Split Fin Physics

[Bob Evans]

Blackwood, You like to frog kick? Well I have the frog kicker fin for you. My new hockey fin cast in this super duper poly-urethane material. I just got home from visiting my Father in Redondo Beach and would have dropped off a test sample for you to try at Dive N' Surf. P.M., me since I will down next week. Thanks for starting this post.

 

[mdb]

Blackwood:

You started a very good conversation asking about the physics of split fins. It seems the thread has now morphed into all fin styles; that is OK, as the discussion is very interesting.

We have some very good contributors with a lot of intelligent input. This talk should help new and old divers when they are considering fin choices. Well Done.

 

[Bob Evans]

Blackwater, Yes, Split fins flat or curved that is one issue. A old review from Spain, but helps direct thoughts on the issue of Split fin designs.Inmersion Features Foil Force Fins....

 

[lmorin]

It is far more difficult than one would think.. as there is not "one" kick...and we have a 3 axis motor, with very large differences in extension and contraction for each. That ends up being the failure of most studies... it becomes depend on the human doing the kicking and what may work well for one fin doesn not necessarily work with others.

It would be possible but costly and time consuming to build and then test all of the possible combinations. And it would have to be done with a consideration for human cycle rate efficiencies. Those are well known for runners and bicycliest, but not for the other half of the muscles being used.

If you first tested humans and determined the right ratio of force that should be used for all the muscle groups... and knew the ideal cycle rate.. and had force gauges to read how a specific stroke pattern caused with a specific fin... then you could take all the fins and test them and compare results and in the end you would still not know if a better fin could be made.

Studies like the one referenced in this thread end up being meaningless to some extent because they only measure what was being done, not what should have been done and are not measuring the capabilites of fins.

You have missed the point. See Henrik's excellent post above. He has addressed your issues.

 

[mdb]

Imorin: Henrik's posts are very interesting and informed. Puffer Fish, and others, also have good thoughts

When we were developing the bio-fin all sorts of parameters were in play, the best design, the angle of the fin, the materials used, length, width, size of the leading outside edge, placement of the inside trailing edge, etc.

Many tests were conducted with actual divers in full SCUBA gear in Japan, the USA, Australia, NZ, and in the UK. Of course divers have different kick styles, endurance, and all that. There are many variables when humans are test subjects.

We also invested in a human size dive robot, so we could have repeatable data based on fin angle, knee bend, ankle bend, kick frequency, etc. The combination of these test methods resulted in the bio-fin design.

Some manufacturers simply choose to copy our style, different materials, but very similar design, others added their own twist, again using different materials. The result is we now have a wide variety of split fins on the market. Some perform quite well.

All fins are not created equal. Choosing the right model is an important choice for a diver. Civil discussions are very helpful and informative. This thread has seen a lot of good input from many perspectives.

 

[John C. Ratliff]

I did some more underwater swimming yesterday, this time on scuba. I'll have more on that soon, probably this evening. But I've found out that my 64 year old body isn't as forgiving as it used to be for these kinds of trails

I would like to say a few words about my other swim analysis that I published on page 12, post #118 of this thread. I took three identical pairs of swim fins (Mares Plana Plus) and modified two of them to different configurations of the same blade (split and scoop). I then tested the three different configurations of the same blade against themselves. What I was testing was concepts, and not different manufacturer's fins.

My swims yesterday tested different manufacturer's fins, and I was able to use an Apollo full-foot split fin too. I'll discuss these swims later.

Mention was made that this is about split fin physics, and not about monofins. Well, there were a lot of ideas and discussions about wings and vortices being bantered about, and because the monofins showed actual vortices by the swimmer's exhalation bubbles being pulverized by the monofins as it passed through their bubble stream, I felt it an accurate representation of fin dynamics. It did show significant thrust on the upstroke too. Some posters had stated that there was "no" thrust on upstroke, and I used this to illustrate that this is not true. I just wanted to make that point clear. And, by the way, what do you think of 100 meters swum in 41.51 seconds (and that was the women's race!)?

Enjoy,

SeaRat

 

[aikiguy]

Having read through the forum here and being in the market for another set of new fins, (my three dive old split fins, Tusa xpert zoom fins don't seem to be doing it for me), we still don't seem to have any scientific data, only marketing claims, and pictures that do not show any measurable understanding even of science.

We as humans are biased to support in argument those positions that we have already taken especially those in which we have a financial interest. So a lot of the "these are great fins, way better than my last set". Seem to me to be people not so subtly being influenced by the money they have already spent on the fins. Almost no one, except for myself has ever reported to have gotten into the water on their first dive with a new piece of equipment and said; "DRAT! I just spent over $100.00 dollars on these fins at my LDS, because they said they were a lot better than the simple mares paddle fins, but I was a sucker and got taken, these are not worth a wooden nickle."

So how can we set up a simple fairly scientific experiment for fins? That is what I would like to do. Otherwise we are all pissing away money on the latest advertising gimmick and the bogus LDS recomendations. Simple science, simple experiments, we can do it, don't let the "experts" in marketing tell you it is impossible. Simple experiments work and work well, if they are thought out and designed well. So we have a bunch of people from all walks of life including some scientists here on this thread. Lets throw out an experimental design on the board here, and have it reviewed by all of us to see if it takes into account the necessary variables, then lets do it. I will contribute to the intellectual exchange of the experiment construction in any way that I can, and will donate a set of Tusa Xpert Zoom fins. Any body else willing to play?


Lets design an experiment that does test them. The closest in this thread is someone that attached some string to their fins in the 1960s. They were not even trying to solve the questions that we have here on the thread.

Then we have to get someone and have him or her try them out blindfolded to have a credible study????

The idea that energy is generated by the fins themselves as a function of either their construction or their configuration is impossible. The energy is created by the person wearing the fins. The translation of that energy into forward thrust can and we hope is effected by the construction and design of the fins, but no one really seems to have anything other than very poor anecdotal data, which is biased in all cases that it is presented in.

So if you are a fin designer lets hear what a real world scientific study would look like. If you are a diver, lets here what you think the study would look like. What do we need to measure, how can we measure it, collect the data, and get something real.

Enough marketing crap. I don't believe anything a single scuba diving manufacture claims anymore. Is a set of $300.00 fins better than a set of $35.00 fins? WHY? Because of the brand name printed on them? NOT a chance.

Sucky Fin owner
Guy

 

[Blackwood]

So if you are a fin designer lets hear what a real world scientific study would look like. If you are a diver, lets here what you think the study would look like.

As much as it's easy to poo-poo the kick robot as being non-human, I like the idea.

My ideal experiment would be somewhat expensive, but it would go something like this:

Build an program a leg simulator capable of repeatable kicks. It should be in undisturbed water, and be attached to something that both measures power into the robot and force generated by the kick as a function of time.

Taking the human out of the equation would in theory allow one to measure what the fins themselves are doing. If you can program a variety of kick types (scissor, flutter, frog, dolphin) as well and alter the parameters (frequency, amplitude, etc.) so much the better.

 

[HenrikBP]

You have missed the point. See Henrik's excellent post above. He has addressed your issues.

Thanks Imorin.

I just want to again point out though how important it is to make that kind of testing *very* focused to be able to truly be able to say "this is what happened and why".

I wonder how much it would be possible to learn from a finite element analysis of fins? I could be a much less expensive way of testing and might make it possible to run many more analyses than with our robot

Henrik

 

[meesier42]

Here is an easy test that everyone can do for themselves, I did it about 8 years ago with 2 other divers and although there was a wide range in the data the final conclusions of fin choice was strikingly obvious and all 3 divers of very different backgrounds and experience found the same fin topped all others. I am choosing not to discuss the brands/models as it is not relevant to this discussion.

The test is simple although it takes many dives and there are some nuances that you need to be aware of

here is how is goes,
locate a dive that is very consistent in conditions, good visibility, lack of current, etc... The goal is to find a dive that you can repeat at least 6 times (depending on how many fins you want to test). The goal is not that this will be a fun dive, or a long one. Its actually best to do this in a quarry that will be absolutely still water. Then find a series of reference points and string a line from point A to B to C and back to A (or however many you have). The goal is that you will swim 800-1000 yards or about 10 minutes, so if you need to loop the circle 2-3 times to get the distance its fine. If you have great visibility and a good reference line to follow use that. Ideally you want to stay shallow like 10-15 feet as your going to be working very hard and you'll burn a ton of air, you will need a full tank.

Now the fun begins, starting with the fin that you normally use (this is your control), drop down to the start point, record your starting pressure, water temp, and tank size. Its a good idea to let the tank set in the water for at least 5 minutes so that the temperature stabilizes and doesn't effect your data (yes, I learned that one the hard way)

Now start swimming the line as hard as you can sustain for the distance. You must note your elapsed time at each reference point. Swim until you have completed the distance and record you finish time and final pressure, its also helpful to note things such as "slight cramp in right leg after 200 meters" or "arches hurt" etc... Then repeat the effort 2 more times, but this time round your goal is to match your previous time to each reference point, our goal is to compare air consumption at a constant swimming speed. Hopefully, by matching the time you will have a single variable, the fins, and a single comparison, air consumption.

Now you have a starting point and you know what your current fins will do. and its time to try another fin model/style. But wait, you have to become accustomed to that new fin before you can test it and have valid data, so go diving. Have some fun, after 4-6 normal dives with these new fins (and ONLY these new fins) you have learned how to use, or more correctly you have trained your muscles to perform the new exercise (however slightly different they maybe), and are ready to test them. So go back to your test loop (you did leave it there right?) and repeat all 3 trials. Now you should have some valid data point to compare 2 fins. If you want to test more fins you have to repeat the 4-6 exercise training dives each time, but you can repeat as long as you want.

Now that you have completed both fins average each fins air consumption and compare. Note- I said air consumption, not Oxygen consumption. If you want odd data points, like one swim was significantly faster, or failed to complete due to severe cramp etc throw that data point out. So long as you use the same tank each time you can simply compare PSI used, if not then calculate the volume used (not going to discuss that here).

A note that I really want to reinforce. The "training" period with each new style/model of fin is key, you must only use that pair of fins for at least 4-6 dives for a total underwater time of at least 3 hours. This is retrain you muscles to perform the new cadence, kick depth, etc that each fin may require. You must really discipline yourself to use these this fin the entire time and not allow any personal preference to effect this training time as it will completely reset the clock. I know its tough as we all like the equipment that we use, but if you want to have good comparison the system must be followed. This "training" period is the reason that nearly all tested fail miserably to completely describe a fin.

If you don't believe the data at the end, repeat the cycle with your original fins to reset the "reference point" as I am sure with focused training you will become better at the effort and the most recently tested fins may have some bias due to becoming more fit and comfortable with the effort.