Split Fin Physics

[mdb]

I may be convinced that curving the blade into an almost half-funnel shape may force more water backwards rather than over the sides. To that end, a wide flimsy center with strong edges with no split may be more efficient.

Blackwood: Perhaps this is close to the concept of the Mares Avanti fin. I remember diving the Avanti on a test dive off Portofino, close to the Mares HQ in Rapallo.

Gianni Garofalo, Head of Research and Development for Mares, explained the concept behind the design of the Avanti fin basically as you describe.

 

[Eric Sedletzky]

Blackwood: Perhaps this is close to the concept of the Mares Avanti fin. I remember diving the Avanti on a test dive off Portofino, close to the Mares HQ in Rapallo.

Gianni Garofalo, Head of Research and Development for Mares, explained the concept behind the design of the Avanti fin basically as you describe.

I thought about the Quatros after I posted about the Vikings. Yes, the mares quatros do that, but the only thing I don't like about them is that the flexible inner part flops as you change directions in kick stroke and there is a resulting dead spot in power while your foot reverses direction and the center baffle of the fin has not retightened with water pressure.
I'm thinking of taking a pair of Apollo Bio Fins and taping the split shut and see how that works. Everything is there for the type of fin I want except for the damn split.

 

[meesier42]

My hypothesis is that it simply decreases the resistance of kicking. It also decreases the amount of water moved, hence the necessity to kick with a higher frequency. The extreme case is: take your fins off. You can kick much more rapidly, but each kick reacts much less force.

I don't suspect that there is any sort of controlled pattern behind the fin, split or otherwise. It's all chaotic. The diagram certainly looks cool, but I have a hard time believing it. I've seen actual wing sections in wind tunnels (which are designed to keep the flow attached as far back towards the training edge as possible) that don't behave as cleanly as well.

I may be convinced that curving the blade into an almost half-funnel shape may force more water backwards rather than over the sides. To that end, a wide flimsy center with strong edges with no split may be more efficient.

Blackwood: Perhaps this is close to the concept of the Mares Avanti fin. I remember diving the Avanti on a test dive off Portofino, close to the Mares HQ in Rapallo.

Gianni Garofalo, Head of Research and Development for Mares, explained the concept behind the design of the Avanti fin basically as you describe.

in fact if you carefully read the U of Buffalo study you will see that is exactly what they showed. They solved the problem of split fins as the extremely high drag that they have, but when you tape the split shut the drag drops significantly and changes the reynolds numbers and the lighthill predictions of the fin. but in the end they then describe the problems with this paddle style of fins and describe the solution to the problems that they encountered, FWIW- in a discussion I had with the author of that paper, he admitted to me that although he did not intend to describe ForceFins as the solution to all the problems they encountered in their testing, he agreed that they do in fact do solve nearly all of the issues they found. However they were not included in that study as they did not conform to the entering hypothesis of the study and FF did not offer funds to be included.

also- at some point I saw mention of Force Fins use of vortex generators to be "snake oil" well I have a couple of comments on that.
First- the concept behind them was reduce the flow drag (aka skin drag) during the recovery cycle of the fin. as shown in the U of Buffalo study, the recovery stroke of a fin produces no real thrust (asymmetric musculature being only one of the reasons) and adds only to the total drag of the overall cycle. By reducing the flow drag on the back of the fin it reduces one component of the drag making the fin more efficient.
Second- the number and location of the vortex generators was tuned, not by Force Fin, but by scientific study. This can be easily seen on the Excellerating Force Fin, as the marine engineering lab worked out those locations are the optimum locations. In fact- Bob Evans will recount the story of the lab calling him and asking him, "do you want functional or pretty?" , in the end they choose function and there are only about 10 total on the fin. The Extra Force went through the same process, it started with about 4 times as many as it has now, but the lab informed him that they were overkill and fewer would actually perform better.

 

[diver_doug]

This reply might be completely worthless as I have no background in physics (well, I've read a number of books on quantum physics, but I doubt the notion of non-local reality has anything to do with split-fins ) Nonetheless, I've been reading this thread since it was originally posted and I feel inclined to chime in with my 2 cents.

It seems to me in thinking about it, that the split fins are creating some sort of controlled turbulence such as a vortex, or something akin to a vortex. Even amongst split-fin proponents it is widely accepted that it takes a couple of kicks to get moving from a stagnant position when using splits. If splits were simply displacing water like a paddle fin this wouldn't make any sense. If, however, the splits are actually creating something (such as a vortex) then this makes perfect sense; a couple of kicks are needed to get the "vortex" going before meaningful movement takes place. This also helps make since out of the notion that splits are more prone to silt-outs. If the splits are simply displacing water the silt-outs don't really make much sense. However, if the splits are creating small underwater dust-devils (so to speak) then, again, the silt-outs make sense. Also, this makes sense of the notion that splits seem to require a more precise kicking-method. If it was simply a matter of displacing water, the kick precision wouldn't really matter, but if you are trying to create a sort of vortex then it does. As a final piece of anecodal evidence I noticed, when using splits, that i didn't feel like i moved until the kick was completed (whereas with paddles I feel like I am moving throughout the duration of the kick). Again, this sensation makes sense if the propulsion from splits is coming from some sort of vortex which is "fueled" by the kick, as opposed to simply being moved by the displacment of water.

 

[Eric Sedletzky]

This reply might be completely worthless as I have no background in physics (well, I've read a number of books on quantum physics, but I doubt the notion of non-local reality has anything to do with split-fins ) Nonetheless, I've been reading this thread since it was originally posted and I feel inclined to chime in with my 2 cents.

It seems to me in thinking about it, that the split fins are creating some sort of controlled turbulence such as a vortex, or something akin to a vortex. Even amongst split-fin proponents it is widely accepted that it takes a couple of kicks to get moving from a stagnant position when using splits. If splits were simply displacing water like a paddle fin this wouldn't make any sense. If, however, the splits are actually creating something (such as a vortex) then this makes perfect sense; a couple of kicks are needed to get the "vortex" going before meaningful movement takes place. This also helps make since out of the notion that splits are more prone to silt-outs. If the splits are simply displacing water the silt-outs don't really make much sense. However, if the splits are creating small underwater dust-devils (so to speak) then, again, the silt-outs make sense. As a final piece of anecodal evidence I noticed, when using splits, that i didn't feel like i moved until the kick was completed (whereas with paddles I felt I was moving throughout the duration of the kick). Again, this sensation makes sense if the propulsion from splits is coming from some sort of vortex which is "fueled" by the kick, as opposed to simply being moved by the displacment of water.

You might be onto something there.
One way to possibly find out would be to inject an ink trail into the oncoming water flow of the fin to visually see exactly what happens.

 

[Kern]

I thought a vortex created by a wing caused drag. If splits create a vortex, wouldn't you tend to be sucked backwards into it, rather than being propelled forward by it.

 

[Eric Sedletzky]

I thought a vortex created by a wing caused drag. If splits create a vortex, wouldn't you tend to be sucked backwards into it, rather than being propelled forward by it.

Are you talking about the same phenomenon as when a boat goes along before it breaks plane and the rolling of water behind the boat onto the transom causes back suction?

 

[diver_doug]

I thought a vortex created by a wing caused drag. If splits create a vortex, wouldn't you tend to be sucked backwards into it, rather than being propelled forward by it.

Maybe it's an inverse vortex, maybe vortex simply isn't the right word...my core point is that I think splits generate propulsion via something more than mere water displacement-I think they actually do create some sort of "force" that propels the diver.

 

[Puffer Fish]

Finally back home and read the Buffalo study.... interesting reading.

I believe the intent and the basic overall work was reasonably done...much of the detail was not.

They did their study with a couple of aspects that would have favored the fins that were already being used by the experienced divers they were testing. I don't believe this was intentional.

By using experienced divers, who already have muscle development for the fins they use, they accidentally predisposed the testing to work best with fins that were optimal for that method.

I would suggest that the first thing one should have done is determine the best kick for each fin, then train people in that kick (using the same methods they did) and then do the testing.

One thing that should have been obvious is that with any flexable fin, the more force applied to it, the more it would change shape, and that shape change would change how effient the fin would be (both thrust, recovery and resistance). What they should have seen was a humped curve (which they did, but tried to make it a smooth line).

Their description of the reduced hip motion with flexable fins would seem to be more from a diver induced, than any actual requirement.

A great example of this is doing a frog kick with standard apollo split fins (one of the three sets of splits fins I have). Trying to do a frog kick with them is almost laughable... but if you try to do a really fast frog kick, they work reasonably well...while stiff paddle fins are almost exactly opposite.

For anyone that happen to be in OZ this last week, they showed the myth busters program on tailgate up versus down in a pickup and is down more efficient. The normal view has always been that a smooth back (tailgate down) or cover is more efficient (after all, isn't smooth laminar flow aways better?)... turns out the the tailgate up is more efficient because the turbulence it creates (talk about non-intuitive).

I do a lot of pool swimming with different fins (from really stiff paddle to really flexible splits)...using do a little over a mile a couple of times a week. What I have found out is that:

1. Different fins require different kicks.

2. Different fins require different muscles.

3. Different fins require different speeds in each sections of the kicking cycle.

If one want to be efficient.

Ironically, my typical speed happens to be right smack in the middle of the testing speeds used for the Buffalo study (around 1.5 mph).

The other aspect of this, is the human efficiency aspects... human studies have long shown that there is an "ideal" cadence rate in bicycling, for each person. It would be reasonable to expect the same thing is true with fins.

Ideally, one would match up their personal cadence effiency with fins designed to provide the optimal thrust at that candence. The first hiccup in that is that optimal thrust assumes one knows their resistance and what their optimal speed should be and then had some way to match up that with their physical muscle structure and fin design.

Lacking all of the necessary information, we are left to argue the issue forever.

I suspect force fins have a different kick skill requirement also, by the way.

 

[Kern]

Are you talking about the same phenomenon as when a boat goes along before it breaks plane and the rolling of water behind the boat onto the transom causes back suction?

I really couldn't say. I'm afraid I haven't been paying enough attention to notice.