Split Fin Physics

[HenrikBP]

He said that I was a classic paddle fin swimmer, with fairly wide strokes. He said that the split fins were best with short strokes within the dimensions of the diver's wake.

To that point - with my *very* limited experience with fins: I started out using a pair of Atomic Aquatic split fins, and found that a short but slightly quicker stroke seemed to work better than a longer traditional flutter kick. Just an anecdotal point of reference.

Henrik

 

[ptyx]

I was disappointed that the standard Jets didn't make it into all of the tests, but I was amazingly struck by the fact that they came out LOWEST in static thrust.

Jim's test, and most of this thread are based on sustained flutters ("shallow, fast kicks" for the sustained thrust). Which is definitely not where Jets and clones shine.
The 'jerk thrust frogkick then rest and glide' style which seems to be favored by divers with heavy gear is a completely different situation. I would be surprised if a single style of fin could be equally efficient at all jobs.

(It doesn't diminish the usefulness or relevance of those studies - one still has to understand the tools before being able to select the right tool for the job. And this thread split=scoop conclusion is the best explanation of split fins I've ever heard.)

Moving heavy gear is one of the primary reasons why people say they use the Jets, and that tiny piece of information would suggest we're all deluded.

It wouldn't be the first time divers are deluded, but I would reserve judgment on that. Noticeably missing from the picture is the efficiency of a 'jerky' frog kick when pushing large amounts of gear compared to a sustained and smooth flutter, and maneuverability at zero speed.

 

[Puffer Fish]

Thanks for the link to Jim's article -- I hadn't seen that one before.

I was disappointed that the standard Jets didn't make it into all of the tests, but I was amazingly struck by the fact that they came out LOWEST in static thrust. Moving heavy gear is one of the primary reasons why people say they use the Jets, and that tiny piece of information would suggest we're all deluded.

Possibly, but not necessarily with regard to the fins.

Jim;s article, at the end does address issue of "feel" and "belief" versus actual numbers. That a fin might feel more effective is not delusional, if it actually feels that way. That reality might be different is a very different issue.

I do a lot of pool swimming, and own a lot of fins....as I find doing laps something like what a gerbil is doing, I entertain myself by doing different strokes and timing things.

I am slowest with a frog kick with apollo fins (but fastest with a flutter), and up until a month ago, jets were fastest. Strange as it may seem, I got a pair of Mares Raptors on sale (I tend to wear fins out over time in the pool and save my apollo fins for diving), and was shocked at how much faster they are with a frog kick... and how hard they were to do a flutter with. These are big, curved, stiff fins...and the split deflects more in the back direction than in the forward...

So my speediest flutter kick fins are:

Apollo yellow's......Tusa split fins...Mares split fins (but tough on the legs)..jets...the fold up fins and some generic paddle fins I wore the name off of.

With a Frog kick it is:

Mares split fins...Jets...fold up fins...generic paddle fins....tusa split fins and Apollo.

I also don't think that maximum speed or maximum thrust is the reason most people buy fins....and the power curve with each fin may be completely different.

Love his report though.. great work.

Now, regarding that Borg cupcake.............

 

[mdb]

John:

Thank you for your latest tests and reports.

I was at Healthways in the early 70's. You have a classic double hose regulator with your SCUBA DELUXE. My first custom wet suit was a White Stag. Noel Brown did the measurements.

We are having a fairly good discussion here about the performance of split fins, and how they work. We have a few yapping dogs, but, for the most part a very civil discussion.

I had an e-mail from Jim Grier today. He is busy and not all that interested in the back and forth that is typical of any internet forum.

Jim did say he would take a look at the thread and contribute when he felt it was appropriate.

Your unbiased and thoughtful input is helpful and appreciated.

 

[Lepomis]

Hi all,

(Note: edited to add links that I couldn't post until after I had made a minimum of 5 posts on ScubaBoard.)

I haven't been a member here or posted on ScubaBoard previously, although I occasionally browse this board and am familiar with it. I first got involved with online dive message boards at scubadiving.com, where I've been posting under the same user name for years, and simply haven't had time to get involved with other sites.

I was alerted to this split fin physics discussion and that is what brought me here and prompted me to add my two cents worth.

It's been interesting seeing the (rather long!) thread and some familiar names and faces, including coldwaterlloyd and mdb. Yes, Lloyd, I also have great, fond memories of our diving on Vancouver Island! We need to get out diving together again one of these days! Thanks to mdb for posting the link to my fin testing results. And thanks to Blackwood for starting a great thread.

Anyway, here's my input for the current thread. I'm a fan of Newton's third law of motion and, like some of the others who posted above, believe that is the primary explanation of fin physics (both paddle and split fins). Here's what I wrote about it, "How fins work", three years ago in a post on scubadiving.com:
http://divertodiver.scubadiving.com/m.aspx?m=25790

The link that mdb posted for my fin-testing reports is an older, somewhat outdated one. A more recent central gateway to my tests is at:
http://www.ndsu.edu/pubweb/~grier/fins.html

In response to replies above about regular Jets, I have further tested them, incidentally, by trying several pairs through the years. I got highly variable results depending on the particular pair being tested, which in turn appeared to vary with the batch of rubber that they were made with (the regular Jets have been made over a long period, in different manufacturing locations including both US and Mexico, and with many different batches of rubber). Depending on the variability in materials, some of the Jets I tested were among the best fins I've ever tested and some were among the worst, with most being somewhere in between!

I posted UW photos of some of my gear testing techniques, including the UW speedometer with air-integrated computer (Suunto Cobra) and stopwatch, with a report on tests I ran of the Halcyon backplate/wings, but that report is currently down with scubadiving.com's archives, which are dead. I might repost some of that later, but don't have time at the moment.

My latest fin-testing report, for the ScubaPro Seawing Novas, Twin Jet Max, and Aqua Lung SlingShots is at:
http://divertodiver.scubadiving.com/fb.aspx?m=153871

Anyway, I gotta run. Will try to check back later ... but not sure when.

DSAO -- with whatever fins work best for you, regardless of their physics
Jim

 

[mdb]

Not sure why you were "censored by ScubaBoard" but nice to see your contribution. In any case, hope your links are posted before long.
If you send them along to me I will see if they can be posted. Good to see you are still actively testing the newest style fins.

 

[HenrikBP]

I don't think the link thing is censorship; in order to minimize spam bots as well as human spammers that will "sign up" for a forum and spew a bunch of garbage posts with links for various product and services (you know what I mean), many BBS softwares will not allow a user to post links or pictures until after a certain number of posts.

Henrik

 

[mdb]

HenrikBP:

Thank you for the info.

I hope Jim will post a few more times so we can take a look at his latest tests. Jim is a true Academic and researcher and it should be interesting to see his take on the newer model fins.

 

[Lepomis]

I agree that "censorship" of the URLs wasn't correct. The system just wouldn't let me post them until after 5 posts. It frustrated me at the time because I was late for an appointment, in a rush trying to submit the post, and it just kept coming back with the error message and wouldn't take the post with any form of link or link-like material included. Sorry that my frustration came through.

I was going to have someone else post them for me, but I figured that would only scatter the information and a simpler solution would be to just get in my mandatory 5 posts and then go back and edit the first one to re-insert the links. I apologize for wasting space with the dummy filler posts, but at least it worked and my main post could then stand on it's own with everything, including the links, present.

[Update: I had added more filler posts to reach the minimum of 5. The additional ones were then deleted by TSandM who helped correct the initial link-posting problem so the links could be inserted. Thanks TSandM!]

DSAO -- and thanks to all for your patience with my being a ScubaBoard newbie
Jim

 

[John C. Ratliff]

Lepomis,

I'm really glad you are here. There is a lot to talk about when it comes to fin hydrodynamics, and you can provide us with some good information. I've been looking at fin design for thirty years, and your ideas about water flow are along the same lines (like my string experiments in the 1970s) as mine.

Today, I was in the pool again, but not to get any numbers. I have bought a pair of used Force Fins, and wanted to try them beside my scoop fins. They will take some time to get used to for me, as they have a very different kick. I do know that their foot pocket is very uncomfortable with only a sock on, so I may try a neoprene bootie noxt time.

I also could not very well compare the Force Fins against my scoop fin, as I brought one scoop and one split fin to the pool. I found that they work well together though.

Also, I bought a pair of Farallon fins with the lower leg support off E-Bay for a nominal amount. I had a pair when they first came out, but they were really bad because the support would not allow the fin to be oriented correctly to push against the water. This is another case of a fin whose engineering principal was great, but the advantage was lost in the manufacture of the fin--lack of attention to details. It has another problem in that the rubber was of two different compounds, and over time the one has separated from the other, resulting in a broken rib on both fins. I plan to try adjusting the support to allow the extension needed for the blade, grind off the current blade and attach another of my choosing. It should be an interesting project, as one major limitation to pushing the fin is the strength of the ankle.

Anyway, I am happy to see you here, and we can have some more discussions.

SeaRat