Freedive fin efficiency test results

[dforcucci]

Hey everyone,
I tested the max output of a variety of freedive fins in the pool with an underwater speedometer! The results are posted on Freedive Wire.

http://freedivewire.com/freedive-fin-efficiency/

 

[Pinniped72]

Very interesting results, with regard the soft vs medium and the same speed results, did you feel that stronger or weaker legs would have given different results, not saying your legs aren't strong, I am sure they are I am wondering if the results would have differed if multiple divers did the tests with all the fins and the average scores were taken? Congratulations on doing the tests, testing freedive fins is long overdue, we are well behind compared to the numerous speed tests done for scuba fins

 

[dforcucci]

Very interesting results, with regard the soft vs medium and the same speed results, did you feel that stronger or weaker legs would have given different results, not saying your legs aren't strong, I am sure they are I am wondering if the results would have differed if multiple divers did the tests with all the fins and the average scores were taken? Congratulations on doing the tests, testing freedive fins is long overdue, we are well behind compared to the numerous speed tests done for scuba fins

There are lots of ways this could be done and of course strength and swim technique of the individual doing the testing is going to influence the numbers. What I did was quick and dirty. If someone else wants to do some testing themselves, I would be happy to loan out the speedometer.

 

[hteas]

Nice study Dave. Similar results to what we did in the pool in Anchorage quite a few years ago, but without the speedometer. Stiff fins, whatever the material used, were slower than the softer blades.

 

[Porky Pig]

Cool that you did this study! As you say, you measured the max speed you were able to attain with the various fins. This doesn't really say much in terms of efficiency for freediving, when we are typically swimming fairly slowly, with the aim of using up as little O2 as possible per distance covered.

Your result that fin stiffness doesn't affect max speed is interesting. You are probably aware that competitive (speed) finswimmers use very stiff monofins relative to freedivers who don't prioritise speed.

 

[cdavis]

Pure speed is a misleading way to rate fins. Drag puts an upper end on speed, so the top speed difference between types of fins is very compressed by the drag limit, ie, it takes a lot more thrust to make much difference in speed and is hard to measure. Further, fins act very differently under different power inputs. Blade thrust comes from the shape of the blade under stress, too much and you overpower the fin, poor thrust. Too little and the fin does not bend enough to produce good thrust. Soft fins bend more easily and are more efficient at low speeds (also easier to push). With correct finning technique they can be as fast or faster than stiff fins. With poor technique, high power input can overbend the blade and you get poor thrust. Stiff fins will produce great thrust if you really put the power to them, but are inefficient at lower power levels(and lower speeds), because they don't bend enough at low power inputs.

Real world examples: I had a set of stiff Specialfins Hybrids that were definitely the fastest blades I ever used and fastest over as long a distance as I could maintain( several hundred yarda), but they were so stiff that they did not work well at low speed and way over powered my legs during a long days diving. I used a set of c4 vtr25 blades for a week of 20-70 ft reef crawling. They were wonderful, super efficient, super easy on my legs, plenty fast enough. Then I took them deep, 100+ ft and had no thrust at all when I make my turn. I was overpowering a very soft fin. Scared me silly till I figured out what was going on.

 

[dforcucci]

Cool that you did this study! As you say, you measured the max speed you were able to attain with the various fins. This doesn't really say much in terms of efficiency for freediving, when we are typically swimming fairly slowly, with the aim of using up as little O2 as possible per distance covered.

Your result that fin stiffness doesn't affect max speed is interesting. You are probably aware that competitive (speed) finswimmers use very stiff monofins relative to freedivers who don't prioritise speed.

Thanks for your comment. This is a complicated subject and doing such a simple test does not do it justice. But I thought it would be a start. One important thing I noticed was that although I felt like I was spinning a prop and going slow with the very soft fins, I actually went faster than what I perceived.

Yes I am aware of the stiff blades for speed fin swimmers. And as you point out we are more into O2 efficiency, not speed. I just switched to a soft monofin from medium and my 75y dynamics were noticeably easier. Bottom line is that for most freediving, even spearfishing, Soft flex might be the way to go.

 

[dforcucci]

Pure speed is a misleading way to rate fins. Drag puts an upper end on speed, so the top speed difference between types of fins is very compressed by the drag limit, ie, it takes a lot more thrust to make much difference in speed and is hard to measure. Further, fins act very differently under different power inputs. Blade thrust comes from the shape of the blade under stress, too much and you overpower the fin, poor thrust. Too little and the fin does not bend enough to produce good thrust. Soft fins bend more easily and are more efficient at low speeds (also easier to push). With correct finning technique they can be as fast or faster than stiff fins. With poor technique, high power input can overbend the blade and you get poor thrust. Stiff fins will produce great thrust if you really put the power to them, but are inefficient at lower power levels(and lower speeds), because they don't bend enough at low power inputs.

Real world examples: I had a set of stiff Specialfins Hybrids that were definitely the fastest blades I ever used and fastest over as long a distance as I could maintain( several hundred yarda), but they were so stiff that they did not work well at low speed and way over powered my legs during a long days diving. I used a set of c4 vtr25 blades for a week of 20-70 ft reef crawling. They were wonderful, super efficient, super easy on my legs, plenty fast enough. Then I took them deep, 100+ ft and had no thrust at all when I make my turn. I was overpowering a very soft fin. Scared me silly till I figured out what was going on.

Thanks for commenting. I agree that once you get up to the faster speeds it may require exponential power to go faster. BUT, there was a measurable increase in speed from from plastic to composite blades. All things being the same, technique, leg power, drag, etc. this result is real.

The test was not misleading. It did what it set out to do. Test the max speed of fins and compare those speeds against blade material and stiffness. Interpretation of the results is up to the reader.

The remaining things you point out are not supported by any empirical data that I know of? "overbend"? That is a relative term.

Your real world example is based on perception and not data. My perception was similar but the data proved me wrong.

 

[DRW]

Congratulations on completing and publishing your test of freedive fin efficiency. I can understand your determination not to complicate matters and to come up with a clear result, but I would like to have seen a more precise definition of what "soft" and "medium" mean in your study when it comes to blade hardness/stiffness. "Soft", "medium" and "hard" are subjective attributes unless there are underlying replicable measurements such as durometer shores or the load required in pounds/kilos to effect a 30-degree bend. For further information, see the thread at https://forums.deeperblue.com/threads/freedive-blades-hardness-numbers.108157/. I wonder too whether it might have been of interest to know the blade dimensions of the fins tested, particularly as you have used a "scuba/military" fin as a control without specifying its length, breadth or stiffness. Such parameters were factored into what might have been the world's first test of fin efficiency, W. G. Fischer's 1956 US Naval Experimental Diving Unit report Comparative Evaluation of Swim Fins, which can be downloaded from http://www.dtic.mil/dtic/tr/fulltext/u2/780665.pdf.

As good researchers always say when they begin a line of enquiry yielding positive results that support the initial hypothesis, "more research is needed". Your pilot test sounds very promising so far! Keep up the good work.

 

[dforcucci]

Congratulations on completing and publishing your test of freedive fin efficiency. I can understand your determination not to complicate matters and to come up with a clear result, but I would like to have seen a more precise definition of what "soft" and "medium" mean in your study when it comes to blade hardness/stiffness. "Soft", "medium" and "hard" are subjective attributes unless there are underlying replicable measurements such as durometer shores or the load required in pounds/kilos to effect a 30-degree bend. For further information, see the thread at https://forums.deeperblue.com/threads/freedive-blades-hardness-numbers.108157/. I wonder too whether it might have been of interest to know the blade dimensions of the fins tested, particularly as you have used a "scuba/military" fin as a control without specifying its length, breadth or stiffness. Such parameters were factored into what might have been the world's first test of fin efficiency, W. G. Fischer's 1956 US Naval Experimental Diving Unit report Comparative Evaluation of Swim Fins, which can be downloaded from http://www.dtic.mil/dtic/tr/fulltext/u2/780665.pdf.

As good researchers always say when they begin a line of enquiry yielding positive results that support the initial hypothesis, "more research is needed". Your pilot test sounds very promising so far! Keep up the good work.

Thanks for the fin test report from 1957! Very interesting. You can see that much more $$, effort and design went into that study. The fin I used for the base line is the UDT Voit which is "A" in the study and had one of the best efficiencies, but maybe not so comfortable. Gives a good cross reference and in hindsight I am glad I used that fin instead of a pair of modern scuba fins.

I used the manufacturers subjective stiffness rating, ie soft, medium, hard. But your point is well taken and if I ever do a more in depth comparison it would be advisable to quantitate the stiffness and blade dimensions.

One of our freedive group members wants to do some testing vertically with depth. And now I have some ideas on what to do vertically! I will post the design in this thread to get input before I run the test.

Thanks for your encouragement and useful input.

 

[cdavis]

"The remaining things you point out are not supported by any empirical data that I know of? "overbend"? That is a relative term."

Sorry if my comments seemed a bit abrupt, no disparagement intended. You are following exactly the path I took in testing fins, with the exception of having a really neat speedometer, and making the same mistakes I did. There is actually a large amount of experimental data that is applicable to thrust in fins, its just not in the freediving world. Think fluid dynamics and airfoil dynamics.

To not understand that the shape of the flexed blade is key to thrust is a major lack of knowledge. I suggest you spend some time reviewing REVANS various posts. He is an engineer, fin designer and builder who has posted a whole lot of interesting and informative stuff. I've tried some of his prototypes, own and dive with one. His knowledge of what makes a fin go is solid.

You correctly show that plastic provides less speed than carbon or fiberglass. Its also modestly less efficient at just about any level of power input. One of several reasons for that is fact that as plastic flexes, it absorbs more energy that it does not give back, relative to carbon or fiberglass. Another is the lack of taper in most (all?) plastic fins. Taper improves a blade's ability to flex properly over a wider range of power inputs.

I guess I should have mentioned, we took videos of several ofthe dives I referenced. You could see very clearly in the vids how the fin was being overpowered.

 

[hteas]

Here are some old results on fin effectiveness. Dave Mullins did repeat dives to 30 m with a variety of fins. He shows the results in graphs. I don't think there is any better answer to the basic question of what matters.
https://forums.deeperblue.com/threads/cheap-fins-or-expensive.75390/

 

[dforcucci]

Howard, Thanks for sharing this.

 

[WynisFish]

Hey everyone,
I tested the max output of a variety of freedive fins in the pool with an underwater speedometer! The results are posted on Freedive Wire.

http://freedivewire.com/freedive-fin-efficiency/

I enjoyed your article and seeing that I need to decide on some fins soon, this was a helpful perspective to consider! Thanks.

 

[Vov]

Bravo, Dave, you have done a great piece of effort to find out what everybody would like to know. I guess the real question in free diving and spearfishing is "Which exactly fins would allow me to dive deepest and stay down longest" and still be able to swim up safely... Clearly that is a tough question but still you have done a great job.
Whether we measure off-the-shelf fins efficiency or custom made, we need to specify a lot of conditions, which may and would jeopardize any test. Because we are different. A soft fin would perform differently if one bends his knees much or almost does not... or if one is 65 kilos and another 100 kilos... One of the most difficult things to learn is to not move one's legs sideways when diving (while paddling forth and back). This skill requires special training and its lack or presence blurs the compared results. So difference in stiffness, angle, width, length, what part of the blade is actually bending, shape of trailing edge, shape and position of rails, shape and stiffness of foot pocket... additionally to parameters already said , all affect performance.
Given the obvious complexity of the task, I would still participate in a wider effort to plan and perform a comparative test. Performance is important because we make thousands of kicks during a 5 hour diving day, so even small differences are repeated many times.
(Of course, safety is more important, so it depends who exactly is using the fins... A pair of super flexible soft fins are of little use to a diver who has gone down to where he is 10 or more kilos of negative buoyancy... ). Thank you.

 

[dforcucci]

Bravo, Dave, you have done a great piece of effort to find out what everybody would like to know. I guess the real question in free diving and spearfishing is "Which exactly fins would allow me to dive deepest and stay down longest" and still be able to swim up safely... Clearly that is a tough question but still you have done a great job.
Whether we measure off-the-shelf fins efficiency or custom made, we need to specify a lot of conditions, which may and would jeopardize any test. Because we are different. A soft fin would perform differently if one bends his knees much or almost does not... or if one is 65 kilos and another 100 kilos... One of the most difficult things to learn is to not move one's legs sideways when diving (while paddling forth and back). This skill requires special training and its lack or presence blurs the compared results. So difference in stiffness, angle, width, length, what part of the blade is actually bending, shape of trailing edge, shape and position of rails, shape and stiffness of foot pocket... additionally to parameters already said , all affect performance.
Given the obvious complexity of the task, I would still participate in a wider effort to plan and perform a comparative test. Performance is important because we make thousands of kicks during a 5 hour diving day, so even small differences are repeated many times.
(Of course, safety is more important, so it depends who exactly is using the fins... A pair of super flexible soft fins are of little use to a diver who has gone down to where he is 10 or more kilos of negative buoyancy... ). Thank you.

Hi Vov,
Thanks for your comments. However I do believe "soft" blades can move us to the surface from depth. At first it doesn't fell like it though.
Dave

 

[Vov]

Entirely true, Dave. I guess I did not say it clear enough. (Since, obviously, English is nOt my mother's...)
I do know that soft blades are generally much better than the others. We are thinking about a limited amount of O2 and efficiency means we count every molecule! The whole point is "How soft, how long, how wide, what part of the blade is actually bending, how the blade is attached to the foot pocket, what rails, how tall rails...) which translate into a whole lot of very interesting , experiments, materials, decisions...

 

[dforcucci]

Entirely true, Dave. I guess I did not say it clear enough. (Since, obviously, English is nit my mother's...)
I do know that soft blades are generally much better than the others. We are thinking about a limited amount of O2 and efficiency means we count every molecule! The whole point is "How soft, how long, how wide, what part of the blade is actually bending, how the blade is attached to the foot pocket, what rails, how tall rails...) which translate into a whole lot of very interesting , experiments, materials, decisions...

I agree, well said.

 

[ronscuba]

Entirely true, Dave. I guess I did not say it clear enough. (Since, obviously, English is nOt my mother's...)
I do know that soft blades are generally much better than the others. We are thinking about a limited amount of O2 and efficiency means we count every molecule! The whole point is "How soft, how long, how wide, what part of the blade is actually bending, how the blade is attached to the foot pocket, what rails, how tall rails...) which translate into a whole lot of very interesting , experiments, materials, decisions...

Good post.

Differences in foot pockets and tendons are a big part of the puzzle as well. People focus so much on the blade, they forget how the FP affects performance. There is more to FP than just weight and fit.

For example, take a soft fiberglass blade that has little taper. Put it in a foot pocket with stiff tendons and stiff sole. You get a fin with soft tips for easy kicking, but the stiff tendons and sole are there for when you need power.

This is what I got when I switched from Leaderfins older all rubber foot pockets to their newer TPR thermoplastic rubber foot pockets. The Leaderfins fiberglass blades are soft. Combined with their all rubber FP, the fin felt too soft. The same blades with their newer foot pockets are awesome.

If your blades have a good taper, maybe you want FP with soft tendons so it does not interfere with the blade bending.

 

[dforcucci]

Good post.

Differences in foot pockets and tendons are a big part of the puzzle as well. People focus so much on the blade, they forget how the FP affects performance. There is more to FP than just weight and fit.

For example, take a soft fiberglass blade that has little taper. Put it in a foot pocket with stiff tendons and stiff sole. You get a fin with soft tips for easy kicking, but the stiff tendons and sole are there for when you need power.

This is what I got when I switched from Leaderfins older all rubber foot pockets to their newer TPR thermoplastic rubber foot pockets. The Leaderfins fiberglass blades are soft. Combined with their all rubber FP, the fin felt too soft. The same blades with their newer foot pockets are awesome.

If your blades have a good taper, maybe you want FP with soft tendons so it does not interfere with the blade bending.

Hi Ron,
Great point. The foot pockets are an important part of the equation. Lots of permutations available.

Your statement "the fin felt too soft" is an important one. How "feel" relates to performance still needs to be researched.

Thanks,
Dave