Pervasive Fallacy about Split Fins

[scubapro50]

there's a lot of discussion about split fins now ... do they reallly work and how do they make a difference in thrust. I can remember when the first Scubapro Jet fins came out in the 60s ....... at first divers made fun of the jets because "they had holes in them and looked funny".

 

[hlsooner]

This debate never ends, but nobody is asking the real question: Do they work? I think the answer, at least for the Scuba Manufacturers, is a resounding YES, split fins work wonders because they fly off the shelves at a price premium over traditional fins, therefore they are working exactly as designed.

As to whether split fins produce more efficient propulsion, we will have to leave that answer to somebody with a supercomputer and the time to do a full Computational Fluid Dynamics Modeling of their use. But since resistance in water is proportional to velocity squared, any small increase in efficiency of propulsion is far outweighed by the fact that going twice as fast takes four times the propulsive force, regardless of what fin you use.

 

[bperrybap]

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.

Craig

Now this is where I think some of the problem is.
The above logic is flawed.

Thrust can produce acceleration but
acceleration is NOT equal to thrust.

Acceleration is defined as:
The rate at which an object's velocity changes with time.
Thrust is defined as:
A force that pushes an object forward.

It is possible to have lots of thrust and have ZERO acceleration.
In fact you could have thrust and be slowing down or even
be moving backwards.

For example, I could strap a rocket onto a huge
boulder and have millions of pounds of thrust and no
acceleration if the rock doesn't move.

Likewise, if I am swimming, at some point, I will reach a
terminal velocity and will stop accelerating and level off into a
constant speed.
This is the point where my fluid resistance or drag
has come into equilibrium with the thrust my fins are producing.
I still have thrust, just no more acceleration.

It is also a false assumption to assume that the thrust
would be the same if the speed is zero or 4 kts (4 kts is FAST BTW).
There could (and will) be all kinds of efficiencies or inefficiencies that only
show up once dynamic fluid resistance enters into the picture.

In automotive terms,
I tend to think of the split vs non split fin thing like a
horsepower vs torque contest.
Would you rather have more torque and less hp or more hp
and less torque?
Which do you think would win a race?
Depends on the type of race right?
Which would burn more fuel?


Again, I'm a big fan of splits and have used a pair on over
120 dives. I really like them.
I'm just not convinced that they are better than non splits
for certain types of things. For the diving I do, they
have been great.
(although backing up, while it can be done, is a PITA)

--- bill

 

[under water]

I'd love to see a real "thrust" test that puts a diver in a harness
and actually measures maximum thrust while holding the
diver back.

Again, I'm a big fan of splits and have used a pair on over
120 dives. I really like them.
I'm just not convinced that they are better than non splits
for certain types of things. For the diving I do, they
have been great.
(although backing up, while it can be done, is a PITA)

--- bill

Hi Bill,
This has already been referenced on this thread.
I'll give the link again. It appears to have been mangled on the first post, although it worked originally

http://www.scubadiving.com/upload/images/pdf/200510_scubalab.pdf

try this if the above just brings you to the top page of the mag

(see page 2 top right) They used divers pulling on guages tied to a pier as one of their tests.

They did concentrate on flutter kicks for most of their test, but they did evaluate alternative kicks as well​

This additional page has several years worth of tests. The link above is the 2005 results.

http://www.scubadiving.com/fins​

Splits seem to be on top of the scoring.​

PS. I agree, Backing up is a PITA, but I attributed that to my technique. One of my instructors who is a seriously technical diver uses splits for wreck diving and he swears that he can do any kick (including backup) just fine with his Scubapro twin jets. (he is just one person though, the referenced link is a measured study.)​

 

[D_B]

Here you go Bill, I find the videos interesting ... http://www.scubadiving.com/2005fintest_protocols

 

[bperrybap]

Hi Bill,
This has already been referenced on this thread.
I'll give the link again. It appears to have been mangled on the first post, although it worked originally

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Cool.
I hadn't seen that test version yet.
Thanks for the reference.

--- bill

 

[MantaRey]

Note that a frog kick, reverse kick, turns ect use the fin exactly as we use boat oars (paddles). Next time you go out in a canoe, try using paddles with splits in them and let me know how they work for you.

Thank you.
How come nobody else ever sees this? It's so obvious.

Sorry to say but I noticed that the only time a split fin works better than a "paddle fin" is when they are on someone who bicycle kicks.

 

[Temple of Doom]

Thrust can produce acceleration but
acceleration is NOT equal to thrust.

True, for purposes of the example of accelerating from a stop, I was illustrating that it is the difference in thrust alone that determines the difference in accelleration between two fins. Thrust has a 1:1 relationship with accelleration.

It is also a false assumption to assume that the thrust
would be the same if the speed is zero or 4 kts (4 kts is FAST BTW).
There could (and will) be all kinds of efficiencies or inefficiencies that only
show up once dynamic fluid resistance enters into the picture.

Exactly why I added the "very, very nearly" immediately after the sentence. At the differneces in swimming speeds (a few kts), the difference in thrust is negligible. Further as it pertains to the discussion, the differneces in thrust at different speeds are effectively equal for both fin types. Thus making the distinction irrelevant to the discussion.

In automotive terms,
I tend to think of the split vs non split fin thing like a
horsepower vs torque contest.
Would you rather have more torque and less hp or more hp
and less torque?

This is also a poor analogy. Horsepower and torque are a 1:1 relationship. All things being equal, the more torque you have, the more horsepower. Horsepower (work) in a car is essentially the torque multiplied by the rpm (force * distance).

A car aplies thrust against a stationary medium, the ground (as opposed to a fluid medium like air/water). As a result of this, in the 1:1 gear (and when the flywheel/clutch/etc are not disengaging the engine from the wheels) the wheels must be rotating in a 1:1 ratio to the engine, or else the car will stall. Different gears merely change the ratio, but the rotations are tied. Due to this, the speed that a car is moving will impact greatly the thrust output of given engine RPM. This is not the case for fins applying thrust to a liquid medium.

In so many ways, how an automobile provides thrust to the ground is a bad analogy for how fins apply thrust to water. There are so many more variations and mechanations in an automobile that it just doesn't work.

The same diver will be able to output the same work (horsepower) regardless of fin. A split fin requires less force to kick and can be kicked more while using a given amount of energy from the diver. A paddle fin requires more force but can only be kicked less using a given amount of energy from a diver.

Regardless of whether a fin is 'high torque/low rpm' or if a fin is 'low torque/high rpm', if both fins were equally efficient they would ouput the same thrust. However, since split fins are more efficient (at forward swimming as is all we're discussing), they output more thrust for the same amount of energy. The 'gearing' of the fin is irrelevant to this.

Again, I'm a big fan of splits and have used a pair on over
120 dives. I really like them.
I'm just not convinced that they are better than non splits
for certain types of things.

Of course! There's multiple factors for determining which fin is 'better'. I don't think you could ever say that one fin is 'better' for all people or situations. What you can do is understand the objective qualities of both fins. In the case of splits, it's been demonstrated consistently and objectively that they are more efficient at forward swimming (and that of course holds true in any drag/current situation).

Craig

 

[BKP]

I don't trust dive magazine tests where advertisers most expensive equipment get's the highest ratings. Since most divers do not know how to use fins properly, a smart engineer would design for the most thrust from bad technique. I would not be supprised if a person with terrible technique went faster in splits. Why would a manufacturer put out an expensive product that requires more skill to achieve results, when the target market has no skill? Much more likely the product gives noticible results to the unskilled masses.

Congrats... Blue Ribbon for the most ridiculously sublime rationalization I've ever seen on this board...

 

[superstar]

It's not the fins, it's your legs. My legs can make any fin work.