Split Fin Physics

[Bob Evans]

bubbletrubble, Some how I have a problem with the second buffalo study. Why did they not test any Force Fin models? Their earlier study included the Original and Pro Model Force Fin in the last lap, and the Jet Fin did not even make the final cut. Why would this subsequent study not include Force Fin products, while including the Jet Fin?

Using the Freedom Information Act we received a copy of the original study. Its adventure is covered in an article by Chris Kostman and posted on his site at The Force Fins Story

The Foil Force is curved not a flat fin with a cut split up the middle. The shape is foiled. I have some ideas on how we might show how water travels across different fins. That would help answer the original question of this thread.

 

[Eric Sedletzky]

Again, I'm not trying to tell people what's best for them. I'm not even expressing an opinion about what's better for me. Just trying to satisfy my own curiosity.

I've had some fun discourse with Bob, one result of which is action on his part to change some of the language on his website.

It's posts like this one that sparked my interest in the subject (name removed):

That post was with reference to a 'paddle vs split' conversation. I don't see how it can be an apt analogy, but the idea is supported by at least some marketing language.

Here's a good diagram from Boeing which shows a wing, flow, and lift.

I modified it to make it like a prop: the 'wing' rotates about the center axis, and the resultant lift (aerodynamic force perpendicular to the freestream, which in this case isn't air flow relative to the plane but rather relative to the prop blades) is in parallel to the direction of motion.

A paddle wheel and a propeller are functionally different and work on different principles. A split fin, as far as I understand, is a modification of a paddle which decreases the work of kicking, but it's not a different paradigm.

I see exactly what you're saying.

Any fin wether it's a split, paddle or whale tail doesn't use the principle of lift. They all use pressure differential. The flat area of a fin is forced through the water at an angle and you have a high pressure side and a low pressure side. With an airplane wing there's a long side and a short side but the wing slices through the air or water straight by which utilizing "lift" to move the wing in the direction of low pressure.

With split fins or any other fins there is no "lift" in the terms of the definition, there's pressure differential. You could screw two pieces of flat plywood onto a pair of shoes and make fins out of those (which would kind of work), but the point being the plywood is flat and there isn't any "lift".
A boomerang has lift. If you look at it, it's shaped like an aircraft wing on the profile. When you throw it straight and flat it want's to go up.

Split fins are nothing more than having two very wide but short paddle fins on each foot, if you look at each side as an individual fin sitting sideways. The split relief of the full force of hydrolic pressure by letting water to pass through, but the two sides are doing the exact same thing as a regular paddle fin and that is creating a pressure differential when forced through the water. With a paddle fin all this force is just put on one larger surface being thrusted backwards instead of two smaller surfaces being thrusted inwards towards each other.

 

[Bubbletrubble]

bubbletrubble, Some how I have a problem with the second buffalo study. Why did they not test any Force Fin models? Their earlier study included the Original and Pro Model Force Fin in the last lap, and the Jet Fin did not even make the final cut. Why would this subsequent study not include Force Fin products, while including the Jet Fin?

Bob, could you please provide citations for the "second buffalo study" and the UCLA study mentioned in the Adventure Corps website interview? This would be very helpful in our discussion.

Is the "second buffalo study" the one published by Zamparo et al. (Eur J Appl Physiol. 2006 96(4):459-70)?

 

[Blackwood]

They all use pressure differential. The flat area of a fin is forced through the water at an angle and you have a high pressure side and a low pressure side.

I suspect that the majority of thrust comes not from a pressure gradient between the back side and the wake side acting across the fins, but rather from the simple reaction force.

When I kick, I impart a force on the water, and Newtonian physics provides an equal reaction force back on the fins.

If forward thrust were primary generated by creating a big wake on the forward-direction side of the fin, wouldn't designers attempt to increase the wake rather than shrink it (as in the case of split fins)?

 

[meesier42]

Bob, could you please provide citations for the "second buffalo study" and the UCLA study mentioned in the Adventure Corps website interview? This would be very helpful in our discussion.

Is the "second buffalo study" the one published by Zamparo et al. (Eur J Appl Physiol. 2006 96(4):459-70)?

the first U of Buffalo Study was completed in 1991, many years prior to split fins of this discussion. This study as I know was never published in its entirety, although and abstract that did not reveal the brands/models was published regarding the basis and findings of the study were. You can see they learned a significant amount about fin swimming in that study and it has been the basis of many of their follow-on events. The reason that the study was not published with names/brands is because that study was sponsored by the US Navy and so they held the release of the information and they concluded that its release would cause significant harm to the manufacturers that did not fair well in the test.

 

[Bubbletrubble]

the first U of Buffalo Study was completed in 1991, many years prior to split fins of this discussion. This study as I know was never published in its entirety, although and abstract that did not reveal the brands/models was published regarding the basis and findings of the study were. You can see they learned a significant amount about fin swimming in that study and it has been the basis of many of their follow-on events. The reason that the study was not published with names/brands is because that study was sponsored by the US Navy and so they held the release of the information and they concluded that its release would cause significant harm to the manufacturers that did not fair well in the test.

@meesier42: I'm still curious about the second Univ. of Buffalo study to which Bob was referring in a previous post. Could you share the citation for that study? Thanks in advance.

 

[meesier42]

let me clarify
the first study was done in 1991, and is not publicly available, no splits were evaluate (none on the market yet)
the second study- was completed in 2003, this one included the Apollo Bio Fin (split). This is available on rubicon library, however its very difficult to read and has to be read many many times to have it make sense. their conclusions basically state the split fins seem to work, but they don't really understand why as it violates most of the predictive models that they have. they did however show that the "propeller" concept is complete bunk and the fins are really just paddle fins with a very low distance per kick and very high kick frequency.

I am not familiar with the study that you posted as I can't find the entire article, only that couple paragraph abstract that have no value, simply states that fins are more efficient than swimming barefoot.

 

[mdb]

Blackwood:

Your thread continues to have "legs" with more input and submitted test data.
There are lot of tests out there, not just Rodales, a number of good independent testers, and some of the more feisty dive mags. particularly in the UK. Most of the results, that I have seen, indicate that a properly designed split fin produces good thrust without undue resistance. Part of this is due to the materials and the design parameter.

Here is a bit of information from the Apollo Japan website:

All natural rubber can be a little heavier on the surface than the plastics used for most fins. However the durability, comfort, flexibility, and responsiveness of 100% natural rubber is unique. Unlike buoyant plastic fins, rubber fins are less likely to break the surface, an action which causes loss of momentum.

The bio-fin is designed with a 20º angle to accommodate a diver's natural tendency to bend slightly at the knees in addition to the obvious bend at the ankle. This angle is crucial to top performance and comfort. The result is better propulsion and the least amount of stress on the body. Apollo's choice of superior material and intelligent design provides divers with quick acceleration and the ultimate in power, speed, and comfort.

Now I know this is a marketing statement, however; the comments regarding choice of materials and shape and design are part of what makes the fin test so well, time after time.

I hope we see some more test data and discussion regarding the materials used etc.

Your OP continues to be of value.

 

[Bubbletrubble]

let me clarify
the first study was done in 1991, and is not publicly available, no splits were evaluate (none on the market yet)
the second study- was completed in 2003, this one included the Apollo Bio Fin (split). This is available on rubicon library, however its very difficult to read and has to be read many many times to have it make sense. their conclusions basically state the split fins seem to work, but they don't really understand why as it violates most of the predictive models that they have. they did however show that the "propeller" concept is complete bunk and the fins are really just paddle fins with a very low distance per kick and very high kick frequency.

I am not familiar with the study that you posted as I can't find the entire article, only that couple paragraph abstract that have no value, simply states that fins are more efficient than swimming barefoot.

*Sigh* So that there's no ambiguity, is this the citation for the "second Buffalo study" that Bob Evans had a problem with?: Pendergast et al., 2003 UHM 2003, 30(1):55-71.

BTW, I did my best to explain the findings of this paper in Post #33 of this thread.

the first U of Buffalo Study was completed in 1991, many years prior to split fins of this discussion. This study as I know was never published in its entirety, although and abstract that did not reveal the brands/models was published regarding the basis and findings of the study were. You can see they learned a significant amount about fin swimming in that study and it has been the basis of many of their follow-on events. The reason that the study was not published with names/brands is because that study was sponsored by the US Navy and so they held the release of the information and they concluded that its release would cause significant harm to the manufacturers that did not fair well in the test.

The bold-faced text above represents a highly speculative statement. Many studies reported in the form of a meeting abstract are never published in a peer-reviewed journal for various reasons. Furthermore, I have never heard of an instance in which a governmental funding source (NSF, NIH, U.S. Navy) has ever suppressed publication of an article in a peer-reviewed journal. That's simply not how the peer review process works. In fact, if anything, there is pressure to publish (rather than to withhold publication) since this provides a public record of the work and can be used in making a case to seek additional funding.

 

[Bob Evans]

Bubblerubble, The U of Buffalo study I got with the FOIA request was being sent off to be classified. You read the true story, Aqua-Corps, Chris Kostman. I ask is why they left out my products in the public study? They did they not test my variable thrust fins or the split fin I make under my own patents receive before others for a curved split fin design, with a power and recovery phase? Might be interesting to post the research on Scubaboard...but then I don't want a visit from the FBI. I think Meesier42 viewed the study while in the Service? If you like to provide lift then I feel the shape has to be curved, not a flat fin with a split down the center.