Split Fin Physics

[meesier42]

@meesier42: Could you please disclose your relationship with Force Fin and Bob Evans? Are you an employee of the company? Have you done any consulting work for Force Fin? Are you or were you ever financially connected to the Force Fin company? Or are you just a happy costumer? Sorry for all of the questions. I'd just like to gain some perspective on your comments. I do want to make it clear that I appreciate your participation on this thread. Thanks for filling in some of the gaps in our knowledge.

I am just a friend of Bob Evans, nothing financial involved. The only reason I started using the fins was doing my own testing and many years later got the chance to know him better.

I get annoyed when something purely marketing is cited as science, specifically when it has been shown to be completely false or defames the name of a friend. I really don't want to completely highjack the thread into a ForceFin verse Split Fin. My issue is with the "mdb" aka Micheal Brennan coming on here citing his marketing information as original work, when his company stole not only the concept, but the marketing from someone else.

the interesting thing about splits is that they do work, in spite of their limitations. To the best I can tell, they work just like any other fin. The "blades" simply flex with much less force and a much larger pivot. Where a standard paddle fin flexes at a point generally 2-4 inches beyond the toes and flex perpendicular to the long axis of the body. Splits' flex parallel to the body along the lateral ribs and when combined with the high offset angle (blade is significantly canted away from the foot) this establishes a high angle of attack with less strain on the ankle. The mechanics are still no different than any other "plank" in the water, the "plank" is pushed through the water at a high angle of attack which forces the water away, the angle of attack with pressure on the face of the plank pushes the diver forward. The reason splits seem so easy to kick is because they don't move very much water. The angle each sides of the fin achieves sends so much water through the split and it just wraps around the back side of the blades and stagnates, very little water actually moves down the blade, much less off the end.

However, to the best of my knowledge, to quantify their function into equations has not been done, I have never even seen a qualitative water flow analysis beyond the bits of string that I have glued to the blade. I know that Mares has a "human leg simulator", but I don't believe they publish any of their information for peer review. I know enough about fluid dynamics, mechanical statics and dynamics to know that even just modeling a fin is extremely complex. As there is nothing linear about a fin, the spring rate isn't consistent and isn't even constant for a single point. I have spoken to some researchers and was told they have attempted to characterize a single fin model using a finite element analysis using video and some expensive software and they ran into so many non-linearities and dependent variables they gave up before they spent too much money/time in an effort they realized would would be such a huge over simplification that it would no longer describe reality or be useful to examine other designs. There are lots of equations that predicts average drag and thrust, but they are predictions and as far as I know are still being refined. (See UofBuffalo study)

all that said, the Foil Force Fin seems to work better, better may be the wrong word- differently may be more appropriate as I have no real way to quantify it as better, I bring this up because it is the ONLY split fin (please correct me if I am wrong) on the market that doesn't use the "Natures Wing" umbrella, and thus it seems to be quite different than the ones that do. when I glued strings to this fin, they stayed pointing at the blade tips, vice the Apollos pointing perpendicular to the split/tips. The direction of the strings indicates direction of water flow, and infers that water moving towards the blade tips is providing thrust. The interesting thing in this experiment was that the Apollo the strings on the back of the blade just wavered in all directions, to me that indicates turbulent water from either a "stall" condition or significant wing tip vortecies. On the Foil Force the strings on the back of the blade mostly pointed towards the tips, the only closest to the bottom of the foot waver a lot (although that didn't surprise me, all the turbulence off the foot). I realize its not scientific or peer reviewed, but interesting nonetheless.

 

[Puffer Fish]

For what it is worth, I agree with Blackwood's vector analysis. Again, FWIW, I did well in fluid dynamics courses at university. In Blackwood's analysis, the net effect of the airplane-like fin shape will have the effect of resisting the deflection of the blade as it pushes against the water. Three things should be noted: (1) the effect is likely to be negligible owing to the relatively slow rate of flow parallel to the camber surface; (2) the same effect can be achieved by simply using a stiffer material; and (3) the effect of turbulence, which has been neglected, is likely to be substantial.

mdb's "explanation" when you strip out all the praise of the inventor, amounts to "fluid dynamics work to create the forward thrust" which is akin to saying "mathematics works to put money in my bank account" - the argument, as presented by mdb, is rubbish.

Now, split fins may work for you, or they may not. If they do - great. To date I have not heard any compelling scientific reason for their use. What I have heard, to date, is pseudoscience.

Note: I find that split fins simplify my choices when pairing up with insta-buddies on dive boats.

Asking someone to explain the hydrodynamics of any fin is somewhat silly. What makes one fin more efficient than another involves some very complex physics. Add to that the different strokes required and I doubt that anyone in a simple post could "accurately" describe all of the details.

What is known, is that Apollo fins have been extensively tested in both the US and Europe...using speed, air usage and thrust and to my knowledge, every testing has shown that they are more efficient, faster and produce more thrust (although there is an inherent compromise between thrust and maximum speed).

What surprises me, is that after all of this, there are still people that believe this is somehow just marketting...

 

[Bubbletrubble]

What is known, is that Apollo fins have been extensively tested in both the US and Europe...using speed, air usage and thrust and to my knowledge, every testing has shown that they are more efficient, faster and produce more thrust (although there is an inherent compromise between thrust and maximum speed).

What surprises me, is that after all of this, there are still people that believe this is somehow just marketting...

@Puffer Fish: The results of Pendergast et al. (UHM 2003 30:5571) are not consistent with your statement. Please download the journal article and refer to the figures for more detail:

• Fins included in the study: Mares Attack, Apollo Biofin, Apollo Biofin with center channel taped up with duct tape, US Divers Blades, Scubapro Jetfins, Mares Avanti Quattro, Oceanic Ocean Pro, US Divers Compro
• Table 2 is interesting. It shows that, compared to the rest of the fins, the Attack, Apollo, and Jets have the lowest O2 consumption values at a velocity of 0.9 m/s. For the record, Jets had a lower O2 consumption than Apollo. This is particularly noteworthy because the aforementioned three fins have dramatically different designs. Attack and Jets are rigid paddle fins, whereas the Apollo is a splitfin.
• Figure 2 is a mess (IMO). It should have been omitted from the article. There are many inconsistencies between the key, figure legend, and text. Shameful really. Fortunately, it's more of a summary figure. There's no new data here...just a re-organization of the data already presented -- this is considered poor form by the way.
• Table 4 shows the relationship between kick frequency and velocity. Also represented is the distance traveled per kick at max velocity. The Attack has the highest max velocity, whereas Apollo and Jets showed slightly lower max velocities. Apollo had the lowest distance traveled per kick at max velocity. The trend observed was that swimmers using more flexible fins had a higher kick frequency but lower distance traveled per kick. This makes sense because the flexible fins typically showed lower thrust.
• Table 5 shows thrust data, the duration of the various finning phases (transition, recovery, power), and Froude efficiency numbers. Jets and Quattros have the highest average thrust. Apollo showed the highest Froude efficiency. Interestingly, taping up the central channel in the Apollo turned the fin into the least efficient by far.
  There are at least two errors in the text relating to Table 5. Not sure what to make of that. I'm going to assume that the data in the table is correct and the text is wrong. Someone really should have proofread the work. :shocked2:
• Froude efficiency: Read this abstract for a brief explanation of how Froude efficiency is calculated. If you have institutional access, you can download the entire PDF. Unfortunately, this paper is not included in the Rubicon Repository. In brief, Froude efficiency is the percentage of the external work of aquatic locomotion contributed by the thrust force that is needed to overcome drag -- yeah, I realize it's a little wordy. Quantitatively, Froude efficiency equals (c+v)/2c, where v is the measured forward velocity and c is the "wave speed" estimated by kinematic analysis, where vertical displacement of hip/knee/ankle in a flutter kick is charted with respect to time using video capture. Yup, Froude efficiency estimation by this method only works for the flutter kick.

I highly recommend that you read the Discussion section in its entirety. After you read the whole thing, go back through the Figures and compare the Apollo Biofin data to the data generated from the Biofin with the center channel taped together with duct tape. I thought this was a very clever way of attempting to isolate the effect of the center channel in a splitfin.

In summary, in the above Pendergast study which looked at 8 different fins:
Are the Apollo Biofins most efficient with respect to O2 consumption? No, the Scubapro Jetfins are most O2 consumption-efficient.
Do the Apollo Biofins have the highest Froude efficiency number? Yes, they do.
Are the Apollo Biofins fastest (have the highest maximum velocity)? No, the Mares Attack fins are fastest.
Do the Apollo Biofins produce the most thrust? No, the Scubapro Jetfins did.

 

[TSandM]

Nothing really to add to the thread, except to thank the posters . . . this has been one of the more informative threads on fins that I have read.

 

[T. Bix]

I have been lurking these arguements for some time and want to thank most of the contributors, some of this is informative ..... some of it is amusing, some of it is shameful but all of it is useful.

Note: I find that split fins simplify my choices when pairing up with insta-buddies on dive boats.

Well if a "scientist" can make such a judgement then that should be good enough for everyone. I have fins that work for my purpose and for cruising the quarries or reefs I like my choice of fins. There are other fins that may be a better choice for tight and confined spots ie: backing, stopping and pivoting. But they are just that "my choice" and they work well for me. I find it interesting that this discussion is continually being rehashed and always ending in something that sounds like I'm rubber and you're glue...... nyah nyah nyah.

 

[Blackwood]

Asking someone to explain the hydrodynamics of any fin is somewhat silly.

I disagree, particularly when hydrodynamics is a major selling point.

What makes one fin more efficient than another involves some very complex physics. Add to that the different strokes required and I doubt that anyone in a simple post could "accurately" describe all of the details.

Sure, no one is going to write state equations for fins. However you could show qualitatively via an isolated variable experiment that there is an additional force associated with the split fin.

Note that wasn't really what I was after. I'm looking for "am I wrong, and if so how/where and how do you know?" The correct answer would be "this math" or "this study" or anything other than patent material that supports the suggestion that lifting fin blades propel the diver.


I'd be highly satisfied with putting a split fin on a sting balance and running it in a water both with the splits free and with them fixed together (in essence the same fin less the inward-rotating blades). If you could build a mechanism to kick the fin (rather than just flow water over it in an appropriate orientation), so much the better.

What is known, is that Apollo fins have been extensively tested in both the US and Europe...using speed, air usage and thrust and to my knowledge, every testing has shown that they are more efficient, faster and produce more thrust (although there is an inherent compromise between thrust and maximum speed).

What surprises me, is that after all of this, there are still people that believe this is somehow just marketting...

That's not what I was saying. I'm asking if their explanation for the increased performance holds water when of mine or meesier42's or a combination of both seems far more plausible.

Here's a link to the 2003 UHM paper by Pendergast et al. (Univ. of Buffalo).
I'm providing this link so that others will view the data and can decide for themselves whether the authors' conclusions have merit.

Thanks!

 

[redacted]

Perhaps it might help to look as some more basic fins to understand differences in how they perform. These are swimming fins, not scuba fins: FINIS - Zoomers Fin Technology

Basically they are the same mold with different materials. The red is stiffer (less flexible) and the blue is softer (more flexible).

I think it is basically a matter of how much force you can store in the fin at high speed and at low speeds, how much effort it takes to load those fins, and what happens to the excess force you apply that the fins can not immediately use or store. Of course, all this is dependent on the kick style being used.

The only wings I see are in the advertisements.

 

[mdb]

My issue is with the "mdb" aka Micheal Brennan coming on here citing his marketing information as original work, when his company stole not only the concept, but the marketing from someone else.

Meesier: This is simply not true. The licensing agreement with Nature's Wing had been signed a year earlier @ The Orlando DEMA show. Mr. Hattori was not at the Los Angeles DEMA show you describe. He stayed in Japan and was working full time on designing, refining and testing the bio-fin.

 

[Blackwood]

Guys, please hold the personal stuff back (or use the PM functionality). It's cluttering the thread.

 

[mdb]

Blackwood:

Will do.

Back to the physics. You started a good thread with lots of good input so far. I hope it continues in the spirit with which it started. As TS&M said this has been one of the better threads on fin performance and characteristics that I have seen on this board.