Pervasive Fallacy about Split Fins

[Splitlip]

There have been several tests that show that divers can sustain higher speed through the water with split fins than with paddle fins. With proper technique they are both faster and more efficient than paddle fins.

So why are split fins not universally accepted?

IMO, it comes down to two problems -----

1. Paddle fins give more control and manueverablity. This is because large paddle fins "grab" the water more effectively and are more easily used to change one's orientation and to do things like back kicks.

2. If you don't use proper technique, then split fins perform poorly. I strongly suspect that this is the reason behind the claims of "paddle fins don't do well in currents". From a physics point of view, the original poster is correct --- there is no difference from the diver's point of view whether he is moving at 4kts in still water, or making 1kt headway against a 3kt current. OTOH, if a diver in a current does not have the discipline to maintain the proper narrow flutter kick and starts trying to use wide powerful kicks, then he will lose speed.
Split fins require more skill to extract their full performance, and a semi-panicked diver's natural response will cause the performance of split fins to degrade dramatically.

Charlie Allen

exactly

 

[Warmwater Wank]

As is often the case when I read threads like this I find myself following "both" sides of the argument. I've had my Jet-Fins (black) since the early 70's and I also own Bio-Fins (yellow) I grew so confused following previous arguments in other threads about which fin type was "best" that I took to using one of each fin to gain the best of both worlds. This worked fine but caused me to swim in circles and sort of corkscrew thru the water (due to tip vortice differential, paddle grab etc.) which tended to complicate return navigation.

Of course I solved that problem by simply swapping fins left for right on the return trip (which also reversed the corkscrewing, thus relieving most of the dizziness). However, after occasionally dropping a fin during the swap over (believe me, this really aggravated the circling problem) I've been searching for a better 2 fin solution. This thread may have provided the answer I'm thinking that if I can just use a pink Jet Fin, the added "speed" of the pink fin will balance out the split fin thrust (and hopefully avoid corkscrewing, though this did allow me to see quite a bit more on my dives). Any one have some size 12 pink Jet Fins out there for sale? Uh, I just need one... // ww

 

[Temple of Doom]

My only reason for creating this thread is that it seems that a lot of poeple like to cite the wrong reasons for disliking split fins. In every thread about split fins vs paddles you'll find somebody bring up this myth.

Dislike them because you gave them a good shot and they weren't for you. Even dislike them without trying them, if the few non-subjective problems are deal breakers (entanglement risk, less fine movement control, etc). But don't dislike them for reasons that are simply incorrect (certainly, please don't propagate that myth to others seeking advice).

I wouldn't want anybody to change their minds about split fins (except maybe opening a closed one by debunking a myth). All I'd want is people to understand that:

If you believe the below assumption, which has been backed up with years of tests done (cited by Frank), then that means that the following conclusions are true.

Assumption: A diver swiming with the same effort can swim faster and thrust harder with splits than with paddles.

Conclusions, all else being equal and the only difference is fin:
1. Split fins are more efficient at swimming.
2. Split fins require less energy to swim at the same speed as paddle fins.
3. Split fins require less air to swim the same amount as paddle fins.
4. Split fins offer more thrust than paddle fins while swimming.
5. The above statements are all true in current and high drag situations.

Of course it doesn't mean that split fins are better, or that you can maneuver better in them, or that they silt less, or they have less entanglement issues. Some of these other 'strikes' against split fins are debateable and personal preference, others are indisputable. Those are valid reasons to not like them. The myth that somehow you get less power out of a split fin is simply wrong.

Craig

 

[bperrybap]

Assumption: A diver swiming with the same effort can swim faster and thrust harder with splits than with paddles.

Conclusions, all else being equal and the only difference is fin:
1. Split fins are more efficient at swimming.
2. Split fins require less energy to swim at the same speed as paddle fins.
3. Split fins require less air to swim the same amount as paddle fins.
4. Split fins offer more thrust than paddle fins while swimming.
5. The above statements are all true in current and high drag situations.

Of course it doesn't mean that split fins are better, or that you can maneuver better in them, or that they silt less, or they have less entanglement issues. Some of these other 'strikes' against split fins are debateable and personal preference, others are indisputable. Those are valid reasons to not like them. The myth that somehow you get less power out of a split fin is simply wrong.

Craig

Its that #4 that causes the most heated debate:
4. Split fins offer more thrust than paddle fins while swimming.

Here are my personal observations.

When I was selecting gear a couple years ago, I spent an entire
afternoon swimming in a pool and switching between splits and
non slits.

What I was clearly able to notice was non splits accelerated
quicker than the splits. i.e. they got me up to full speed from
a dead stop in 1 or 2 less kicks than the splits.
They didn't seem quicker, they just seemed to get me up to
speed faster.

I also noticed that under extremely hard kicking, the splits
didn't work as well as under moderate to heavy kicking.

Also, as a test I would empty my BC and then remain vertical
in the water and kick to see which fin would "lift" me higher
out of the water. It was quickly noticeable that with a hard
single or double kick the non splits would lift me further
out of the water.

However, in real tests under water, once I was moving,
it felt like it took much less effort to sustain the speed with the
splits.

So my conclusion was that non splits can transfer more torque
to accelerate you up to speed quicker than splits. However,
once moving, the torque is not as big of an issue and
splits are more efficient at keeping you moving at constant
speed and can even top out at a higher speed than non splits
depending on energy exerted.

I'd love to see a 10 ft test. i.e. have a diver hovering and
when given the signal, time him on swimming horizontal 10 ft.
I'm guessing non splits would easily win that one.

And that is where I think the issue is.
Acceleration vs speed vs efficiency. Typically, in any design you
can't have all three.

All the tests and research I've seen since then doesn't
seem to contradict this. None of them actually test acceleration.
The tests done are for maximum speed
and efficiency at speed over time.

So, after all my playing around back then, I did go with splits
since my diving is nice and slow. I'm happy just looking
at pretty fish and taking pictures.

But I'll admit that based on my testing, that if I were having
to push/pull/roll up into zodiacs, I might get some non splits
to give me that extra push to help get up out of the water over
into the boat.

--- bill

 

[Temple of Doom]

I'd agree that paddles allow you to get bursts of higher thrust, but not sustainable. Acceleration = thrust. If split fins have been shown to have greter thrust, that means they have greater acceleration. When you kick through the water with a certain energy, you're generating thrust. That thrust is pretty much the same whether you're moving at 4kts or 0. ScubaDiving did tests where they tied divers to a scale, the split fins pulled harder than the paddles.

Of course you're not imagining things when you notice a difference over the first few kicks. It's because while the average thrust/accelleration for paddles is lower, it's pretty uneven. With paddles you get higher thrust/accelleration while kicking, but less during recovery, the average is less thrust/accelleration than splits, but very close to the start the total thrust could be greater over the same time.

The thrust output of either fin over time throughout the kicking cycle looks like a sine wave (peaks and valleys all of the same height/depth and seperation). The thrust graph of paddles would have fewer, but bigger hills and deeper valleys, while split fins would have more but smaller hills and shallower valleys. With paddles, the peak may be higher than the peak of a split fin, but the average thrust is lower than the splits.

Obviously where you'd notice a difference is over the first couple kicks. The peak of the thrust is obviuosly the kick, and the valley is the recovery. After one kick, but no recovery, you'd have all peak and no valley the average thrust/acceleration over that time would likely be higher than splits over the same time. After two kicks, but before the second recovery, you'd have 2 peaks and 1 valley, the average might still be higher than the splits over the same time. By three kicks and two recoveries, splits could likely have left the paddles behind in total thrust.

However, it's important to note that after each full kick/recovery, the paddles will have generated less thrust/accelleration than the paddles. If this were not so, then it would stay true over the entire swim, and paddles would be faster.

--

Also, it's important to note that there's nothing wrong with a fin that requires more effort, but less often. That's not what makes them slower. The issue comes down to the design of the fin itself and its efficiency. Right now the best split fins translate more of the kicking energy into forward thrust than the best paddles, but there's nothing preventing a paddle fin to be re-engineered to be more efficient than splits, and therefore be the faster, higher thrust fin.

Craig

 

[Adobo]

I've read many people make the argument that split fins are faster and/or more efficient. I've always wondered why it matters even if it was true. There is one thing that seems almost universally accepted, paddle fins are the choice of divers that require control and ability move front/back and pivot turn (tech, wreck and cave). Not many will argue that split fins are marginal at best for use in moving in any direction but forwards. (I've seen only less than maybe 5 people claim any proficiency in doing a back kick or a helicopter with split fins.)

So extend that idea into real diving - you are doing a reef dive where there is a slight current behind you. You swim up to something you want to look at. Because of the current, you will drift past whatever it is that you are looking at. To have another look, what do you have to do in your split fins? You have to swim around.

Extend the idea to another scenario - you are looking forward but something of interest is to your right. How do you re-orient yourself so you can go to whatever it is that you want to look at in your split fins? You have to swim around in an arc. Alternatively, you will use all kinds of body gyrations to get your body pointed in to the right. Lots of hand sculling and thrashing about.

Let me put it in a slightly different way... if your car had great gas mileage and accelerated pretty well but it didn't turn well and did not go backwards, would you still consider that car to be the most efficient simply because it has the best miles per gallon?

P.S. If the objective is finding optimal efficiency in going forwards, divers should start with trim and weighting. I would be willing to bet that an overweighted diver with poor trim will see far more benefits fixing those issues than he/she would see simpling by switching from one fin style to another.

 

[Temple of Doom]

Extend the idea to another scenario - you are looking forward but something of interest is to your right. How do you re-orient yourself so you can go to whatever it is that you want to look at in your split fins? You have to swim around in an arc. Alternatively, you will use all kinds of body gyrations to get your body pointed in to the right. Lots of hand sculling and thrashing about.

You're going off the topic of the myth, and into the valid, though debateable reasons for not liking split fins.

Question for you, your description of diving with split-fins sounds as if it's from your experience with them, how many times have you used split fins?

Craig

 

[Adobo]

You're going off the topic of the myth, and into the valid, though debateable reasons for not liking split fins.

Question for you, your description of diving with split-fins sounds as if it's from your experience with them, how many times have you used split fins?

Craig

10 or so dives in the ocean. Roughly the same in the pool. Used twin jets mostly but also used twin speeds on one ocean dive and two pool sessions. The pool sessions were mostly to figure out if I could learn to back kick and helicopter. Incidentally, I remember the first time I went diving, the question on my mind was, "how the heck do I change directions?" On that dive, the issue was a lack of knowledge/skill as opposed to fin choice. The later pools sessions took place after I learned how to do back kicks, etc. using paddle fins.

My reply was an attempt to take the conversation back a step. This thread is focused on the myth. My point is that whether or not there is truth in the myth, in the grand scheme of diving, it doesn't really make a difference. There are far more crucial criteria to use to decide whether or not split fins are right for the diver. (i.e. need softer fins due to joint issues, need heavier fins due to floaty feet with a drysuit, need fins you can maneuver in, need fins that present a minimal entanglement hazard, need a fin that is inexpensive, and so on, and so on). And even in the area of needing increased efficiency, an individual diver has other areas to focus on that will provide more benefit than simply switching from one fin to another.

 

[Temple of Doom]

My reply was an attempt to take the conversation back a step. This thread is focused on the myth. My point is that whether or not there is truth in the myth, in the grand scheme of diving, it doesn't really make a difference. There are far more crucial criterion to use to decide whether or not split fins are right for the diver. (i.e. need softer fins due to joint issues, need heavier fins due to floaty feet with a drysuit, need fins you can manuever in, need fins that present a minimal entanglement hazard, need a fin that is inexpensive, and so on, and so on)

Fair enough, I agree wholeheartedly that you have to weigh all the factors. I just think it's important to not propagate myths. Your previous post sure seemed to have a lot of myths in it. You pretty much entirely ruled out the possiblility that split fins can go backwards at all and claimed they can't turn well (not that they can't turn "as well", just that they can't turn "well"). That post sure seemed to be propagating a bunch of myths.

I found another post by you that seems to contradict your previous post:
http://www.scubaboard.com/showthread.php?p=1672075&highlight=split#post1672075

I was in the pool today (all my dive buddies are out doing their own thing) and saw a pair of twin jets laying around. I strapped them on just to see what I could and couldn't do with them.

First of all, in comparison to jetfins, twin jets make you feel as if you don't even have fins on. Frog kick with them on seems to be okay but a little sluggish at first. To my surprise, back kicks are completely doable with twin jets. Again, a little sluggish and sloppy in comparison to jetfins but completely doable. Helicopter turns are also doable but far sloppier when compared to jetfins.

Overall, I would say that I was completely shocked with what I was able to do with those twin jets. Would I trade in my jetfins? No way. But certainly, I would encourage the "personal preference" divers to try to learn how to do alternative propulsion techniques in their split fins. My guess is that most will be able to do some measure of back kicks and helicopters with a little practice. It might not be adequate for diving overhead environments but completely usable in recreational environments.

So, even after your first pool session you realized that the splits were completely useable in recreation environments, a far cry from a car that can't turn well and can't go backwards...

It's important when you're weighing the pros and cons of something to be honest to yourself and others. People tend to over-exaggerate to support their preference about split fins. Doing so only weakens the good points they have. You have good points about why splits aren't as good, stick to them.

Craig

 

[D_B]

When I took the Equipment Specialty class, my instructor (he's a sailor and I'm and ex-sailboat designer) said that splits employ the same kind of high pressure/low pressure physics that creates lift on an airplane wing or a sail, so that in addition to the "pushing" of water that you get with a paddle fin, you are also creating lift that "pulls" you through the water. This supposedly balances the fact that you have lost some of the "push" due to water escaping through the split.

In either style of fin, a stroke creates "tip vortices", which are really lost energy. The fin that reduces this kind of vortex, and conserves the energy in the stroke, overcomes part of the inefficiency of a fin.

exactly the point I was making ... the flow through the split redirects water tip ward improving efficiency