 50-52 for use with 7 mil boots and we're smooth! :cool:

I just got done doing my bathtub statics, and was reading in this month's SCUBA magazine, (they should call it the Belize chambe of Commerce meeting minutes...) where an article looking at the future of fin design was pretty stalled, yet small thrusters were on the horizon.

Yeah. OK. :hmm The gills-in-a-T-shirt was pretty sexy though.
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