Split Fin Physics

[meesier42]

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

I wondered this question myself and looked into studying it while I was still in college, as we were able to model a waving plate fairly easily. I got about as far as shooting video of a fin and doing some basic spring force measurements. After showing it to my professors and some mechanical types they told me to move on. The major issue we found was that the fin is too complex a shape and the spring rate was so non-linear (the same point wasn't even linear) that they believed it would take a lot of time to simply describe they 3 dimensional map of the spring rate and the shape before we could even model it.

 

[Blackwood]

I wondered this question myself and looked into studying it while I was still in college, as we were able to model a waving plate fairly easily. I got about as far as shooting video of a fin and doing some basic spring force measurements. After showing it to my professors and some mechanical types they told me to move on. The major issue we found was that the fin is too complex a shape and the spring rate was so non-linear (the same point wasn't even linear) that they believed it would take a lot of time to simply describe they 3 dimensional map of the spring rate and the shape before we could even model it.

I'm not sure I understand why we would care about spring rate. Seems that stiffness (which is probably constant enough) would be sufficient. I know Bob has fins that have "snap" to them at the end of the power stroke, but in general I would assume we could neglect spring-effects in such analyses. Am I missing something?

 

[redacted]

I'm not sure I understand why we would care about spring rate. Seems that stiffness (which is probably constant enough) would be sufficient. I know Bob has fins that have "snap" to them at the end of the power stroke, but in general I would assume we could neglect spring-effects in such analyses. Am I missing something?

I believe I can see where spring rate can be a pertinent variable and where both extremes are probably undesirable. Think of it like the burn rate on gun powder. If it burns too slowly, you may not get the potential energy from the charge before the projectile leaves the barrel. And if it burns too quickly, it can damage the weapon and produce other undesirable side effects. And I'm sure there is a range where it basically works OK and "maximum efficiency and performance" may be heavily effected by the specific user and kick style mix.

 

[lmorin]

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................................

You have made this way, way too complicated. The task is too complicated and you have inserted a "training" obligation. Thus, manufacturers will always have an "out" by saying the diver didn't take the time or make the effort to adapt to the particular fin.

The first point of any test must be to compare the basics of the fins. Personally, I'd start by measuring drag. This could be done very easily in a pool where the water flows linearly from one end to the other, like a wind tunnel. Used a variety of fins, all the same foot size. Put a dummy foot (no ankle) in the fin pocket; same dummy for each test of each fin. Attach one end of a thin rod to the heel of the fin, with the upstream end attached to a device able to measure the force of pull by the fin. Turn on the water flow and measure the pull force. It will vary from fin to fin. Modify fins as desired, by turning paddles into splits and v.v. Or, by shaving off extra fin-lets, inserting stretchy panels, etc. Compare full foots to open heels and re-affirm the documented observations that full foots perform better based on objective tests of speed and thrust ScubaLabs Protocols and Results

 

[mdb]

For those interested in independent fin testing, here is a link to tests done by Jim Grier. Jim is a retried college professor and a very unbiased individual. He is an academic, as well as a diver, and was seeking answers.

www.ndsu.edu/pubweb/~grier/fins-in-depth-2002.html

Jim's bio: James W Grier

I submit this to our discussion as one more data point and reference.

 

[coldwaterlloyd]

I had the pleasure to dive with Jim a few years back in the Saanich Inlet , Vancouver Island . What an asset to our environment . Hello Jim if you are out there .
I have to chirp in here , I own a pair of Scubapro XL Jet Fins , Apollo XXL xt pro Fins , love em both , depends on what I am doing and what kind of mood Im in . I also get the same power with the Apollo XT Fins as the Jet Fins , all body mechanics.

 

[TSandM]

Thanks for the link to Jim's article -- I hadn't seen that one before.

I was disappointed that the standard Jets didn't make it into all of the tests, but I was amazingly struck by the fact that they came out LOWEST in static thrust. Moving heavy gear is one of the primary reasons why people say they use the Jets, and that tiny piece of information would suggest we're all deluded.

 

[redacted]

Thanks for the link to Jim's article -- I hadn't seen that one before.

I was disappointed that the standard Jets didn't make it into all of the tests, but I was amazingly struck by the fact that they came out LOWEST in static thrust. Moving heavy gear is one of the primary reasons why people say they use the Jets, and that tiny piece of information would suggest we're all deluded.

I'm pretty sure tractors have a lot less thrust than race cars. Yet when you want to pull a heavy load, it is the tractor that gets the job. The race car would just be spinning its wheels.

 

[meesier42]

You have made this way, way too complicated. The task is too complicated and you have inserted a "training" obligation. Thus, manufacturers will always have an "out" by saying the diver didn't take the time or make the effort to adapt to the particular fin.

The first point of any test must be to compare the basics of the fins. Personally, I'd start by measuring drag. This could be done very easily in a pool where the water flows linearly from one end to the other, like a wind tunnel. Used a variety of fins, all the same foot size. Put a dummy foot (no ankle) in the fin pocket; same dummy for each test of each fin. Attach one end of a thin rod to the heel of the fin, with the upstream end attached to a device able to measure the force of pull by the fin. Turn on the water flow and measure the pull force. It will vary from fin to fin. Modify fins as desired, by turning paddles into splits and v.v. Or, by shaving off extra fin-lets, inserting stretchy panels, etc. Compare full foots to open heels and re-affirm the documented observations that full foots perform better based on objective tests of speed and thrust ScubaLabs Protocols and Results

the intent has nothing to do with what marketing BS that the manufactures can use, but to give the individual, aka you, a real comparison of 2 fins and avoid most of the pitfalls of the Scubalab BS that fails even the most rudimentary of scientific methods.

the point is that YOU have to find the best fin for you and your style of fin. as in the end it doesn't matter what the robot or any other method says is the more efficient fin if that fin doesn't fit you. The really important thing to glean from any study is the bio-mechanics that describe how the human/fin interface works and why certain designs fail while other succeed. For example, every thing that I have ever seen says that the Jet fin is a terrible fin, but yet it excels at performing certain functions, but then the same can be said about Split Fins.

 

[John C. Ratliff]

I've had a few minutes to put together my fin trials from last Sunday now. These are very rough estimates, but were the best I could do given a period of time, seven different fins, and a single tank of air. A few of the times are shaky, as I had a hard time deciphering my notes from the small pad I took them on with a pencil.

What I did was to bring all these fins to the pool, and throw them in on the bottom. I had one fin that I did not swim as a pair, but as a single with another Jet Fin, the Jet Fin Split fin. I found that fin in the river last year, and decided to swim it for the first time. It was as good as the Jet Fin, I'll say that. I then jumped in with my BC and a single tank with a double hose regulator on, and started timing my swims while counting the number of strokes. One stroke was when my right foot was on the down stroke (flutter kicks, this time).

The interesting thing to me was how consistently I swam this distance (100 yards) with these fins. There was a bit of difference, but not the wider range I thought I would see. I swam the Super Stag fin as I had found a pair on E-Bay last year, bought it and been pleasantly surprised at how supple the blades still were, after about thirty years in a box (the heel straps had to be replaced though). I had modified a pair of Super Stag fins in the 1980s with my scoop design, and swam the two just to see any difference (there was a bit of a difference, which amounted to about 57 strokes in a mile).

The Quattro Excel looks good in time, and bad in number of strokes; I don't place too much on these numbers as this was my first swim, and the paper was not too legible for this set.

I was pleasantly surprised by the Apollo Full Foot Split Fins. These were not my fins, as another diver (Ron Peters, who does write on this board) loaned them to me during the pool session. He was pretty well convinced of their effectiveness, and this small swim seems to bear him out. He told me that he had been a "paddle fin" enthusiast, and had bought these fins for his wife. He then tried them out, and was very surprised at how effective they were, so he bought another pair for himself. So he wanted me to try them; he signaled me over to him underwater. I handed Ron my fins with my booties stuck into them, and swam the 100 yards with the Apollo Full Foot Split Fins. Afterward, he said that he had swam beside me during my swim, both to look at my equipment (my Para-Sea BC and double hose regulator, a Healthways Scuba Deluxe) and to watch my stroke. He said that I was a classic paddle fin swimmer, with fairly wide strokes. He said that the split fins were best with short strokes within the dimensions of the diver's wake. I don't know about that, but I do know that these fins were considerably smaller and lighter than the other fins, and yet provided about equal thrust. Ron, thanks for the loan of those fins.

Again, the scoop fin did well too. This was not the more sophisticated ones, but my third try at the scoop concept in the late 1970s.

As a critique of my own timing, I was using a Seiko dive watch with a sweep second hand. This is not too practical for these tests, and may have led to some wrong readings. So while this table is interesting to me, I do feel a need to repeat it sometime with a different timepiece, and perhaps with my better regulator/computer so that I can get air consumption data too.

SeaRat