Reverse engeneering

[Lovre]

Hi,
Some time ago I started a thread about making fiberglass diving fins. Since then I'm looking for the right way to make it. But I haven't go far ...
Maybe some one knows how to achive sort of performance that you can see on this video :

Diving fins video

And some view of what am after you can see at the pictures bellow :


By lovre at 2008-02-15



By lovre at 2008-02-12



By lovre at 2008-02-12


By lovre at 2008-02-15

[JRL]

Flexible vinyl comes to mind. Same stuff used on carbon dress up kits for car interiors. Only manufactured with some type of vacuum/pressure set up.

[Petros]

I once many years ago worked for one of the larger diving equipment manufacturer as a design engineer. I had designed a new fin before I came there, it one of the reasons they hired me (my name appears on the patent). It was the best design at the time (about 1986), but poor alterations to the design done by others at the company resulted in only modest improvements in performance. Soon I got fired over it and then my employer sued me to sign over the patent to them (thanks a lot).

In investigating the design I came to the conclusion that most fin designers do not have a clue as to how swim fins generate thrust. It is a complex interaction of flow and surface, most fins mostly only generate turbulence and drag. The modern designs do not impress me either, whatever improvements were gained by accident IMO. It is pretty clear to me they are gaining any thrust improvement with vortex generated lift. Not very efficient, but effective.

I had actually made several fins with ABS sheet and solvent glue that generated so much thrust a full ankle high boot-like "foot" is required to keep them under control. I keep thinking someday I may take up my interest in swim fins and actually produce one that improves thrust and efficiency quantum leaps ahead of anything available. Because the market is controlled a few big players I likely would lose money on it again (like I did the first time). So I am gun shy about attempting it again.

The carbon graphite pattern is only a marketing gimmick, the reason to carbon graphite is to save weight. On a fin used underwater how much difference can that make? You can control the stiffness of any material with thickness or flexible fiberglass inserts. I would think what you want is a shape that does not distort under load, but still maintains an good angle of attack to the flow direction around the surface. There are a number of ways to achieve that with various materials. I thought of making some test samples with laminated wood frames with fabric covers. A lot easier to make and test than fiberglass or carbon. Mass Production would best be done by injection molding of course.

But I would not waste time copying other people's bad designs. looking at the video indicates to me that there is a lot of lost energy, there is indications of flutter (unstable flow over the fins), and as stated above a lot of lost energy (wasted effort). More efficient fins can be smaller, will use less oxygen from the user, allowing longer, deeper dives, or allowing a SCUBA diver to stay down longer (they would use their air up slower).

[Landlubber]

Petros know what he is talking about. There is also a large amount of body oxygen used up in trying large fins, the leg muscles work way too hard trying to maintain constant work effort. Smaller ones certainly are the go for improved overall efficiency. I have tried just about every fin available in Australia, I used to own a dive shop, so trying them was no expense, the largest were actually the worse overall, yet did have enormous power sometimes, the end result though was lost time through massive oxygen consumption, both freediving and on tanks.

I am sure there is a market for a technically correct fin. There are many rec divers out there today, and they seem happy spending money on dive gear.
Look at all the Cucci suits today, black with a yummy yellow stripe has long disappeared, I would not be too afraid to come up with a marketable, technically designed , comfortable, Powerful dive fin, as long as the in water testing was completed. Go for it.

[Lovre]

Thanks guys for your comments.

I'm talking about freediving/spearfishing fins - long blade is essential. I tried a lot of plastic, fiberglass and fiberglass-carbon diving fins and I must say that carbon really pushes you best, and if its not too stiff, saves you a lot of oxygen.

Problem I have is to determine what cloth to use - should it be unidirectional inside maybe ? What g/m2 and how many layers to use not to make it to stiff ?
Sea water resistance - which resin ? Which resin is good for its flexibility ? Can any epoxy-carbon laminate be bent like blade on second picture - of curse it can't thats the problem I want to solve.

The advantage of carbon on other materials is not that its light - but that it has almost zero hysteresis (plastic deformation) - meaning that it gives back almost all energy you put in by kicking your legs.

[Petros]

Originally Posted by Lovre

Thanks guys for your comments.


The advantage of carbon on other materials is not that its light - but that it has almost zero hysteresis (plastic deformation) - meaning that it gives back almost all energy you put in by kicking your legs.

So does metal, how about stainless steel blades?

[Lovre]

Originally Posted by Petros

So does metal, how about stainless steel blades?

Metal ?????
Metal has a lot of plastic deformation. If you bend metal it will not go back to its original shape but will stay bent. You mean that its hard to bent it maybe, but it has nothing to do with carbon being good choice as blade material.

[Petros]

Originally Posted by Lovre

Metal ?????
Metal has a lot of plastic deformation. If you bend metal it will not go back to its original shape but will stay bent. You mean that its hard to bent it maybe, but it has nothing to do with carbon being good choice as blade material.

What are you talking about? This is simply not true. You are incorrect. The metal springs in your car keep from bottoming out for many many thousands of impacts over many years, including the flat leaf springs in most trucks and older cars and trucks, metal springs close the valves in almost all internal combustion engines for trillions of cycles, the metal springs in the old mechannical watches ran for many years and years, the tiny metal springs in your keyboard keep popping those keys back up despite much use, abuse and dirt and dust.

A thin metal blade will reliably flex in a swim fin without plastic deformation if made of the proper metal and not overloaded. Even so you can always bend it back even if you do overload it (though it will not look as nice after that).

There are very few things that are as reliable, inexpensive and has been pressed into service as a spring than metals. Some metals are better than others, but ALL metals (even lead) will spring back to the original shape almost exactly as long as you stay within the elastic limit of the metal. That is the way all springs work. If you over stress the metal (past the "yield" point) than yes you will get plastic (permanent) deformation. But it will still spring back less the amount that the metal was permanatly bent, and it will still be almost as strong as the unbent metal.

All solid materials behave in this way, though some do not flex much (like glass or concrete) before failure, but all solid materials have their elastic limits before yielding into the plastic deformation mode or into failure (since concrete and glass has no capacity to plasticly deform). Even wood can make a good spring, ever seen a wooden archery bow? Been around for a looooog time.

There have made some carbon graphite springs for certain applications (mostly aerospace) that have also been used in a few consumer products (golf club shafts, fishing poles, etc.) but this is expensive and has a fatigue limit (life) that is lower than most metal springs. And if over loaded it simply fails, there is little to no plastic deformation.

You clearly do not know what you are talking about if you think metals only plasticly deform. If all of the examples all around you do not convince you than you can go look up any first year engineering text book on mechanics of materials.

Metal would make a fine swim fin blade, as long as it is thin enough and made of corrosion resistant metal.

[Lovre]

Ok. You are completely right. Making fins from metal. Bravo

Anyway, I don't think that you had any intention of helping But thanks anyway

[Petros]

I would be happy to offer help but you did not ask what design I think would be better.

If you are intent on copying someone else's bad design go ahead. I would use bi-directional material with epoxy resin. Polyester resin is too brittle and would crack with constant flexing. If you can measure the thinkness from the fins you like that will give you an idea of how think to make the blade. The same thickness from similar material will have similar flex properties.

Good luck

[Lovre]

Thank you.

But of course you can offer me advice on making something better. If you have some ideas you are free to share them,if not nevermind - you helped me already.