Split Fin Physics

[lmorin]

SeaRat - I think one thing you're showing with your test is that you can generate more power from your kick when the curve of the fin matches the natural strength curve of your kick. We have more power in the forward kicking motion than when we pull our kick back. Add some resiliency, or Snap to that fin blade, and you'll be able to sustain the speed even longer.

I think that what the physicists will tell you is that generating power with fins is a myth or perhaps a misapplication of terms. All energy comes from the kicker through the kicking stroke and is related to the physical energy put into by the person. Without fins, there is an inefficient translation of that energy from an up/down leg/foot motion into forward body motion. So, it would seem to me that a "good" fin would be one that effectively makes that translation. Any characteristic of fin design that increases translation efficiency (defined as proportion of up/down leg energy converted into the energy of forward propulsion) would be good.

 

[mdb]

Imorin: Good comment. What are your thoughts about the split fin designs that provide forward thrust with both the up and down stroke?

Paddle fins provide thrust only on the down stroke. The up stroke is known,commonly, as the "recovery" stroke.

The materials used are also a part of the equation. High grade polyurethane, expensive but very responsive and very durable, 100% natural rubber, expensive to mold in compression molding, but very comfortable and effective, long lasting and durable.

You are certainly correct in that the energy comes from the kicker and the kicking stroke. What are your comments on the various designs available to translate the kickers energy and stroke in the most effective manner?

 

[meesier42]

Just looking at the orientation of fins, it appears self evident that a airfoil type effect that produces propulsion due to low and high pressure areas can't be occurring. For that to happen, the foil surface would have to be oriented perpendicular to the diver, not parallel as in the case of fins. If there is any foil effect, it's not being used to propel the diver.

What they're describing would be like an airplane with the wings trailing sideways behind the plane.

Fins run parallel to the diver, so any foil effect produced by split fins would be producing sideways forces.

no, fins running parallel to the diver is a situation that really only occurs at rest. if in fact the fins stayed parallel to the diver you would not be able to go anywhere. The reality is that the fin flex to achieve a high angle of attack. per the UofB study, they showed that the more the fin flexes the more efficient that fin will be. in the case of split fins, the flex point is parallel to the diver, but when combined with the angle that the split fins angle away from the toes established a positive angle of attack that directs water roughly behind the diver. for a very good diagram of this refer back a handful of post.

Imorin: Good comment. What are your thoughts about the split fin designs that provide forward thrust with both the up and down stroke?

Paddle fins provide thrust only on the down stroke. The up stroke is known,commonly, as the "recovery" stroke.

you need to stick to marketing, split fins do not provide anymore thrust on the recovery stroke than any other fin, this has nothing to do with the fin. The lack of propulsion has everything to do with the fact that the human leg is simply incapable of providing the required strength to overcome drag. You need to actually read the UofB study they clearly state this as fact and that NO fin in that study produced a thrust greater than its drag on the recovery stroke. they further conclude that due to the muscles of the human leg that no fin will ever produce adequate thrust on the recovery stroke and the fin should relieve load on the recovery stroke to minimize the effort required to return to the top of the power stroke. This is a point of fact, that fact is no longer opinion and any opposing opinions are pointless.

 

[Abaco24]

130 plus replies on a split fin topic? Really ? It's a scuba fin for God's sake. I have a pair of Mares for $80 and love them, relax and go in peace :blessing:

 

[mdb]

M: You do not have a clue. You continue to contaminate a fairly good discussion with your obnoxious comments. The "study" you continually refer to is very old.

Some of the more objective posts will provide some real information. If you wish to call someone names, perhaps it can be done in person.

 

[mdb]

130 plus replies on a split fin topic? Really ? It's a scuba fin for God's sake. I have a pair of Mares for $80 and love them, relax and go in peace :blessing:

Yes, go in peace is the right comment. Some will and others will not. Some folks do get their panties in a major twist. So be it.

This thread has been, for the most part, a good discussion.

 

[John C. Ratliff]

SeaRat - I think one thing you're showing with your test is that you can generate more power from your kick when the curve of the fin matches the natural strength curve of your kick. We have more power in the forward kicking motion than when we pull our kick back. Add some resiliency, or Snap to that fin blade, and you'll be able to sustain the speed even longer.

I'm not sure who is asking from Bob Evan's computer, but I don't believe this totally. It's not that we cannot generate the power on an upstroke, it's that the orientation of the fin is not as good for generating backwards thrust on the upstroke. But in order to use the flutter kick in any kind of balance, you need to have some thrust on the upstroke too. If you look at the Navy diver using Duck Feet fins many, many years ago, you'll see that the upstroke fin is providing some propulsion, but not as great because of the angle. It also provides balance to the stroke.

When I was training as an instructor for NAUI, we had to do a rescue through about 200 yards of surf, doing in-water mouth-to-mouth all the way in (and yes, in those days it was really done by the instructor trainees to another trainee). During the first part of that exercise, I lost one fin (my first Jet Fins). Kicking with only one fin is no fun, and shows that the balance of the upstroke is pretty vital to the propulsive effort.

Now, I have used Force Fins, and tested them years ago (although not all models). I found that they are very effective on the downstroke, but almost totally ineffective (by design) on the upstroke. My first try with them was memorable, as my feet came completely out of the water on the upstroke. I was used to having that resistance, and that propulsion, from the upstroke. I learned with some time with the fins that if they were to be maximally effective, the downstroke had to be very hard (getting the maximum flexion of the blade) and the upstroke very easy (as there was no effort to the upstroke). It is a very different style of swimming, but once adapted to it, the fins do very well. But I have not made any comparisons with other brands that I have times for.

Imorin: Good comment. What are your thoughts about the split fin designs that provide forward thrust with both the up and down stroke?

Paddle fins provide thrust only on the down stroke. The up stroke is known,commonly, as the "recovery" stroke.

The materials used are also a part of the equation. High grade polyurethane, expensive but very responsive and very durable, 100% natural rubber, expensive to mold in compression molding, but very comfortable and effective, long lasting and durable.

You are certainly correct in that the energy comes from the kicker and the kicking stroke. What are your comments on the various designs available to translate the kickers energy and stroke in the most effective manner?

If you look at the diagram below, you will see that there is a component backward for the upstroke, even with the flat blade fin. Yes, there is force generated on both strokes, as I tried to explain above. If you will look at the wake turbulence of the 100m finswim that was posted on the Texas Finswimming website (see the photo below) you can see that there is force from the monofin both in the down and upward stroke. This shows by the symmetry of the bubble path that the finswimmers generate. By the way, what do you think of swimming 100 meters in 41.51 seconds?

The split fin does orient the fin to provide more backwards-looking thrust, just as the scoop fin does. It probably works best with a fast kick, rather than a slow one, as the water is pushed against harder with the faster kick. By this I mean that because the water is a fluid it would more easily escape through the split if pushed slowly, verses a rapid thrust.

SeaRat

 

[David Wilson]

The materials used are also a part of the equation. High grade polyurethane, expensive but very responsive and very durable, 100% natural rubber, expensive to mold in compression molding, but very comfortable and effective, long lasting and durable.

Indeed. Many divers probably don't care what goes into their fins these days when they're manufactured, but some do. I admire Japanese diving equipment companies such as Apollo, Gull and Diveways for insisting on traditional natural rubber when producing fins while many major diving equipment manufacturers in the western world limit their offerings to composite fins, a veritable "dog's dinner" of synthetic materials, whose only virtue is their compatibility with current airline baggage allowances.

"Modern" materials such as thermoplastic elastomer (TPE) are often presented as exciting "space-age" innovations when more discerning end-users will perceive them to be trade-offs rather than improvements. TPE probably represents a dream to cost-cutting producers because of its relative ease of manufacture, but unlike natural rubber it sags, warps and needs to be propped up by plastic formers when it is deployed as a fin foot pocket material. I wince when retailers describe TPE and oil-derived materials used in fin production as "rubber" when their source is the lab technicians' test tube and not a Malaysian or Indonesian rubber tree. I suspect many recreational divers think their wholly man-made fin foot pockets are natural products.

I'd like to see traditional-material fin manufacturers make more of the natural origins of their products in the fifteen countries of the world where natural rubber fins are still produced. I believe at one time that there was a natural rubber symbol in the same way as there were quality marks for other natural products such as leather, wool and cotton. Something should be done to stop manufacturers and retailers using the word "rubber" indiscriminately to describe what are basically soft plastics. If that proves too problematical, then the designation "natural rubber" should be protected and confined to products made from the sap of the rubber tree. Then there's the protection offered by National Standards agencies. Both Russia and Malaysia have National Standards for fins and both countries' Standards for fins confine their scope to natural rubber fins and do not cover fins made from other materials. Some of us still want our fins to continue to be made the way they used to be made, with the best quality traditional materials, because they meet our particular, individual needs and priorities, which we know are not the same as everybody else's.

 

[meesier42]

M: You do not have a clue. You continue to contaminate a fairly good discussion with your obnoxious comments. The "study" you continually refer to is very old.

Some of the more objective posts will provide some real information. If you wish to call someone names, perhaps it can be done in person.

very old? that study is barely 7 years old and has been backed up by at least 3 other study's including ones performed only last year, those studies examine the physiology of the swimmer leg not the fins they use. not to mention that 2003 study is the ONLY peer reviewed objective scientific analysis of fins available, so if you have something more recent please provide it. The fact remains that the human has not evolved in the last 7 years and baring some odd mutations is unlikely to ever change to benefit a swimmers leg.

I'm not sure who is asking from Bob Evan's computer, but I don't believe this totally. It's not that we cannot generate the power on an upstroke, it's that the orientation of the fin is not as good for generating backwards thrust on the upstroke. But in order to use the flutter kick in any kind of balance, you need to have some thrust on the upstroke too. If you look at the Navy diver using Duck Feet fins many, many years ago, you'll see that the upstroke fin is providing some propulsion, but not as great because of the angle. It also provides balance to the stroke.

When I was training as an instructor for NAUI, we had to do a rescue through about 200 yards of surf, doing in-water mouth-to-mouth all the way in (and yes, in those days it was really done by the instructor trainees to another trainee). During the first part of that exercise, I lost one fin (my first Jet Fins). Kicking with only one fin is no fun, and shows that the balance of the upstroke is pretty vital to the propulsive effort.

Now, I have used Force Fins, and tested them years ago (although not all models). I found that they are very effective on the downstroke, but almost totally ineffective (by design) on the upstroke. My first try with them was memorable, as my feet came completely out of the water on the upstroke. I was used to having that resistance, and that propulsion, from the upstroke. I learned with some time with the fins that if they were to be maximally effective, the downstroke had to be very hard (getting the maximum flexion of the blade) and the upstroke very easy (as there was no effort to the upstroke). It is a very different style of swimming, but once adapted to it, the fins do very well. But I have not made any comparisons with other brands that I have times for.


If you look at the diagram below, you will see that there is a component backward for the upstroke, even with the flat blade fin. Yes, there is force generated on both strokes, as I tried to explain above. If you will look at the wake turbulence of the 100m finswim that was posted on the Texas Finswimming website (see the photo below) you can see that there is force from the monofin both in the down and upward stroke. This shows by the symmetry of the bubble path that the finswimmers generate. By the way, what do you think of swimming 100 meters in 41.51 seconds?

The split fin does orient the fin to provide more backwards-looking thrust, just as the scoop fin does. It probably works best with a fast kick, rather than a slow one, as the water is pushed against harder with the faster kick. By this I mean that because the water is a fluid it would more easily escape through the split if pushed slowly, verses a rapid thrust.

SeaRat

SeaRat- discussing monofins is not the topic and they really do not apply here. sure its interesting but visual symmetry is not equal to thrust symmetry. this would be more obvious if you saw the same swimmer at equal depth swim face down and then face up, the power stroke is hugely more powerful and even in the best finswimmers the recovery stroke may provide some really significant power its a mere percentage of the thrust generated on the power stroke. Although I don't have a link for you, I have seen a video that traces the position of a monofin swimmer at speed, and it was obvious that this person traveled much further on the power stroke than the recovery, I know it talked about the difference but I do not remember the numbers, but I believe it was about 30% further. I also remember them discussing that if the swimmer did not already have significant forward momentum that the recovery would have even less distance.

The concept of balancing the power stroke and the recovery stroke of a fin is bogus. This is afterall how many paddle fins work and it has been shown to false, the fact is that the human body will provide some strength into the fin on the recovery stroke, but as shown the thrust generated does not overcome the drag generated on that portion of the stroke. Studies have shown that the power stroke last only ~30% of the total cycle time, the "transition" period (the time the fin is essential not moving at the top and bottom of the kick) takes up approximately 15% total cycle time (although most of it is at the end of the power/down stroke), leaving 45-55% of the cycle time being used in the recovery period. For the Apollo Bio-Fin the data showed that 29% cycle time on the power stroke, 56% on the recovery, and the power stroke produced some 40% higher thrust. so yes some thrust can be produced in the recovery stroke, but it is marginal and acts more to increase fatigue than propel the swimmer.

 

[Bob Evans]

I respect your comments, drawings, photo's, and will try to share what I've learned through my fin development.

I think you'll find you actually get more water moving, or propulsion from a Force Fin when it snaps on the recovery and forces water through the split, on the upstroke. It is the fin movement or snap doing the work, not your legs. That is the beauty of the design. I've transfered propulsion from resistence or power of your legs to the movement or snap of the fin blade. Its also why with all your experiencing powering flat blades, you popped the surface when you first tried Force Fins.

We tried my first polyurethane Force Fin (Dick Anderson turned me onto Urethanes 1971), in my mentor's, Bev Morgan of Kirby-Morgan Diving Systems fame, home pool. His first comment was that the curved backside allowed a recovery stroke just like a dolphin. He had just returned from Hawaii where he had seen some slow motion 16mm film of dolphins swimming from a Navy study. To the naked eye it looks like they are powering equally on both the upstroke and down stroke. Actually, they muscle their tail harder downwards, and drop its angle of attack to quickly pull
the fin back up using less effort on the recovery. They don't lose any propulsion during that recovery.

Like dolphins we have more strength when we kick forward. (In response to the resistence of the fin blade, our ankle and knees lock, and we're using our quadriceps, instead of our hamstrings.) Force Fins' wing tips bend inwards to match your natural strength. You can power a flat fin blade for a sprint, but when it comes to covering distance you have to work with your body.

A diving physician told me that the recovery would aid in oxygenation of the muscles. When we kick up and down equally, we oxygen deplete at a faster rate. This adds to the efficiency of a curved fin in use, over that of a flat fin.

This information has come from my customers and many mentors who know what they are talking about. I am an artist. The shapes flow out of my hands, and my customers who are more knowledgeable about the science behind what I do give the explanation.

For those like you who want to go all balls out, powering up and down, I do make the SD1, Excellerating Rip and Multi Force Fins. For me, its all about what the customer wants.