Pervasive Fallacy about Split Fins

[Temple of Doom]

Recently quoted in a thread about split fins:

basically if you have to push through the water (against a current, with lots of mass on you, or bulky with a drysuit) then you need stiff fins that will get you through

i think of stiff fins as low gear and split fins as high gear ... high gear is useless to climb a hill

my style of diving is definetely low gear

This oft-quoted myth/fallacy about split fins. It sounds plausible, but it's actually a terrible analogy. I'm not focusing on the specific poster, this argument comes up time and again.

It's a terrible analogy for three reasons:

1. A current isn't at all like a hill. To go the same speed relative to the ground that you're pushing against on a hill, you have to exert more energy. To go the same speed relative to the water you're pushing against in a current requires no more energy than if the water were dead calm. It's the most basic relativity. When you're not moving against a current (drifting with it), the water's velocity from your perspective is zero. Going half a knot against the current does not make the water suddenly act any different from moving behind you at half a knot. Just like when you move at a half knot speed compared to still water.

The analogy would be valid if it compared still vs. current diving to driving on the highway at 50 vs. driving on a treadmill rolling at 50 in a wind-tunel blowing at 50. Of course in either situation, the forces acting against the vehicle would be equal so the analogy, while valid, would only prove the point that it's trying to disprove, that current has no effect.


2. Increased drag does not affect fin thrust or efficiency, only velocity. The 'conclusion' of the analogy seems to imply that the amount of drag the diver is overcoming has an effect on the thrust output of a fin. This is incorrect. For both types of fin, when you kick with the same energy the fin will produce very close to the same amount of thrust whether you're tied to a pier, pushing a board through the water, or swiming naked with only your fins. If a split fin makes you go faster than a paddle with the same amount of energy expended in one situation, it will make you go faster in all situations (well ok, when tied to a pier you wouldn't be faster, but you'd be pulling harder). So, a split fin which goes faster using the same energy, would always require less energy to go the same speed, regardless of the situation. Thus a faster fin (with the same effort) means a more efficient fin, always.


Now my very favourite reason the analogy is bad:

3. Split Fins *are* Low gear! It's paddle fins that are high gear! Don't you see that while kicking with the same energy split fins are the faster, easier strokes, while paddle fins are the slower, harder strokes with more 'resistance'? Low gear is more, easier rpms, while High gear is fewer, harder rpms.

The 'gearing' really doens't matter though. When the energy going into the fin is the same, all that matters is the efficiency. One could modify a split fin to have the same efficiency as a paddle fin (I'm thinking a little electrical generator that would steal energy and provide it to your light), and while the fin would require the same higher amount and easier effort of kicking (thus same energy), you would now be going at the same speed as the paddle fins.

--

Now I'm not going to delve into the legitimate reasons to not like split fins. There are several: don't prefer the kicking stroke, frog kick is harder so silting issues can occur, entanglement problems, etc. Those are all legitimate concerns based on preference and fact (excellent reasons to do anything). They are unlike the above concern, which is just all around bunk.

Craig

 

[fisherdvm]

Split fins only work if they are brightly colored. Pink ones go fastest. It has to do with the intrinsic resonance of the H20 molecules, which closely matches the light spetrum of pink - which borders on ultraviolet.

It is complex physics, and I don't expect you to understand it.

 

[The Kraken]

ToD,

I think this is the part of the equation that generates so very much argument:

"For both types of fin, when you kick with the same energy the fin will produce very close to the same amount of thrust . . . "

I think the whole argument revolves around the hypothesis that since split fins are split, there is a loss of velocity because of a loss of efficiency generated by the split in the fin. Conversely, a blade type fin generates more velocity given the same input force due to a contiguous force surface.

Then throw into the mix different types of kicks and you can see from whence the miasma is given life.

This is just one of those issues in scuba diving discussions into which I elect not to enter.

the K

 

[sandmanz32]

Split fins only work if they are brightly colored. Pink ones go fastest. It has to do with the intrinsic resonance of the H20 molecules, which closely matches the light spetrum of pink - which borders on ultraviolet.

It is complex physics, and I don't expect you to understand it.

 

[Diver Dennis]

To go the same speed relative to the water you're pushing against in a current requires no more energy than if the water were dead calm.

Isn't the whole idea that you are trying to move forward relative to the bottom, not the water? Let's say you are traveling north along a reef at .5 knots. If there is a current against you going 1 knot and your energy output is the same, you would be going backwards at .5 knots. Correct?

I should add that I've never tried split fins so I'm just commenting on the physics here.

 

[D_B]

Are there any pilots out there that can see that the splits on the fins are to increase the efficiency of the fin, in the same way the slots in slotted flaps do for flaps on aircraft? .. notice the slots in a jet fin, do they look familiar?
... air or water, the same effects apply


(unless their pink )

 

[Temple of Doom]

Isn't the whole idea that you are trying to move forward relative to the bottom, not the water? Let's say you are traveling north along a reef at .5 knots. If there is a current against you going 1 knot and your energy output is the same, you would be going backwards at .5 knots. Correct?

Of course. But you're not moving against the ground, you're moving against the water.

If you can move against still water at 4.0kt in split fins and 3.0kt in paddles, then in a 3.5kt current you're going backwards in paddles and forward in split fins.

I think the whole argument revolves around the hypothesis that since split fins are split, there is a loss of velocity because of a loss of efficiency generated by the split in the fin. Conversely, a blade type fin generates more velocity given the same input force due to a contiguous force surface.

I understand that that is the counter-argument, but it's wrong. Well it's wrong if you accept that you can move faster with split fins (using the fastest split-fin kick) than you can swim with paddles (using the fastest paddle kick). Most people in the argument will accept that, and then say that things are different in a current or that it's different against a lot of drag, that's not true.

With fins: faster = more force per energy in

Craig

 

[fisherdvm]

It is similar to the "vortex" theory of Pre- carburator tornado gas saving adapter. You can save gas on your car by putting one of this into the air intake of your car. The turbin like fins causes a swirling vortex of air to enter the engine. This will improve your car's mileage at least 5%. I have two in series on my car, and get almost an extra 20 miles on a tank full.

Split fins, if used properly, will create small tornado like swirls of water behind you. If used accordingly, the vortex will increase your SAC rate by at least 5%.

 

[The Kraken]

Of course. But you're not moving against the ground, you're moving against the water.

If ya want to get back to the shore or where the up line is, ya are !!!

the K

 

[halocline]

Maybe there's a small analogy with bicycle gears. Split fins present less resistance to your leg muscles and so you can kick faster with less fatigue (per kick) just like when you're in low gear with a bike, you can peddle faster with less resistance. It is true that peddling a bike, there's a "sweet spot" of peddling speed that offers the most torque for the least effort. But, taking this analogy further, that would imply that splits are more appropriate for slow going against increased current, kind of like switching to a lower gear against a head wind. Since it seems that splits are not very good in that situation, maybe the whole analogy doesn't work.

I have a feeling that most divers who like splits experience the benefit simply as less effort, because their legs are meeting less resistance. This lets them kick faster, which (supposedly) makes their effort more efficient. Personally, I bet in most cases they just like having less resistance, a feeling of more freedom of movement, and less likelyhood of cramping.

But, take someone with really strong leg muscles, and I would imagine that they'd move alot faster with paddles; they're simply pushing more water with each kick. I honestly don't see how it could be any different.

I've never used splits, so for me it's all a guess. I really like the control I get with my paddles (avanti quattros) so I have no interest in trying splits.