Split Fin Physics

[Blackwood]

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.

To create fluid dynamic lift, you need to turn the free stream, and you can do it with a flat surface. Go stick a board (or your hand if you consider that to be flat) out of a moving car window at a positive angle to the oncoming flow. It will lift. It's not as efficient (L/D) as an airfoil, but it will do it.

 

[meesier42]

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.

not speculative at all, see http://www.scubaboard.com/forums/5085461-post34.html

in this case, those who are they decided that the information held in that report offered the our military a significant advantage in tactics and they chose to hold the information in house. per the above linked letter they seem to also believe that no manufacturer should be able to use this information to their benefit and ForceFin has done a good job of keeping what information they have fairly close as well.

I would like to push this thread back to the topic at hand and continue what has been the single best discussion of split fins I have ever seen on this forum. if you would like to continue the discussion of FF please post over on the ForceFin Forum and we can discuss those fins in detail, but I would love to keep this thread on topic.

 

[Bubbletrubble]

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.

Bob, after reading the Aqua Corps interview transcript, I have to say that it's completely irrelevant whether or not the records sent to you were intended for Navy legal counsel. You made the FOIA request and they sent you the info. Period. If the Navy wants to make a big deal about a clerical error, then let 'em. If you obtained the information under the auspices of the FOIA, then the material is considered in the public domain. Even if the material were subsequently classified, I think you would still have a right to possess and share the information -- it's not your fault that the government opened up a public accessibility "window." Even after the info became classified, the government would have to go through legal means to restrict your rights to the material. I'd be surprised if they would waste time/energy/money doing that. After all, we're talking about a scuba/snorkeling fin study...hardly an issue of national security.

I imagine the Navy could argue that keeping the fin study results under wraps will hinder spread of intelligence info to other countries regarding our under water operational capability...but that's still quite a stretch. The fins in the study are all commercially available. Other countries can buy them and test 'em for themselves.

Why not release the results (post them on your website and/or here on SB) and subject the data to some scientific scrutiny? I'm sure that the study would prove to be very informative in the context of this discussion. If the Navy makes a big stink about it, you can let everyone else know. "Here's the super-secret study that the U.S. Navy doesn't want you to know about..." Sounds dramatic, doesn't it? Pretty good publicity if you ask me. Alternatively, you could request that the Navy grant Pendergast permission to publish the results in a peer-reviewed journal. It would be highly unusual for a researcher to turn down the opportunity to get another publication on his CV.

not speculative at all, see http://www.scubaboard.com/forums/5085461-post34.html

That letter was written almost 17 years ago. If the other fin manufacturers feel threatened by research that old, then that's a sad commentary on the state of the advancement of fin technology. Heck, I bet that most of the fin models tested aren't even being sold anymore.

I find it shocking that manufacturers don't have a testing facility similar to what Pendergast used in the 2003 study. How are fin manufacturers testing their fin designs anyway? I guess it's probably easier and cheaper to just market the $#&% out of them with pseudo-scientific data. Disappointing from the point of the consumer...but probably not unheard of in the athletic equipment industry.

 

[scubafanatic]

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.

Last week I just picked up my FOURTH set of Apollo Bio Fins....just had to chime in here!

 

[fnfalman]

I think that the paddles do displace more water. For the life of me I do not understand the reasoning behind ships having propellers nowadays instead of the paddle wheels. Don't they understand that you need to have paddles to displace water so that you can generate propulsion, especially against currents?

 

[Blackwood]

I think that the paddles do displace more water. For the life of me I do not understand the reasoning behind ships having propellers nowadays instead of the paddle wheels. Don't they understand that you need to have paddles to displace water so that you can generate propulsion, especially against currents?

Where do you get the idea that blade fins:split fins:addle wheelropeller is in any way an apt analogy?

I value any substantial input you, as an engineer, may have. Do you have some insight into how split fins operate like propellers? Nobody in this thread (including at least two split fin manufacturers) seems to be able to explain that idea.

If you're just trolling, please don't.

 

[fnfalman]

Where do you get the idea that blade fins:split fins:addle wheelropeller is in any way an apt analogy?

I value any substantial input you, as an engineer, may have. Do you have some insight into how split fins operate like propellers? Nobody in this thread (including at least two split fin manufacturers) seems to be able to explain that idea.

If you're just trolling, please don't.

They don't work like propeller per se, however they do work quite well. The analogy to propeller is that the propeller doesn't displace water the way a paddle wheel does, yet it works quite well. Well enough to displace paddle wheels from ships.

Technology moves on and new things get discovered and invented. I'm a chemical engineer and not a fluid mechanics engineer, so I'm not the one who would be calculating Bernoulli's equation or Stoke's equation all that fluently. But if splits were not to work, then none of the divers who use splits would have been going anywhere underwater.

If the mock-up to believed then there's a pressure gradient building up with the Delta P being at the greatest magnitude at the tips of the fins. But unlike an airplane wing, where the Delta P lifts the plane up, this Delta P is directed rearward into propulsion.

 

[Blackwood]

They don't work like propeller per se, however they do work quite well. The analogy to propeller is that the propeller doesn't displace water the way a paddle wheel does, yet it works quite well. Well enough to displace paddle wheels from ships.

When I read the propeller posts in conjunction with a mention of lift, I think of airplane propellers. They are fundamentally different. Split fins are not. That's where, to me, the analogy breaks down.

But if splits were not to work, then none of the divers who use splits would have been going anywhere underwater.

Of course not. Note that this thread isn't intended to show that split fins don't work. That would be ridiculous. It's to explore HOW they work, and the validity of some of the marketing language.

If the mock-up to believed then there's a pressure gradient building up with the Delta P being at the greatest magnitude at the tips of the fins. But unlike an airplane wing, where the Delta P lifts the plane up, this Delta P is directed rearward into propulsion.

I don't believe the mockup, but if delta-P were directed rearward, you'd be going backwards :mooner:

 

[Mike Boswell]

Discussions of sails and wings and fins and propellers are often accompanied by descriptions of the well-known Bernoulli principle, showing how an airfoil with a flat bottom and a curved upper surface generates lift due to the higher airspeed and reduced pressure on the top of the airfoil.

This principle is taught in schools and colleges and is well known generally by the public at large. So, when an aerospace engineer friend of mine asserted that this well-known principle is wrong, I was skeptical but I decided to look into it for myself. Who, I asked myself, would have the final word on how an airfoil really works? Maybe NASA? For those here with a scientific interest, here is what I found:

Bernoulli and Newton

 

[lmorin]

According to one of the tests mentioned above, splits were turned into paddles by closing the gap with duct tape. One could also run this test backward by cutting a pair of paddles down the middle thereby turning them into splits. So, what does that prove? Only that splits are not special--they are just a variant on the basic paddle. Instead of being more or less rigid across the breadth of the fin, splits consist of two "demi-fins", each rotating slightly on a lateral axis in addition to moving up and down with each kick.

Now, my wife uses Mares Volo Race full foots that seem unbelievably floppy when she kicks. So floppy that other divers who have watched her swim have been horrified. Nevertheless, she has no difficulty keeping up with anyone else with other fin types regardless of water conditions. The point is, the splits have a built in "floppiness," too, by virtue of the split. Instead of the floppiness being 100% up and down, as it is in the Volos, splits have both up and down plus medio-lateral floppiness. The axis of the medio-lateral floppiness is along the stiff outside edge of the fin.

It seems to me that as long as a split fin conserves the energy imparted by the swimmer and then displaces water in the direction opposite to the swim orientation with the same efficiency as the paddle fin, then the two types should perform equally. However, given that some of the split fin kick energy is turned from an up/down stroke into medio-lateral rotation, it would seem to me that average splits would not transmit the energy as efficiently into forward motion as a paddle fin.

Also, a critical part of any swimming efficiency analysis is the amount of drag created by the fin in question. It seems to me that any fin with a split would have more drag than one that without, simply because of the small currents/eddies coming from the two edges of the split in the fin. Most fins have little devices on them that, in all likelihood, increase drag rather than decrease it, despite what the manufacturers say. This includes buckles, water guidance channels or little fins, and especially the rather large empty space around the foot pocket on the underside of most open-heel fins. If someone/company actually wanted to corner the fin market, they'd test all available fins with an artificial foot and leg under standard laboratory conditions and end all this "discussion" once and for all. Right?