Pervasive Fallacy about Split Fins

[radinator]

If you wave either piece of paper as hard as you can, it will be just that, as hard as you can. Whether you're trying to wave a playing card or a hang glider as hard as you can, that means you can't do any more. It's your body's max energy output.

Going 'as hard as you can' is a terrible way to measure efficiency, especially if that speed is fast with one item (the small piece of paper) and slow in the other case (the large piece of paper). There is a fundamental difference here, in that when you go fast you are spending a lot of effort just overcoming the inertial of your limbs. For the paper in this example you have to move your hand back and forth quickly, overcoming the hands inertia.

 

[Temple of Doom]

Going 'as hard as you can' is a terrible way to measure efficiency, especially if that speed is fast with one item (the small piece of paper) and slow in the other case (the large piece of paper). There is a fundamental difference here, in that when you go fast you are spending a lot of effort just overcoming the inertial of your limbs. For the paper in this example you have to move your hand back and forth quickly, overcoming the hands inertia.

Of course! But the energy your body is outputting is equal in both cases!

Naturally there's a sweet-spot to deliver the most energy to the fin/paper, but that doesn't change the amount of energy the body is generating. A diver swimming his hardest will generate the same amount of energy whether using splits or paddles. How much of the energy going into and out of the fin is variable, but I tend to think of the efficiency of a fin includes the efficiency of the kick required to use the fin, don't you?

Craig

 

[Benthic]

Granted this is from the site of the company that holds the split-fin patent, and have a vested interest in getting you to buy split-fins, but it's still anecdotally worthwhile.

While I think you should take the testimonials with a grain of salt. I'm sure they may be a little overly-enthused, holding splits up as the Best Fin Ever™. However, I do think that the soldiers and officer listed (who agreed to have their name and rank next to their testimonial) did not outright lie about their personnel's use of the fins. And perhaps that this is a credible answer to the above quoted question.

http://split-fins.com/press.htm
(look at the bottom: Military Testimonials)

So I think it's safe to say that at least some Special Forces personnel (including SEALS), have used and like split-fins. Further, the SEAL who talks about extensive testing he had done (including VO2 Max) warrants at least some interest.

Craig

Testing by the SEALS huh? I'm not sure that their forte is equipment review...

What we really need is a true scientific review, performed by someone who has a background is hyperbarics and physiology. Where might I find something like that...?

Oh wait, it's been posted here already hasn't it? Recently even. You can find it right here. But you didn't read it then, so I doubt you'll read it now.

I'll not quote the entire paper, but I will point out that in the discussion section Pendergast states that stiff rigid fins are more efficient than flexible ones. He even says that in the abstract (for those that don't want to read the entire paper). These tests included the Apollo split fin, and Pendergast even went so far as to duct tape the split closed for part of the trial. He himself says:

"Fins designed on the basis of physical principles for airfoils or propellers do no necessarily lower the energy cost of swimming (Apollo vs Appolo taped). This
demonstrates that, based on our current understanding of the physics of underwater swimming with fins, theoretical models have to be evaluated empirically as was done in this study."


These are research results that were published in the Undersea & Hyperbaric Medicine Journal and is the result of research done with the support of NEDU & NAVSEA on Contract 33199C0028. This isn't the output of someone's marketing department.

As they say..."In God We Trust--All Others Bring Data."

Brian

 

[radinator]

IA diver kicking their hardest will generate the same amount of energy regardless of what is on thier feet. How much of the diver's energy is translated into thrust is dependant on the efficiency of the fin.

If you disagree with that statement, then I'm not sure if there's a way to convince you otherwise. A diver can only output as much energy as a diver can output (did I have to just say that?), the speed the diver is going based on that output (all things being equal except fin) is dependant on the efficiency of the fin.

If what you say were true, then a bicycle would not need gears. A rider would be able to put the same energy into their motion regardless of the speed of the pedals.

The fact is, just as engines have a power curve where the maximum power is given at a certain RPM (requiring cars to have transmissions) so it is with biological systems such as the diver. There is a certain speed of moving the legs that has the most power delivered.

This concept was very important when the Gossamer Albatross was developed, since human pedal-power is what powered that aircraft across the English Channel.

Likewise, moving your legs at different speeds will produce different power characteristics. Since the different fins will cause the legs to 'max out' at different speeds, the power will be different.

This is just fact. It can be measured. Do a web search on {'power curve' 'cyclist'}. Here is a link illustrating the difference.

 

[Temple of Doom]

Likewise, moving your legs at different speeds will produce different power characteristics. Since the different fins will cause the legs to 'max out' ad different speeds, the power will be different.

This is just fact. It can be measured. Do a web search on {'power curve' 'cyclist'}. Here is a link illustrating the difference.

Of course the power characteristics are different, but the energy output by the body is the same. Here's a quote from the site you linked to;

"because the energy used in accelerating and decelerating massy components (legs) begins to take up all the energy produced."

The "energy produced" is the same for all points of the curve you linked to. What is different is the efficiency of using the gear.

 

[DiveMom1927]

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.

I conquer with this..... Bright Pink Baby! Yeah.... But please note that a bright Orange Snorkel is only necessary to equal out the H20 molecules of the pink light spectrum... This will ensure balance and the conservation of energy in your dive.

 

[BKP]

Testing by the SEALS huh? I'm not sure that their forte is equipment review...

What we really need is a true scientific review, performed by someone who has a background is hyperbarics and physiology. Where might I find something like that...?

Oh wait, it's been posted here already hasn't it? Recently even. You can find it right here. But you didn't read it then, so I doubt you'll read it now.

I'll not quote the entire paper, but I will point out that in the discussion section Pendergast states that stiff rigid fins are more efficient than flexible ones. He even says that in the abstract (for those that don't want to read the entire paper). These tests included the Apollo split fin, and Pendergast even went so far as to duct tape the split closed for part of the trial. He himself says:

"Fins designed on the basis of physical principles for airfoils or propellers do no necessarily lower the energy cost of swimming (Apollo vs Appolo taped). This
demonstrates that, based on our current understanding of the physics of underwater swimming with fins, theoretical models have to be evaluated empirically as was done in this study."


These are research results that were published in the Undersea & Hyperbaric Medicine Journal and is the result of research done with the support of NEDU & NAVSEA on Contract 33199C0028. This isn't the output of someone's marketing department.

As they say..."In God We Trust--All Others Bring Data."

Brian

Striking similarity to the idea that helicopters were aerodynamically impossible...

...that is, until they built it... and flew it...

 

[Soggy]

Striking similarity to the idea that helicopters were aerodynamically impossible...

...that is, until they built it... and flew it...

huh?

 

[BKP]

huh?

If you can't make the connection, PM me... I'll draw you a picture...

 

[Benthic]

Striking similarity to the idea that helicopters were aerodynamically impossible...

...that is, until they built it... and flew it...

Nice rhetoric. In the passage I quoted Pendergast simply said that the physical principles that apply to airfoil and propeller design may not necessarily translate directly to the design of fins. I don't know how the heck you leapt from that to 'aerodyamically impossible.' :shakehead

Look at his results. It's right there in black and white. Paddles are more efficient than split fins. However I see that you, and many others have your minds made up. So I'll go away and quit bothering you with documented and published empirical data. Feel free to resume the objective speculation at your leisure.

Brian