Pervasive Fallacy about Split Fins

[Teamcasa]

Originally Posted by Lobbster
splits are for people who cant use proper fins, eg. small women who dont have the leg muscles to use real fins. being a man i use cressi reaction's. there great fins.

What exactly is a "proper" fin? And what are you overcompensating for?

Now boy's stop fightin'

I too use the new Cressi Reaction Fins. They do work great, I have not tried split fins, they just look like broken paddles to me !

And as a DORK diver, I subscribe to the theory of WGAD when it comes to fins.

Dave
DDHF

 

[Temple of Doom]

I dont' really want to go too far into the silting qualities of splits vs fins, perhaps that's for another thread, but ClayJar's post is really interesting.

I will gladly concede that split fins may provide greater cruising thrust. It naturally follows, however, that there is increased potential for silting.

I'm pretty sure I know what you mean. You mean that with greater thrust, if you point that greater thrust at the ground, there will be more silt, right? This is certainly true. Of course though it doesn't mean at all that splits mean more silt, only the potntial for more silt.

While flutter kicking, if the only difference is the fin (same diver, same trim, same forward velocity), then with splits you're expending less energy to get that same thrust in the forward direction. It follows logically that with a paddle fin, to get that same thrust in the forward direction, you're expending more energy.

Where is the extra energy going in paddles? Why it's going up and down and left and right of course. This will lead to greater chance of silting, as more water is shooting away from the fin in unwanted directions. The more efficient fin will allow for the lowest amount of silting with the right technique, because you have to push less water to go where you want. This isn't disputable.

HOWEVER, That isn't to say that paddles will always silt more! I have no idea of the efficiency of frog kicks vs flutter kicks at slower speeds, I haven't seen any data. It could be very likely that frog kicks using paddles are more efficient than the flutter kicks of splits at low speed. Naturally frog kicks can't propel you as fast as flutter kicks, but they may be more efficient at slow speeds. If the frog kick were more efficient at caving speeds, then they would allow for the lower amount of silting at that speed. This isn't disputible either.
(There, now that the cavers have untied their long hose nooses, I can continue. )

In other words, could split fins simply be reducing the penalty divers would be paying for inclined trim, which results in less incentive for the divers to learn horizontal (non-silting) trim? If that is the case, it could be said that the "problem" is that the fins are too good for the inclined divers' benefit.

This is a really neat point! It may very well be true, and may be the reason that so many divers feel that splits are worse for silting (because they see so many people roto-ing the ground going the same speed as the padle guys). However, that doesn't mean that splits lead to more silting than paddles! It only mean that splits with bad technique silt more than paddles with good technique.

Craig

 

[waynel]

My science boils down to this. I'm 51 years old and the knees are not what they used to be. I'm in great shape for 51 but knees and back are often the downfall of even the best conditioned athelete. Split fins don't hurt my knees if I have to do some serious finning. Avoiding pain and maximizing pleasure becomes the number one criterion as you age. So, young guns...there will come a day when speed, etc. no longer matters.

 

[Charlie99]

Okay, here's an interesting idea for everyone to ponder and discuss:

• For a given amount of effort, split fins can provide more cruising thrust than paddle fins, or split fins require less effort to produce a given amount of thrust.
• When diving in inclined trim (i.e. upright or "leaning forward", as opposed to horizontal), to maintain depth while kicking, a diver must be negatively buoyant equal to the vertical component of the thrust vector. (Thrust_v = Thrust * Cos(Theta), where Theta is the inclination from horizontal)
• If the diver in inclined trim is kicking at all while maintaining constant depth, the diver must maintain constant thrust in order maintain constant depth.
• In split fins, the effort required from the inclined diver to maintain constant depth (by kicking to provide constant thrust) is significantly less than that required if paddle fins are used.
• Therefore, divers in inclined trim who are diving split fins may well have significantly less motivation to correct their inclined trim.
• Therefore, any perceived difference in the rate of silting divers between those wearing splits and those wearing paddles may be a secondary effect of the efficiency of the split fins.

In other words, could split fins simply be reducing the penalty divers would be paying for inclined trim, which results in less incentive for the divers to learn horizontal (non-silting) trim? If that is the case, it could be said that the "problem" is that the fins are too good for the inclined divers' benefit.

Here's an alternate explanation:

• Unlike paddle fins, split fins generate vortices that travel long distances.
• If a diver is horizontal, they don't cause a problem
• but if they are negatively buoyant and inclined to maintain depth, the vortex can hit and disturb the bottom even if the diver is several feet off the bottom.

I have seen a diver with split fins (unknown make and model) disturbing the bottom about 20' back and 7 or 8' down from him, so I know at least one variant of split fins sheds a very long lasting vortex.

 

[ClayJar]

I don't at all want to suggest that I know more about your experiences than you, but do you think the following perhaps better explains what you've noticed?

(If you're viewing this as a single post, go read the original, which I haven't quoted here, as I'm postulating a rather completely orthogonal idea.)

Upon thinking of what you wrote (and I had indeed thoroughly read your earlier posts, which were also quite interesting), I found something new to toss in to better illuminate the discourse:

Up until this point, most of my thinking for this thread had been centered on the "power stroke" -- i.e. the part of the kick in which a fin is thrust against or through the water in order to provide the greatest portion of the motive force of the kick. I imagine that is what most people think of when they think of the thrust of a fun. However, it's plain to see that this is not the only significant phase of the kick.

There is also the portion of the kick which immediately precedes the power stroke, in which the fin is moved to the position from which the power stroke is performed. I'm going to call this the "recovery stroke", as I haven't any more reasonable terms handy (feel free to change the terms).

Using this nomenclature, for a flutter kick, there are two power strokes and two recovery strokes. The major pair is on the fin's "down" motion (toe-side toward heel-side) motion, and the minor pair is on the fin's "up" motion (heel-side toward toe-side). For a frog kick, there is but one power stroke and one recovery stroke.

It has been mentioned that split fins excel at what I have termed the minor power stroke. Much less considered, judging by the discussions I've read, are the recovery strokes. Split fins provide much less "purchase" on the water, as has been mentioned ad nauseum in the many debates about thrust. However, that also means that there is significantly less resistance during the recovery stroke.

Consider, for example, cruising. The faster you cruise, the more flow there is past your fins. Split fins impede flow through the plane of the fin much less than paddle fins. The amount of force a paddle fin will require during the recovery stroke to properly position it against the water will become substantial well before it becomes an issue to a split fin.

Of course, as I pointed out, different kicks have different power and recovery strokes. For a small flutter kick, the recovery may be but a pivoting motion around the ankle, but what is the effect of diver-relative current on that motion? For a frog kick, on the other hand, the recovery may be drawing your feet toward you, pivoting the ankles, and positioning the fins more or less perpendicular to the oncoming current.

While the power stroke may me invariant with respect to oncoming current, the recovery stroke (or pre-power-stroke positioning phase, if you will) would seem highly likely to be considerably affected by forward velocity. Naturally, the confounding factors of fin and kick styles will cloud the issue, but perhaps this could account, at least in part, for what I seem to perceive.

I'm pretty sure I know what you mean. You mean that with greater thrust, if you point that greater thrust at the ground, there will be more silt, right? This is certainly true. Of course though it doesn't mean at all that splits mean more silt, only the potential for more silt.

[...clipped for space...]

However, that doesn't mean that splits lead to more silting than paddles! It only means that splits with bad technique silt more than paddles with good technique.

That was indeed all I was attempting to state with that post.

Here's an alternate explanation:

• Unlike paddle fins, split fins generate vortices that travel long distances.

I would love to get some splits into a "smoke tank" to be able to study that.

If I assume the flow off a paddle fin is more turbulent than the flow off a split fin, that would seem to support both concepts. The paddle fin may be less efficient due to the turbulence, and the turbulence is likely to dissipate in three dimensional space before it travels as far as shed vortices. (Following distances in aircraft are not based on the problems from general chaotic turbulence but on organized wake vortices.)

I have seen a diver with split fins (unknown make and model) disturbing the bottom about 20' back and 7 or 8' down from him, so I know at least one variant of split fins sheds a very long lasting vortex.

I must say that I have seen the same. (Actually, a funny-sad story -- my significant buddy's parents came out of the spring all upset that these guys had silted it all up. I'd been watching, and the cave divers didn't silt anything. It was the flow from their own fins. They never accepted that.)

 

[highdesert]

splits are for people who cant use proper fins, eg. small women who dont have the leg muscles to use real fins. being a man i use cressi reaction's. there great fins.

Tell me you're joking ...

Then again, I believe you're actually serious.

 

[MauiScubaSteve]

There's plenty of other reasons why they not be the better fin for any given situation, but refusing to believe the results confirmed by test after test is curious to me.

Craig

Look at the test videos from scubadiving magazine. The testers are racing and huffing and puffing and straining, hardly what skilled recreational divers do. With proper paddle or blade fin technique, I do not bend my knee much (I do not flex the calf muscles much). With less muscle flex I have less O2 usage by the body. The reason I refuse to believe the test results is because they do not conform to my own personal experience, in the ocean while recreational diving with splits vs with non-splits. If anyone, with all your dives or all your book learnin' choses splits, so be it. The beliefs of all the people will never all be the same, and test results are not the end all of any argument/discussion.

I suggest you hold out one hand, palm up. Now into this hand, wish for a world where everybody thinks like you. Hold your other hand similarly under your naked bum and see which hand fills up first.

 

[Temple of Doom]

I suggest you hold out one hand, palm up. Now into this hand, wish for a world where everybody thinks like you. Hold your other hand similarly under your naked bum and see which hand fills up first.

Honestly, I don't want everybody to think the way I do. Opinions are like arseholes, everybody has them, and they all stink (inluding my own). Naturallly everybody likes their own brew, hence the amount of hot-air being blown around all the time.

I'm not talking about subjective opinion though, I'm talking about objective fact.

If you can go faster/pull harder with splits by using the same energy as you do with paddles, then you're going to use less energy to go the same speed.

This isn't a matter of opinion, it's fact.

If you choose not to beleive that fact, then it's up to you, but whishing the fact away isn't going to make it untrue. Further if one were to make statements which contradict that fact, I and others would feel an obligation to correct the fallacy.

I encourage anybody to have whatever opinion they choose of split-fins, regardless of whether those opinions are based on fallacies or truths. However, spreading fallacies about anything in diving will result in a well reasoned, objective correction from me when I know the truth.

Craig

 

[MauiScubaSteve]

:mooner: Did I miss something, where is the objective energy usage data. It would be hard to argue the fact that a number of tests have crowned splits as faster with the bicycle/flutter kick, but do I really see any hard science that split fin usage results in longer dives / less air consumption / less fatigue, which would indicate less energy usage? How would you account for all the variables; profile, current, tide, temp, training, skill, nutrition, pms, cheating spouse (task overload?), yada yada yada. Your facts are not exactly the facts in my opinion, and with less than 24 dives your truths are definately not my truths!

 

[Temple of Doom]

How would you account for all the variables; profile, current, tide, temp, training, skill, nutrition, pms, cheating spouse (task overload?), yada yada yada.

Do you really think all those variables really matter as to whether one fin is more efficient than another?