ajduplessis
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Friends dont let friends use SF
Split Fin Bashing?
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I'll respectfully disagree with one major point of this fine post. Splits most certainly CAN do specialty kicks like helicopter turns and frog kicks (and I'm told back kicks--I can't do these yet in any fin). They aren't quite as effective at these techniques but they do work.
I agree with the rest of your post.
As the kids say "hater's gonna hate" and as has been said in this thread "it's the internet" people always yell more stridently when they aren't face to face, for some reason.
On any given recreational charter boat, you're going to find all kinds of different fins. Use the ones that work for you and let everyone else have their fun too. Win win.
I'm a professional, graduate-trained, fluid dynamicist. Think "aeronautical engineer" without all the airplane stuff...just the fluid flow over wings and other objects. Here is how i believe split fins work, and why they can work well in restricted conditions.
Your normal paddle fin just pushes water, and you go the other way. Pretty simple idea. What makes some work better than others is the relative flexibility and springiness, which depends on your legs and ankles...so some work better for some folks than for others. There may be also advantages of shorter/wider, or longer/thinner, depending on the kind of diving and kicking/kicks. The disadvantages are that the power stroke is really only in one direction (when your foot is moving forward), so any drag on the fin when returning your foot to the power position (like ready to kick a football) is power/energy lost....hence vents, weird shapes, odd springs, etc, to try and maximize the power stroke and minimize the losses on the return stroke.
The split fins do push a little water around, but not much. They tend to work on a very different fluid dynamical principle: vortexes. One relentless principle of things moving in fluids is the "starting vortex" and the "ending vortex", both of which are usually just pure losses of energy....your effort has gone into spinning the water rather than moving it in the opposite direction to which you are trying to travel. These vortexes are spun off the surface as the boundary layer builds and attaches (at the beginning of the movement) or is thrown off as the movement ends. With a paddle fin, and a big, slow kick, you spend a lot to time pushing water, and only make a starting vortex and an ending vortex at the beginning and end of each occasional kick. The trick with the split fin is that two, smaller vortexes are spun off (one from each half of the blade) on each start/stop motion,,,,and these two vortexes are counter-rotating, so push some water between them. With each kick with a split fin you thus get a little bit of water being pushed, and also some counter-rotating vortexes to push some more water...and both effects help you move forward. To maximize your speed, you need to generate a lot of vortexes, hence you make many, fast, small kicks. Lots of flutter, lots of speed forward. A great big wide kick with a split fin inefficiently pushes water, and inefficiently uses the counter-rotating vortexes. Again, the forward kick allows the fin to bend more, so more vortex generation; the return kick has little of this.
The patent for split fins is not based on this concept, of course, but on "nature's wing," the idea that each half of the blade slicing through the water at an angle generates lift in front of it and a pressure void behind it, so the lift causes the fin to go forward. My problem with this explanation is that the lift does not develop until the boundary layers are developed...which means the vortex has been shed. The whole wing idea is a steady-state flow idea; fins are in a constantly accelerating/decelerating flow....just not the same.
So there you have it. Split fins can work well if they are fluttered rapidly, which means a small but fast cycle, because you want lots of vortexes being shed per unit time. Paddle fins can utilize big, wide kicks if needed, with reasonable efficiency.
I own several brands of split fins and paddle fins, and have never been able to do a decent frog or back kick with the splits.
Your normal paddle fin just pushes water, and you go the other way. Pretty simple idea. What makes some work better than others is the relative flexibility and springiness, which depends on your legs and ankles...so some work better for some folks than for others. There may be also advantages of shorter/wider, or longer/thinner, depending on the kind of diving and kicking/kicks. The disadvantages are that the power stroke is really only in one direction (when your foot is moving forward), so any drag on the fin when returning your foot to the power position (like ready to kick a football) is power/energy lost....hence vents, weird shapes, odd springs, etc, to try and maximize the power stroke and minimize the losses on the return stroke.
The split fins do push a little water around, but not much. They tend to work on a very different fluid dynamical principle: vortexes. One relentless principle of things moving in fluids is the "starting vortex" and the "ending vortex", both of which are usually just pure losses of energy....your effort has gone into spinning the water rather than moving it in the opposite direction to which you are trying to travel. These vortexes are spun off the surface as the boundary layer builds and attaches (at the beginning of the movement) or is thrown off as the movement ends. With a paddle fin, and a big, slow kick, you spend a lot to time pushing water, and only make a starting vortex and an ending vortex at the beginning and end of each occasional kick. The trick with the split fin is that two, smaller vortexes are spun off (one from each half of the blade) on each start/stop motion,,,,and these two vortexes are counter-rotating, so push some water between them. With each kick with a split fin you thus get a little bit of water being pushed, and also some counter-rotating vortexes to push some more water...and both effects help you move forward. To maximize your speed, you need to generate a lot of vortexes, hence you make many, fast, small kicks. Lots of flutter, lots of speed forward. A great big wide kick with a split fin inefficiently pushes water, and inefficiently uses the counter-rotating vortexes. Again, the forward kick allows the fin to bend more, so more vortex generation; the return kick has little of this.
The patent for split fins is not based on this concept, of course, but on "nature's wing," the idea that each half of the blade slicing through the water at an angle generates lift in front of it and a pressure void behind it, so the lift causes the fin to go forward. My problem with this explanation is that the lift does not develop until the boundary layers are developed...which means the vortex has been shed. The whole wing idea is a steady-state flow idea; fins are in a constantly accelerating/decelerating flow....just not the same.
So there you have it. Split fins can work well if they are fluttered rapidly, which means a small but fast cycle, because you want lots of vortexes being shed per unit time. Paddle fins can utilize big, wide kicks if needed, with reasonable efficiency.
I own several brands of split fins and paddle fins, and have never been able to do a decent frog or back kick with the splits.
It escapes me why you ↑↑ would have any split fins. As for those of you that are sure that you have mastered the split ... you're still messing up my hair.
victorzamora
Contributor
Tursiops, I liked a lot of your post. The fluid dynamics I enjoyed. "Vortexes" was like nails on a chalkboard, though, considering it's "Vortices" but I'll let that one slide 
One more thing you missed was that paddles also produce two small vortices. Paddle fins even have little winglets (they're stiffeners, really) that help. I dive OMS Slipstreams, for instance. They have three holes so that on a "downstroke" you can get an energized boundary layer and maintain a more laminar flow over the top. Yet, due to pressure differences, you still get two counter-rotating vortices.
I don't know if I still have the video, but I caught my wife in bad trim at Vortex Springs (awesome, right?) and you could clearly see the vortex shedding....one off of each edge of each fin. I didn't get a video of it, but the same could be seen in the haloclines in Mx.
ETA: Splits and paddle fins work on the same principal. Those vortices are a direct result of the creation of lift. The difference between splits and paddles is that the paddle fin can be thought of as a each being low-AR wing with a chord line parallel to your body while splits can be thought of as each having two high-AR wings with a chord line perpendicular to your body. Either way, pushing water around is still just "lift" and they're both caused by the same principal.
You're right. You can move your legs and feet in the motion of a frog and helicopter turn while in split fins. However, the absolute lack of any kind of propulsion, control, or results for your effort make them utterly useless AT those kicks. So, you can "do" a frog kick in splits.....but the splits won't "do" a frog kick in that you get nothing useful for your effort. For that reason, I believe that saying "splits can't do frog" is an accurate statement for the VAST majority of split fins.
One more thing you missed was that paddles also produce two small vortices. Paddle fins even have little winglets (they're stiffeners, really) that help. I dive OMS Slipstreams, for instance. They have three holes so that on a "downstroke" you can get an energized boundary layer and maintain a more laminar flow over the top. Yet, due to pressure differences, you still get two counter-rotating vortices.
I don't know if I still have the video, but I caught my wife in bad trim at Vortex Springs (awesome, right?) and you could clearly see the vortex shedding....one off of each edge of each fin. I didn't get a video of it, but the same could be seen in the haloclines in Mx.
ETA: Splits and paddle fins work on the same principal. Those vortices are a direct result of the creation of lift. The difference between splits and paddles is that the paddle fin can be thought of as a each being low-AR wing with a chord line parallel to your body while splits can be thought of as each having two high-AR wings with a chord line perpendicular to your body. Either way, pushing water around is still just "lift" and they're both caused by the same principal.
You're right. You can move your legs and feet in the motion of a frog and helicopter turn while in split fins. However, the absolute lack of any kind of propulsion, control, or results for your effort make them utterly useless AT those kicks. So, you can "do" a frog kick in splits.....but the splits won't "do" a frog kick in that you get nothing useful for your effort. For that reason, I believe that saying "splits can't do frog" is an accurate statement for the VAST majority of split fins.
Tursiops, I liked a lot of your post. The fluid dynamics I enjoyed. "Vortexes" was like nails on a chalkboard, though, considering it's "Vortices" but I'll let that one slide
LOL! I agree with you...but my spell-checker doesn't. I let it win this time.
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Apparently, you haven't tried doing frog with Subgear's XP splits. No, they're far from ideal for that job. OTOH, they're also far from "utterly useless" for the job. Visit us, and I'll be happy to show you that it's absolutely feasible to do frog kicks in splits (though I prefer paddles for that job).
My biggest complaint about split fins is that they work very poorly to stabilize the diver on the roll axis. They simply don't have enough resistance -- which, of course, is why people like them. The lack of resistance makes them very easy to kick, and that's nice for people who don't have very strong legs.
But new diving students often struggle with the tendency of the large "keel" they have on their back to roll them over. This is especially bad if they are in shop BCs that either don't fit well or don't stabilize the tank firmly (which is true for the majority of them). With paddle fins, the feet can easily serve as horizontal stabilizers, and the diver really doesn't even have to think much about it. I have repeatedly seen that, when I take the splits off students and put them in my paddle fins, the hand-waving diminishes or even stops, because they are much more stable.
If a certified diver, after class, finds he is having a lot of trouble with kicking his paddle fins and wants to change to splits, I don't have a problem with that. But I have a real problem with shops putting OW students into splits. I know they do it because the profit margin is much higher, but it does the students no service at all.
But new diving students often struggle with the tendency of the large "keel" they have on their back to roll them over. This is especially bad if they are in shop BCs that either don't fit well or don't stabilize the tank firmly (which is true for the majority of them). With paddle fins, the feet can easily serve as horizontal stabilizers, and the diver really doesn't even have to think much about it. I have repeatedly seen that, when I take the splits off students and put them in my paddle fins, the hand-waving diminishes or even stops, because they are much more stable.
If a certified diver, after class, finds he is having a lot of trouble with kicking his paddle fins and wants to change to splits, I don't have a problem with that. But I have a real problem with shops putting OW students into splits. I know they do it because the profit margin is much higher, but it does the students no service at all.
If split fins have a superior mechanical advantage, I wonder why more fish don't have split fins. It seems to me that it would have been a pretty good evolutionary advantage to be faster and use less energy than predators or prey.
During a DEMA show in Orlando, a dive shop owner friend of mine got us tickets to dive Disney's The Living Seas. In order to help maintain a stable environment for the fish diver must wear the equipment Disney provides. When I dove it they used split fins. I tried doing the frog kick, the modified frog kick, the modified flutter kick, shuffle kicks, helicopter turns and backward kicks and the performance of split fins was sub-par compared to ScubaPro Jets. I couldn't backward kick which I can do without fins as well as when wearing paddle fins. I know a girl cave diver who wears splits and can backward kick, but her kick isn't at all pretty as she seemingly forces the fins to obey and reverse.
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