Split Fin Physics

[HenrikBP]

A diving physician told me that the recovery would aid in oxygenation of the muscles. When we kick up and down equally, we oxygen deplete at a faster rate.

Just to that specific point; easing work (decreasing muscle contraction) to allow blood flow through muscle tissue will indeed improve recovery. However, since you are mainly using different (opposite) major muscle groups on the up-stroke vs. the down-stroke, I'm not sure how much of a difference it really makes.

The overall work load of the power stroke, the decrease of workload on the recovery stroke, as well as the time of contraction vs. relaxation phases is what would dictate how well the various muscle groups would recover.

Henrik

 

[mdb]

John:

Your data and experience continues to keep this thread on track with a lot of additional information. At Apollo Sports: Dive Gear there are a number of videos showing the bio-fins in open ocean with full scuba gear. You can clearly see the fin in motion on both up and down strokes.

I would be interested in your comments, if you find the time to take a look.

You elevate this discussion with your calm and rational approach. Thank You.

 

[lmorin]

Imorin: Good comment. What are your thoughts about the split fin designs that provide forward thrust with both the up and down stroke?

Paddle fins provide thrust only on the down stroke. The up stroke is known,commonly, as the "recovery" stroke.

The materials used are also a part of the equation. High grade polyurethane, expensive but very responsive and very durable, 100% natural rubber, expensive to mold in compression molding, but very comfortable and effective, long lasting and durable.

You are certainly correct in that the energy comes from the kicker and the kicking stroke. What are your comments on the various designs available to translate the kickers energy and stroke in the most effective manner?

What I think or believe is of no real import. My fins are tomato red and black. That's why they are so good for me

I continue to be amazed that no gear manufacturer has studied fin performance in an experimental test setting that would include an artificial foot/limb, artificial current, etc., while monitoring propulsion force generated to learn how it varies with type of kick, phase of the kicking stroke (wouldn't you really like to see a set of curves, each one showing the forward propulsive force being generated by a single fin moment-to-moment across the entire stroke cycle, the series of curves representing the behavior of 100 different old and new model fins, with one set of curves showing the results from a slow speed kick stroke, one from a medium speed kick stroke and one from a high speed kick stroke), material of manufacture, and design. This is not a difficult task and I'd think the Navy would want to know the answer even if the manufacturers do not.

 

[mdb]

Imorin:

You are spot on. Apollo designed a full diver sized robot that was able to replicate different kick cycles. The robot had a natural shape and knee and ankle bend etc. Both real time and slow motion film was made. This was part of the development process.

The fin diving robot was later sold to another SCUBA manufacturer once the testing was complete. Apollo had spent quite a bit of money researching design parameters and proper materials. The robot was sold for a pretty tidy sum.

 

[Blackwood]

Thanks again for the mostly on topic (with the exception of some recent "this is a stupid thead" stuff), mostly civil posts.

The discussion about recovery with respect to fin style is interesting even though I frog kick almost exclusively and get no forward propulsion from the recovery phase (quite the opposite in fact).

 

[meesier42]

problems with robots......... the point is that while they may look cool in the lab, they do not translate to the human leg.

 

[redacted]

blackwood- as you know, frog kicking is an entirely different beast that requires a completely different style of fin. the muscle groups used are not even the same, even if you were to compare the muscles used on the recovery of a flutter kick to the ones used for a frog kick.

problems with robots

1) good luck showing that the robot actually simulates the range of motion of a diver. there are too many variables, you can't even video two world class swimmers and replicate the perfect kick much less the entire diving population, or someone with knee, hip or other range of motion problems.
2) how do you quantify ankle flexion- the answer is simple, you can't. here you can't even get a consistent measurement off a single swimmer, each ankle is different, often by 10 degrees or more
3) you can't quantify the ratios of quad, calf, hamstring, ACL, PCL, MCL, strengths. as again each person is different
4) how can't quantify knee bend for optimum power, you can quantify how much knee bend the robot uses for optimum, but that doesn't translate to a human leg.
5) robots do not have blood flow, do not generate lactic acid, and are not limited by Oxygen uptake.
6) the ideal kick for one fin is not representative of the ideal kick for a different fin, so even though you may believe that you have removed the human variable, in fact you have just replaced it with another unquantifiable variable.

Yes, you are right. The use of such a robot has it's limitations, depending on how much work was done to test the similarities and differences with human subjects. The big advantage it has over humans is repeatability of testing where the only independent variable has to do with the fins under test.

Perhaps it would be better to drop the whole scientific approach and simply resort to selected testimonials and some catchy demonstrations.

 

[Notso_Ken]

There is no "lift" contribution to you being propelled forward. If you examine the diagrams purporting to show "lift" it becomes intuitives obvious to the most casual observer that the "lift" is perpendicular to the direction desired to travel!

No, there is a MUCH simpler explanation of how fins propel you without invoking Bernoulli. Just invoke Newtonian physics, and the principle of equal and opposite reaction.

You kick up and down, the fin bends, and the angle of the fin pushes water backward. The equal and opposite reaction of the water going backward is you going forward.

Ken

 

[HenrikBP]

Yes, you are right. The use of such a robot has it's limitations, depending on how much work was done to test the similarities and differences with human subjects.

Limitations yes, but fortunately not nearly as many as Meesier had in his post pre-edit - as per your quote. If he were correct we'd be "up the creek" with regard to design of such complicated devices as knee replacement prostheses etc.

Fortunately range of motion, muscle strength, axes of rotation etc. can be approximated (bell curve anyone ...) closely enough that certain conclusions can be made wrt. what's being tested.

As with all research it is of course important to *start* with 1) a clear definition of what you're trying to prove 2) designing your study to eliminate/minimize variables and specifically look at what you're trying prove and 3) resist the temptation to draw conclusions from your data that your study was not designed to look at

Henrik

 

[Puffer Fish]

What I think or believe is of no real import. My fins are tomato red and black. That's why they are so good for me

I continue to be amazed that no gear manufacturer has studied fin performance in an experimental test setting that would include an artificial foot/limb, artificial current, etc., while monitoring propulsion force generated to learn how it varies with type of kick, phase of the kicking stroke (wouldn't you really like to see a set of curves, each one showing the forward propulsive force being generated by a single fin moment-to-moment across the entire stroke cycle, the series of curves representing the behavior of 100 different old and new model fins, with one set of curves showing the results from a slow speed kick stroke, one from a medium speed kick stroke and one from a high speed kick stroke), material of manufacture, and design. This is not a difficult task and I'd think the Navy would want to know the answer even if the manufacturers do not.

It is far more difficult than one would think.. as there is not "one" kick...and we have a 3 axis motor, with very large differences in extension and contraction for each. That ends up being the failure of most studies... it becomes depend on the human doing the kicking and what may work well for one fin doesn not necessarily work with others.

It would be possible but costly and time consuming to build and then test all of the possible combinations. And it would have to be done with a consideration for human cycle rate efficiencies. Those are well known for runners and bicycliest, but not for the other half of the muscles being used.

If you first tested humans and determined the right ratio of force that should be used for all the muscle groups... and knew the ideal cycle rate.. and had force gauges to read how a specific stroke pattern caused with a specific fin... then you could take all the fins and test them and compare results and in the end you would still not know if a better fin could be made.

Studies like the one referenced in this thread end up being meaningless to some extent because they only measure what was being done, not what should have been done and are not measuring the capabilites of fins.