FINS ... in Depth
by Jim Grier, Dept. of Biological Sciences, North Dakota State
Univ., Fargo, ND, USA
with a contribution by Mike Collyer, Dept. of Biological Sciences, NDSU
I tested a variety of different scuba diving fins many ways with several divers over several weeks. Although already shedding light on a number of performance characteristics (of both fins and divers), including differences beyond just the basic style of fins, these results are still incomplete and tentative. However, further tests are planned and this report is online, active, and will be periodically updated and revised as new data are obtained. These are independent tests that are not associated with any manufacturer, distributor, or sponsor. The original data are available via links for anyone who would like to form their own analyses and conclusions. Life is just a series of approximations; I have tried to take fin testing to the next stage.
Last revised/updated: 16 January 2003
Originally posted 20 October 2002
(Original report as of 16 January 2003; minor editing and
miscellaneous clean-up work is planned)
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Table of Contents
Either scroll down or click on the sections you want.
To get back here, scroll back, or use the back-to-top links at the end of each section.
- Introduction and Background
- Methods: overview, challenges/limitations, and general descriptions
- Methods: details
- Fins that were tested (to date)
- Results (to date)
- Miscellaneous preliminary aspects and evolution of techniques
- A controlled, balanced, randomized experiment with measurements done blindly,
by Mike Collyer
- Ad libitum measures: Maximum speed
- Air consumption
- Static thrust
- Correlation between the different performance measures
- Physical measures -- putting fins behind the 8 ball! (methods and results)
- Pool tests -- for what they're worth (methods and results)
- Discussion (including tentative conclusions and suggested future directions)
- General comments: overall comparisons between different types of fins
(open heel vs full foot and traditional paddle vs splits vs pivots vs ...),
leg/muscle strain, interesting differences between otherwise similar fins,
and the difference between how fins feel like they are performing versus
how they are actually performing
- Familiarity and previous experience (or lack thereof) with particular fins?
- Puzzles, including my old Contempro full foot fins and Force Fins
- The human (diver) factor!
- Other elements (comfort, fit, buckles, appearance, cost, etc.)
- Kicking into the future
- Links (including to the original data and a summary of other links and references)
- About the Authors
Fins are a major component of one's dive gear -- part of the engine that moves a person as an extension of the diver's legs and muscles -- so it's important to know what works best for different divers and circumstances. (What is best for one might not necessarily be best for all.) However, most divers have only used or tried one or a few kinds of fins themselves and they don't have the time, access to a wide variety of brands and models, or inclination to spend valuable dive time trying other fins. Information from gear manufacturers and distributors is understandably focused on just their products, often is self-serving with a lot of hype, and has to be carefully interpreted. ScubaLab has provided a valuable service to the diving community by testing and reporting on many kinds of gear, including fins. But even their tests cannot cover everything; and questions remain. I wanted to do further tests, mostly to satisfy my own curiosity but also my results might be of help to other divers as well as gear manufacturers and distributors.
I became interested in pursuing some of these questions and began comparing a wide variety of fins in different ways. Education is a process whereby you learn that what you previously thought you knew, was wrong. I used to think that fins were just fins; any old fin would work as well as any other; and it was just a matter of how you used them. Well, after pursuing the matter of differences among fins (and divers) over the past couple years, including several weeks of recent, extensive fin testing, and finally getting techniques and tools that provide good resolution (e.g., to 1 psi in air consumption and 0.05 mph in speed), it is clear that there are many differences. In addition to differences among different categories such as split, paddle, pivot, and others, I found differences even among fins of the same design, even out of the same molds, but made of different materials or with other slight differences in construction. I also discovered that different divers may respond to the same fin in different ways and for different reasons.
It appears that there is no one "best fin for all divers and all times and places." Just as a variety of fins and webbed feet occur in nature among different fishes, amphibians, aquatic birds, mammals, and reptiles, with different shapes and degrees of flexibility, different fins undoubtedly work better for different divers and different circumstances. This includes variety among divers with different physical conditions, motivations for diving, and types of diving that range from light recreational to working with heavy loads. Numerous factors are involved and the picture is very complex.
I dive a fair amount, both recreationally and as part of my work and interests with a wide variety of animals (both terrestrial and aquatic). While on a dive trip to Puget Sound in August 2000 I saw the new split fins for the first time and became curious. They appeared strange to me. I tried a pair that I borrowed from another diver on one of the boats, but wasn't sure whether I liked them or not. When I got home I rented a pair from a local dive shop and did some crude tests (by dragging a stick to make a line on the bottom of a local lake then timing dives with an underwater stop watch and looking at air consumption using a standard SPG, marked in 500 psi increments). The split fins were comfortable, worked, and initially seemed faster and more efficient than my old paddle fins. So I bought a pair (ScubaPro Twin Jet graphites, the grey ones) and used them off and on over the next year and a half, along with other kinds of splits (including a pair of Atomic Aquatic Splitfins that I won for a report that I posted here on D2D) plus my old paddles (Cressi Maxirondines [no longer produced by Cressi]) which I still liked.
However, particularly as a scientist by profession, I'm an eternal skeptic, not only about what others say but even of my own thoughts and ideas, and I'm constantly checking and rechecking things. Either as planned dives or in the course of other diving, I continued to pay attention to fin performance. For example, while doing long distance dives for underwater lake measurements using GPS, I switched among different fins for different segments of the work and kept track, for example, of the distance covered per tank under similar conditions but with the different fins. I also occasionally took different fins to the pool for routine workouts, followed discussions (and heated debates!) about fins on D2D, read the dive magazines, previous tests (e.g., see references at end of this report), and advertisers' messages, etc. In general, the split fins seemed to perform better ... but not always, and there were some puzzling questions. Occasionally the old paddle fins gave better speed or better distance on a tank of air; results in the pool were not clear cut; and different fins of the same basic type sometimes seemed to do better than others of that type. Some divers continued to vigorously defend one kind of fin while others strongly disagreed. What was causing all the disagreement, questions, and confusion? This sort of thing starts nagging and gets under a person's skin!
When the offer to join a ScubaLab fin-testing team going to Guanaja, Honduras, was announced here on D2D, I applied and was fortunate enough to be picked as one of the members. That provided valuable experience and insights, plus led to the development of much better fin testing techniques, but it also left many questions unanswered. In Honduras we only tested some of the most recently designed fins. Because of logistical constraints and limited time and numbers of divers, we couldn't test even all of the latest crop that had proliferated, let alone compare them with previous models, many of which remain popular, even "standard," with divers around the world. We also stumbled across (or were reminded about -- we should have known from previous knowledge among divers) the differences between full-foot and open-heel/adjustable-strap fins.
Testing fins in Honduras. We recorded data on slates using waterproof paper.
I returned from Honduras wanting to pursue some of these issues further. I thought a lot more about the questions, possible ways of testing them, bought a Suunto Cobra dive computer so I could measure air consumption with better resolution -- to 1 psi (absolute accuracy is only to 10 psi, that is, to the real pressure in the tank, but these relative measures should be good to 1), and started borrowing and buying as many different kinds of fins as I could get my hands on. I roped in a number of interested and willing dive buddies to help. And the whole thing soon got out of hand. (This project, however, has been only part of my overall diving; I squeezed these tests in among many other kinds of dives as well.) Mike Collyer, one of the graduate students in our department at the university and one of my dive buddies, became interested and involved with some of the fin testing dives and much of the discussion on design and data analysis. He analyzed and wrote up the results from our controlled experiment. (Note added in response to a reader's comments: Mike is working on other subjects for his graduate work [see link at bottom of this report, under Authors]. This project was extracurricular for both Mike and me, although it provides a service role, one of the three components of our university mission [teaching, research, and service], by making this information available to the general public and business/marketing world.)
Two of the volunteer divers, Laureen and Matt, who helped with the fin testing in Ontario and Minnesota.
I want to make it clear right up front that all of this testiing was entirely independent and of my own doing. Although I received much help and input from many people (see Acknowledgments), I (and nature!) are responsible for the outcomes, interpretations, and any mistakes that I might have made. I am not working for or representing any particular manufacturer or distributor, directly or indirectly, had no sponsors, and did not get paid for this. (In fact, I spent a lot of my own money to accomplish it!)
I am neither for nor against any brand or model of fins. Since I already owned or purchased several of the fins, I didn't care which of them performed better in particular ways or even if they were all the same; I simply wanted to find out! ... as completely and objectively (quantitatively) as possible, given my available time and limited resources. I didn't care where the chips fell; as I sometimes tell my university students, although I might be mean, at least I'm fair -- I'm mean to everyone!
This report is only a "work in progress" and far from definitive. It is only a rough start, very incomplete, and probably will never be finished. I had neither the time nor resources to test all of the fins that are currently available. And reality with its numerous unexpected problems, including weather and scheduling of volunteer divers (see Methods), routinely interfered with plans. Thus, even the tests of the fins that we (I and several volunteer divers) did accomplish, as represented in this report, are far from having as many replications and being as complete or balanced as I had hoped. Numerous gaps and questions remain. I encourage skepticism. I myself view these results as tentative and subject to change. If you want the final word on the subject, this isn't it and you might as well hit the back button now. However, I expect to continue squeezing in future fin tests among my various dive plans. With the wonders of modern computers and the internet, this report can remain active and changing. I'll revise and update it as appropriate.
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The fins were tested in freshwater lakes in northwestern Minnesota and nearby Ontario, Canada, starting in July 2002. The methods involved an ongoing period of development and evolution in an attempt to improve the techniques and in response to results and new questions as they arose.
I initially wanted and planned to test the fins using proper experiments with a large number of replicates, using a large number of volunteer divers, with randomized trials, balanced data (with equal numbers of trials across all fins and factors), and blind measurements (in which the divers did not know which fins they were wearing during any particular trial). I and Mike Collyer, a graduate student in our department at the university, did manage to successfully accomplish such an experiment in one component of the project. But logistics, weather, being able to borrow some of the fins only at particular times when they weren't being used otherwise by their owners, and problems scheduling divers made a proper experimental approach impractical on a large scale. Furthermore, it takes a lot of time to run just a few tests (and air! ... including underwater work associated with the tests, in addition to the tests themselves, I and my buddies used well over 60 tanks of air). So I had to settle for getting measurements whenever, with whomever, and involving whatever fins I could get my hands on. That resulted in a hodge podge of data that can be interpreted only to a limited extent, mostly only in a simple descriptive terms and not subject to the rigorous statistical tests that I originally desired. But that's life; one can only do the best possible job with the available resources and make the best of the outcomes.
Divers who I was able to get to participate were mostly males, in good physical condition, of various ages and weights, and experienced, active divers. We also primarily tested the fins using the basic flutter kick. I wanted to test the fins using other kicks, such as frog and dolphin, but that would have doubled or tripled the necessary number of tests. We were already limited by time and logistical constraints. Life is full of trade-offs, so we had to pass on trying other kinds of kicks for now. The basic flutter kick is the main one used by most divers most of the time anyway; and I consider the other kicks to be relatively unimportant in the overall picture.
The two biggest challenges involved weather and scheduling of divers. During a summer when much of the United States was suffering from drought, we had abnormally wet and unsettled weather in this region with frequent storms involving a lot of rain, wind, and lightining. As a result, I had to postpone or cancel several planned tests. And when a planned set of dives were interupted by storms, it often proved difficult or impossible to arrange new times with particular divers, particularly when either they traveled to where I was or I had traveled to their location. I estimate that because of these problems, I was able to get only about half to two thirds of the measurements that I otherwise could have obtained. Oh well.
An example of the weather we frequently encountered! This approaching
storm cut short a series of tests in Ontario.
In addition to weather problems, I also encountered a number of other unexpected, even bizarre situations that proved challenging to deal with. Those events included problems that ranged from a large school of bullheads (fish) stirring up the silt to horses in the water! (For a description of those encounters, click on: bullheads, horse crap.)
Types of tests
It is difficult to measure the performance of fins (or any other gear for that matter) in a standardized manner that simulates real-life diving conditions, which is what we really want to know about. To cover the array of situations, one needs a variety of tests.
Imagine a continuum involving the use of fins with two extreme (and mostly unrealistic) end points: at one end is a diver moving through the water at the maximum speed he or she can achieve. The other extreme involves a diver kicking his or her fins but not going anywhere. Normal diving is somewhere between those two extremes, with the diver moving at a slow to moderate speed and occasionally stopping then starting to move again. In addition to the speed of movement (and related issues such as the flow of water over the fins, drag from the diver's body and equipment, etc), there is a matter of what might be best summarized as "load" (which involves more than just weight per se). Loads can range all the way from very light, such as easy recreational diving in calm, warm, tropical waters, through increasingly difficult situations involving current, colder temperatures, additional gear, and heavier actual weight. Really heavy diving can include, for example, some technical, commercial, and heavy recovery diving. Furthermore, there are numerous other factors related to such things as physical condition of the diver, gender, psychology, previous diving experience, and on and on! Trying to design tests around all of this is like being a mosquito at a nudist camp: where do you start?!
I initially planned to test just in the middle of the continuum, that is, measuring air consumption of different divers using different fins under a standardized set of conditions (constant depth and speed). I then decided to also look at one of the end points of the continuum, maximum speed. I subsequently added tests also for the other end of the spectrum: measuring thrust under static, stationary conditions. Neither of the extremes are realistic for true diving. Maximum speed cannot be sustained for more than a few seconds and it burns through energy and air like you can't believe. Most divers can run up to maximum speed only around 10-15 times on an entire tank of air (normal air; one can do more with nitrox). And since we were doing 4 replicates per test (details in sections below), we could only get about 3 tests per tank on the average. It's kind of like drag racing with an automobile -- you can get up to a high speed, but only for a short distance and you don't really go anywhere! How realistic is that? Why bother? (And I was asked those questions by other divers several times!) At the other end of the spectrum, why would one want to be stationary under water, just kicking fins in one place to measure thrust? That's like a tractor pull. However, I figured that the combination of information from different points on the continuum might help paint a better overall picture about any given fin's performance characteristics under different conditions. And, just as in the world of vehicles where there is a range from slow but powerful tractors and trucks to small, fast race cars, is there a similar range for fins? In addition to the endpoints, I also looked at the middle, more realistic part of the spectrum. Even looking at three points represents only a start. There is also a point between the low end and middle, i.e., moving slowly with lots of gear or otherwise doing heavy work, perhaps for a sustained, long period of time. Higher up the scale, past the middle but below the maximum possible speed, there is the realistic situation where a diver may be in current and have to put out increased speed/effort for a considerable length of time. Maybe I, or someone else, will devise tests for these intermediate conditions (I have some preliminary thoughts, under the conclusions) and run them later. In the meantime, I had to start somewhere.
I ended up running three types of tests, at the two end points and middle of the continuum: maximum speed, air consumption rates under constant conditions, and stationary or static thrust. Trying to find the best way to accomplish each of those measures required some trial and error; some ways worked better than others.
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FINS THAT WERE TESTED
First off, it is important to recognize that all of the measurements I conducted involved both task loading and a level of diving skill to accomplish properly. ALL of the tests reported here are vulnerable to artifacts and misleading measurements if not done properly, carefully, and with much attention to details.
Miscellaneous preliminary tests
I started planning and developing these tests from where we left off in Honduras. The Honduras tests were great because they were significant improvements in technique and design over previous testing. The ability to have several divers participate at one time in a randomized fashion was great, and something I hoped to achieve in my tests. Unfortunately, the participation of lots of divers at the same time was not practical in my situation and those hopes fell apart. However, I was able to take some of the techniques much further than what we were able to do in Honduras.
One of the questions that arose following our Honduras tests related to the speed of swimming for the air consumption (efficiency) tests. We had used a standard speed of 1 mph. A couple of the fins didn't hold up even to that speed very well, without a lot of effort and fatigue on the part of the divers. But, for most of the fins, 1 mph didn't seem to be a problem. In fact, we questioned whether it was sufficient to really test the fins. So I decided to increase it in my tests to 1.2 mph, and marked my line (with tied strips, so one can reach each one at 10 second intervals while watching a stop watch). I discovered that 1.2 mph was too fast for several of the fins I tried to measure. Thus, if I was going to continue to use a measured line, I would have to change the intervals for either the markers or the stop-watch times (e.g., same marks on the lines but at 12 second intervals).
Marked lines work great. I improved the system so it could be set up simply and cheaply using cement blocks, empty bottles for buoys, and line, in a manner that could be set up anywhere (and the main line could easily be taken down when not in use, so as not to get snagged by passing fishermen). The following drawing shows the setup.
Diagram of underwater measured line for measuring air consumption at constant speed and depth.
In the end, however, the measured line still required a lot of work and effort, including a boat to help set the cement blocks. And the lake where I set mine up got an unexpected algae bloom, along with unusually frequent winds that kept the sediment stirred up, so the otherwise good visibility there was gone for much of the summer.
I next tried a modification of my towed-GPS technique. (For a previous report on that, click here. GPS signals do not go through water; so use of them for diving requires that the unit stay on the surface and be towed.) Instead of using my canoe to carry the GPS unit, I rigged up a simple little low-drag float that I could attach to my dive flag. (Note: aside from measurements using this GPS unit, ALL measurements made in this project were done without carrying the flag, which would add one more drag factor. We simply anchored the dive flags and conducted tests around them, unhampered by having to carry the flag.)
I made the float from an empty, plastic one gallon water bottle. I tipped it on the side with one edge down, to serve somewhat like a keel of a boat, added sand for ballast, then filled the remainder with the expandable foam from a can, like used for insulation or to fill gaps in things. Before the foam hardened (which takes several hours), I pushed in a flat groove on the top of the container to make a slot for the GPS unit to set in. Then I made a little wet suit carrier or harness for my GPS unit from an old three-fingered dive mitt, by cutting out holes for the display, control buttons, and antenna. To use it, I double-bagged the unit in plastic freezer bags, to keep it waterproof but at the same time allowing the display to be seen through the bags and the buttons could be pushed for resetting, etc. When finished and in the water, with the flat cap at the end of the bottle, the device looked like a masked pig head!
Photos of the GPS floating rig. The green arrow marks the line, slightly visible through the sides of the bottle, between the sand in the bottom and the light-weight, expanded foam in the top. Also note the flat groove at the top made for the GPS unit to sit in. The floating unit could then be tied to the dive flag float, for easy towing behind the dive flag.
GPS unit on float for measuring distance and speed.
This rig worked great, and I plan to use if for other purposes in the future. But for testing fins it had two problems: (1) it was vulnerable to drifting on windy days and (2) the towing line, which added an unknown amount of drag and also caused occasional entanglement and disruption of measurements.
I eventually abandoned both the measured line and GPS float for these studies, and instead used a combination of depth gauge, UW speedometer, and stop-watch to standardize depth, speed, and distance.
Speed tests were done using borrowed UW speedometers (not currently available on the market) like we used in Honduras. The diver holds the speedometer out ahead of himself or herself, lays out flat for maximum trim, and builds up speed to the maximum that can be achieved. This is a very intensive use of energy and air and, after "hitting the wall," requires that one stop and recover aerobic respiration, like a cheetah having to stop and rest after it has chased prey in a rapid burst of speed.
Maximum speed, like stationary thrust tests (see below) do not simulate normal diving. These speeds cannot be maintained for more than a few brief seconds at most and cause a huge oxygen debt that has to be paid by resting afterwards. But maximum speed does push fin performance to the limit, which is one of the things that I wanted to test.
The manner in which one kicks and attempts to reach maximum speed is important. It is often stated that split fins require a different style of kicking than paddle fins, using shallower, faster kicks for the split fins. However, wider kicks (and legs spread apart in the water) = increased drag in all cases. Thus, to the extent possible, (some paddle and other stiff fins don't reverse direction as well as more flexible fins) the kicks need to be the same, or else one may be testing kicking style rather than fins! The problem of shallower-faster vs wider-deeper-slower kicks involves not only fins but legs (mass, length) and muscles (strength, ability to reverse contractions, etc). But water is water, as we all know, and has a lot of resistance -- although not many divers stop to think about the implications for depth of kick.
There are trade-offs between water's resistance on spread legs and our natural inclination to speed up or increase thrust by taking wider strides (which doesn't make much difference in air, such as when walking). The speed generated by kicking is also like boat propellers: the faster they turn the faster the boat goes. The bottom line is that maximum speed is reached with as shallow and fast of kicks as one can accomplish with any particular set of fins. (The same applies to air consumption and even static thrust [which generates some of its own flow over the fins]. The large resistance of water against the legs is involved in all cases.) The final consideration involves momentum: it takes a little time to accelerate one's body mass and you can't hit maximum speed without building up to it.
Thus, my instructions to all test divers, which always worked if they consciously thought about it (and which they didn't need to waste time discovering for themselves) was to stretch out flat for maximum trim (minimum drag), start out slowly and watch the speed climb using whatever kicking was most comfortable (often a wider, slower kick), then gradually go faster and faster, with an all-out final push using shallow, fast kicks regardless of which fin one was wearing (i.e., paddles as well as splits -- the same kicking technique works best for all, plus it needed to be standardized).
Air consumption was measured to 1 psi using an air-integrated Suunto Cobra computer. Changes in tank temperature, such as when the tank is taken into cooler water from the air or when it moves from warmer surface water to colder deeper water, affect the pressure of the tank and have to taken into consideration. Otherwise temperature changes in pressure can be mistaken for, or confound, diver air consumption.
I initially assumed that pressure changes resulting from temperature changes would occur rapidly since both the metal tank and water conduct heat rapidly and differences in the air inside the tank should mix quickly from internal convection. I figured that by the time a diver was in position and ready to start a test run, the tank's pressure would have come to an equilibrium with the ambient water temperature and not be a problem. However, that was not the case. The greatest changes did occur rapidly, but the final stabilization took longer. The problem was not truly appreciated until part way through the tests when enough data had accumulated to see that the first readings on tanks were higher and out of the normal range for the subsequent readings, indicating that pressure changes were still occuring from changes in tank temperature.
I then measured the time required for the pressure to stabilize by taking a tank-BC-and regulator to depth in cooler water, from warm air in the shade on a hot day, while breathing from a second tank-BC-regulator setup. Details are shown with the Air consumption data. We subsequently made sure that tanks were cooled at depth before starting tests, to avoid misleading data. Tanks were initially cooled to surface water temperatures by putting and keeping them underwater until needed, with a minimum of 15 minutes. Then we made sure at least 5 minutes at depth had passed before starting tests.
To conduct a run, the diver first established neutral buoyancy, horizontal posture, with arms extended forward and hands coming together to form a point in front of the body, with the underwater speedometer held in one hand and the gauge console in the other (with the hose running above the shoulder and arm) so that speed, depth, tank pressure and a stop watch attached to the console could all be seen. The starting time was written on the data form, along with an even numbered psi that was lower than the current reading (to allow time to start everything). Then the diver would start moving and establish the desired depth and speed, while watching the psi readings on the computer. As soon as the chosen psi was reached, the stop watch was started. The diver then proceeded at constant depth and speed until exactly 5:00 minutes had passed. As the stop watch hit the 5:00 minute mark, the psi was read and recorded. The depth and speed were maintained at 15 (14-16) ft and 1.00 (0.95-1.05) mph. Keeping a precise depth and speed (whether with a measured line and stop watch or by depth gauge and speedometer) is difficult and required practice to accomplish. After doing an outbound run, everything was repeated for a second, return run.
Traveling at 1 mph for 5 minutes covers a distance of 440 feet, nearly 3 times the distance of 150 feet used with the measured lines (both in the Honduras tests and the measured line I used in Minnesota). The longer distance permits a greater length of air consumption and reduces the effect of intitial start-up effects (there is a time lag between the start of exercise and the increase in respiration), which should result in overall better accuracy. At the same time, the increased amount of time (5 minutes vs 1 minute and 40 seconds) required to conduct each leg of the test does not add significantly to the overall time required to do the tests, since most of the time involves setting up for the test, changing fins between tests, etc.
For final comparisons of air consumption among different fins, rather than using raw air-consumption or psi rates per se, consumption was related to a diver's resting air consumption rates (measured) and converted to surface air consumption (SAC), using 34 ft for 1 atm in fresh water.
Static thrust was measured by having the diver push against a wooden bar that was connected with a line to a digital scale which measured up to 50 lb, read by an observer above water. A simple PVC pipe frame held the scale on top, above the water, and an underwater pulley at the bottom to connect the line from the diver to the scale. The frame had two PVC pipe extensions that rested on the top of the dock or pier; and the framework was tied down with ropes at both the top and bottom to hold against the pull of the diver. The whole setup could be easily taken apart for transportation or storage.
Measuring thrust in a static position lacks much of the water flow over the fins compared to normal movement through the water (although some water movement is generated even with the diver in a fixed position). Thus, this test, like maximum speed tests, does not reflect realistic diving conditions per se. Nonetheless, it should measure fin performance at this end of the continuum. It's sort of like testing jet engines by bolting them to the ground.
The photos below show the equipment disassembled, assembled on land; and attached to a dock or fishing pier. A second wooden bar, placed well behind the diver, held the ropes from the bar apart and created a space where the diver could work without hitting the rope, as shown in the pictures below.
Static thrust measuring set-up.
Static thrust equipment ready for use, with line and push-bar on the surface just before the diver descends.
The diver would get in position by taking ahold of the bar floating on the surface, descend with it to a depth of about 15 ft, establish neutral buoyancy and a horizontal position, then begin kicking while pushing against the bar. Arms needed to be fully extended to avoid creating false readings from arm movements. As with speed and air-consumption tests, these tests required some practice and skill to accomplish properly. Both the diver and the person reading the scale need some initial instruction and training to make the measurements valid, reliable, smooth, and free of misleading artifacts (such as from pushing against the bottom, jerks, or arm and body movements).
The readings on the scale typically bounce around while the diver gets in position and builds up to higher levels of thrust. Divers were instructed to build up first to a level that they could maintain and hold for awhile, a flat or plateau level, then do a run-up to their maximum exertion, after which they release the unit (which floats back to the surface). The pattern of readings, including the final release that drops back to 0 thrust, allows the person recording data to know what the diver is doing and when to properly record the information, as in the following diagram. (Also note: the electronic scale had an automatic shut-off after a period of time with no readings, so it had to be turned on anew each time before the diver started to descend, to avoid misleading zero readings [e.g., if turned on after the diver had descended].)
Diagram from a hand-drawn instruction sheet, showing a chart of how the data were generated and measured.
Maximum thrust measures, we discovered, were reached with the same kind of kicking as for maximum speed, that is, with shallow fast kicks, even though the flow rates (and drag) across the diver and fins would be different between the two types of tests (rapidly moving during speed tests vs staying stationary [hence, "static"] for the thrust tests).
First, a word of caution: data involving small sample sizes, nonrandom samples, unequal sample sizes, and without blind measurements must be treated carefully and tentatively. Some analytical techniques, such as combining unbalanced data into larger groupings or using inappropriate analyses, can create misleading outcomes. (In some cases, for example, unbalanced data can even cause an apparent reversal of outcomes as a result of some groups contributing too much weight to the outcomes, a well-known phenomenon called Simpson's paradox. [For references, click on 1, 2, 3, or do a web search for Simpson's paradox.]) Also, small sample sizes are more vulnerable to statistical Type II errors than larger samples. In plain English that means that, if one does not find a significanct difference among groups, that doesn't mean it doesn't exist but that you simply might not have detected it. Small sample sizes can also create problems in the other direction, basically for knowing whether differences that do occur are meaningfully significant or not.
Our random, balanced, experiment involving blind measurements, as written up by Mike Collyer below, should be fairly sound. However, the remaining data in this report should be viewed as somewhat less reliable because of the ad libitum nature of the measurements. Accordingly, we have tried to avoid "over-analysing" these data or using inappropriate statistical techniques on them. I have mostly just used descriptive analyses. I believe that the descriptive approaches are meaningful and can be used for tentative inferences, but they should not be given too much weight. Further studies are needed to strengthen and increase the confidence in these results.
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I tested whatever fins I could get ahold of, including some that I already owned, purchased for testing, or borrowed from other divers. I tested the ones I already owned (cr-max, sp-tj-g, ap-blu, and contemp [see below for full names]) first and most often (using them as "standards" to compare results from other fins and to double check that results were consistent from day to day, which they usually, but not always, were). Then I added others into the testing off and on depending on their availability and choices by volunteer divers.
A line-up of some of the fins and gear, ready for a day's testing.
In order from left to right: (see list below for full names) ap-blu, sp-tj-g (white spots?**see note below table), cr-max, ma-quat, ma-volo, da-tigers, contemp, sp-ts-ff, sp-tj-mblu, sp-tj-mblk, and ff-orig.
The full list of fins, by acronym in alphabetical order, full name [categories: oh=open heel/adjustable strap, ff=full foot, p=paddle, s=split, o=other--including pivots, Force Fins, and misc old style types] and comments:
a?-sj Aqualus Super Jet [oh,o]
("?" because I can't find out anything about the company)
I got these from eBay while trying to find a set of ScubaPro Super Jets; they are similar to the Scubapro regular jets.
*see comments below table
aa-sf Atomic Aquatics Splitfins [oh,s] al-blds Aqua Lung Blades [oh,p] The original Blades, NOT Blades II (which, based on our Honduras testing but not tested here, don't seem to perform as good as the originals) ap-blu Apollo bio-fin pro, blue [oh,s] Intermediate stiffness, between the more flexible original black model and the new XT model (next) ap-xt Apollo bio-fin XT [oh,s] The new, recently released XT (for eXtra Torque), stiffer than previous blue and original black models. contemp Contempro [ff,p] An old full foot fin that I bought years ago, and I believe no longer available. I picked up these fins on sale for less than $20 for use in swimming pool workouts. I bought them from a Sherwood dealer and think they were marketed by Sherwood at one time. Made in Mexico. cr-max Cressi-Maxirondine [oh,p] My old, favorite fins, used for years, before the arrival of split fins da-tigers Dacor Tigers [oh,o] ff-orig Force Fins original [oh,o] ff-td Force Fins Tan Delta [oh,o] ma-quat Mares Quattro [oh,o] ma-volo Mares Volo [oh,o] oc-v12 Oceanic Vortex-12 [oh,s] sh-mag Sherwood Magnum [oh,p] sh-trk Sherwood Trek [oh,s] sp-j Scubapro Jet The old jet fin sp-tj-blk Scubapro Twin Jets Black [oh,s] The black ones; stiffer than the graphites; sink sp-tj-g Scubapro Twin Jets Graphite [oh,s] The grey version; more flexible than the black model; Monprene; float sp-tj-mblk Scubapro TJ full foots black [ff,s] Black color; Monprene sp-tj-mblu Scubapro TJ full foots blue [ff,s] Blue color; Monprene sp-ts-ff Scubapro Twin Speed (silver) [ff,s] New full foot fins from Scubapro
*see comments below table, including regarding source
sp-ts-oh Scubapro Twin Speed (blue) [oh,s] A new open heel split fin from Scubapro
*The Scubapro Super Jet fins have been difficult to find in the United States. I have a friend who operates a dive shop in Canada who carries and uses them; but they seem to be unheard of state-side. I ran into a pair of "Super Jets" made by Aqualus and got them on the chance that they might be the same as the Scubapro Super Jets, but they are only roughly similar, not the same. I also can't find out anything about the company, Aqualus -- if anyone knows about them, I'd be curious (send me an e-mail).
The Scubapro Twin Speed full foot model also has not yet been available in the United States. They are produced in Italy and became available in Europe, I believe, starting around last May. They are shown on Scubapro's Italian web site (click here -- note, the site seems to work best on Internet Explorer; I've had problems with it on my version of Netscape), but not yet on the United States web site. The pair that I got was sent to me by another diver who got them out of Europe.
**sp-tj-g ... the white spots painted on them. Those are my old sp-tj-g fins. I painted the white spots on them partly to help identify them when around others with the same fins and partly for what I call "walleye spots." Walleyes (fish) are mostly grey and often difficult to see in the water. But they have white spots on the bottoms of their anal and caudal fins, probably serving for communication (so other fish in their school can easily spot and follow them). If you know what to watch for, it also makes it more likely that you will see them while diving. I often dive with other, newer divers and, short of going to bright orange or some other highly visible fin, I decided to put the white spots on mine so new dive buddies can see and follow me more easily underwater. In addition, it serves as a little topic of discussion, so I can say something about fish and biology -- as I just did here. :)
Gear laid out for another day of testing. Long Lake, Clearwater County, Minnesota.
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Miscellaneous preliminary aspects and evolution of techniques
I went through much trial and error trying to find out what worked best and what didn't. For example, 1.2 mph is too fast to be sustained for some types of fins (but not others). In addition, although measured lines and the GPS unit worked, and could be used, I found it easier and more practical to use the UW speedometer to accomplish various facets of the tests. The effects of temperature changes on tank pressure had to be measured and accounted for (by stabilizing tanks to ambient water temperature) before obtaining correct air-consumption measures, as discussed above.
Many of the tests were run in deeper, open water, and sometimes in poor visibility, with the diver having no visual references, going only by depth gauge and speed, navigating by compass, etc. Although that worked, it proved better to run tests parallel to shore, say along a slope or where the bottom could be easily seen below, and with reasonable visibility (at least 10-15 ft). That made the job of maintaining a constant depth easier and also less boring, with passing vegetation and fish to watch. Incidentally, with many of the tests, particularly at maximum speed, we often passed through schools of minnows or other small fish, sometimes with so many fish in the water that it created an effect like in space-travel movies or some computer screen savers where stars approach from a point in front and rapidly pass by on all sides, except that they were fish instead of stars!
I encountered several problems with weather and schedule conflicts among volunteer divers. Although many divers wanted to help and be able to try different fins themselves, the scheduling often simply didn't work out. I also discovered that tests require much more time (and air -- requiring frequent runs back to the dive shop to refill tanks) than I anticipated. As a result, fancy, complex experimental designs involving a large number of fins, divers, and factors proved impractical. Instead, we had to settle for either ad libitum tests or smaller-scale experiments. In the future, when experiments are contemplated, they should be broken into relatively small modules involving just a few fins and divers per module as was finally done with the controlled experiment reported in the next section below.
(If interested in the original miscellaneous data and progression of early stages, click on: Miscellaneous, preliminary data -- web page or Miscellaneous, preliminary data -- Excel file. Those files include a lot of explanatory comments.)
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A controlled, balanced, randomized experiment with measurements done blindly,
by Mike Collyer
(Note: this was just a small, extracurricular project. I am working on other topics for my graduate research [see link at bottom, under Authors].)
Abstract of this section.
We conducted an experiment to compare maximum swim speeds using four different types of split fins: sp-tj-blk, sp-tj-g, ap-blu, and sp-ts-oh. We used an experimental design that allowed us to draw conclusions about fin speed independent of factors that could affect fin speed such as fatigue, order of comparisons, and variation in fin performance. In addition, we were able to conduct these tests without the diver (Mike) knowing which brand or model of fin he was testing. The ap-blu fins performed the best in terms of speed and were relatively consistent. The sp-tj-g were always the slowest fins, which also was fairly consistent. The sp-tj-blk and the sp-ts-oh fins gave intermediate and more variable speeds. We found an unusual result where the sp-tj-blk fins showed the most change from one run to another on a given day.
This experiment was an attempt to compare fin speed for four different split fins, sp-tj-blk, sp-tj-g, sp-ts-oh, and ap-blu, using a randomized, balanced, design with blind measurements, and repeatable, standardized conditions. The design should also be applicable to future tests with other fins, other divers, and other locations.
Th experiment was conducted at Turtle Lake in Becker County, Minnesota. It involved one test-diver (Mike) and one helper-diver (Jim).The fins were carried to the dive site in a large plastic storage box, with holes to let water in and out, then submerged and weighted on the bottom, where fins could be exchanged without the diver seeing them.
The test-diver was unaware of which fins he was wearing. The test diver swam to and from the test site underwater using his own fins, which were then exchanged for test fins by the helper-diver. Fins were placed on the test-diver by the helper-diver as the test-diver docked himself by holding onto a cinder-block anchored in the sandy substrate at a 20-foot depth. After fins were on, they were tightened for proper comfort and fit by the test-diver reaching back (while not looking). The test-diver then made four successive swim trials (two away and two back) at a depth of 15 feet. Each trial consisted of the diver increasing speed slowly with full leg-strokes until his speed plateaued (generally around 1.5-2.0 mph), after which he switched to shortened and faster strokes. This pace was continued until his maximum speed was reached (or rather, until his maximum speed could no longer be maintained). After each trial, the test-diver stopped to recover his breath until he felt ready to continue, usually requiring a few minutes.
Speed was determined by an underwater speedometer held outstretched together with the depth gauge of the regulator (in a triangular fashion with the hands of the diver forming the apex of the triangle that was in the same plane as the body imagine a diver belly-down on a table with his hands directly outstretched in front of his face). After the four trials, the test-diver recorded each maximum speed on an underwater writing tablet, coded to fin only by letter so the diver wouldn't know which fin he was wearing. The helper-diver recorded the time departed, time returned, and overall air consumed (including recovery time) during the four trials. Then, while the test diver was docked at the cement block with his eyes closed, the helper-diver exchanged the test-diver's fins to the next pair, as predetermined by the randomized selection process.
This procedure was repeated a total of 16 times (64 total speed trials) over two days (8 times per day). Because we tested four pairs of fins, we logically performed two sets of four trials per fin pair per day. We randomly assigned the order of fins for the first four runs and subsequently changed the order of successive runs such that for every four runs, the order of fins was different, no fin followed any other fin twice, and each fin appeared in each position (first, second, third, or fourth) after a total of 16 runs. (For those of you who love statistics, this was a 4x4 Latin square design.)
In summary, we had four sets of four runs of four trials with each of four fin types appearing in each of four positions. The reason for doing such an approach is that if fin A causes such fatigue that fin B has a low score after, and we never ran fin B before fin A, then we would not know if there was an order effect. By having each fin in each position at least once, and not having any two-fin-sequence repeated, we can statistically test to see if there is an order effect.
We initially, following some preliminary runs on an earlier date, planned to perform four runs per tank of air, but that didn't work. We were able to make only three runs per tank; thus, three tanks were used per day with the last tank only used for two runs. However, that did not impact the results; in addition to fin types, we were also able to analyze day of testing, tank used, number of runs within tank, number of runs within day, and sequence of each fin within runs. (One of the great aspects of such a balanced, proper design is that it permits all of these factors to be analyzed! For those of you who love statistics: this was a nested ANOVA with main factors fin type, sequence position, day, and tank, with runs nested within tanks and days.) The analyses are explained below.
There was a significant difference among the four types of split fins with the Apollos finishing significantly faster than the other three. The day of testing, tank used, and fin sequence position had no effect on the speeds recorded. The results of these speed trials are shown in the following figure.
Box plots representing maximum speed recorded for four types of split fins: sp-tj-blk, sp-tj-g, ap-blu, and sp-ts-oh Boxes represent the central 50% of the values and bars extend to 100% of expected values. An asterisk indicates a value that is an outlier. A line across a box represents the median value that falls within the central 50% of the values. For each fin type, there are 16 values (each fin tested 4 runs with 4 trials per run).
We also considered the changes in speed of the four fins over consecutive runs. Changes, in terms of either loss or improvement, was calculated as the difference between the average speed of a fin between two runs for each day. A negative value indicates a loss between the two runs, a positive value indicates improved performance (in other words, we subtracted the first average from the second average). We assume that sequence position had no effect (see above) and that subtracting average scores was, therefore, an appropriate measure for run to run changes. The results are reported in the following table; we report the differences as percentages of the average speed recorded in the first run.
Loss or improvement in speed for a given fin between
first and second runs. A negative value refers to a reduced speed; a positive value to improvement. Values are represented as percentage of average first speed.
Fin (all open heel) Day 1 Day 2 sp-tj-blk -12.15% -4.21% sp-tj-g +1.75% +2.98% sp-ts-oh +0.51% +0.58% ap-blu +1.68% +0.86%
A loss of speed from one run to another could result either from a form of fatigue on that fin or perhaps an unintentional bias or discrimination against that fin. Although we attempted to conduct the tests blindly, and I did not know which fin was which in terms of brand or model and was not familiar with the sp-tj-blk (in fact, I had never even seen it before and didn't know of its existence -- Jim slipped it into the mix as a truly blind test!), I nonetheless quickly associated the performance of particular fins with the feel of them on my feet (and it was not possible to maintain completely blind measures, regardless of not knowing which actual fin I had on). I tried conscientiously to reach the maximum possible speed on every single run; but because the sp-tj-g and sp-tj-blk are virtually the same fin except for composition, the poorer performance of the sp-tj-g fins during my runs with them might have "polluted" my expectations and effort with the sp-tj-blks. Similarly, the experiences of the runs with the sp-tj-blks might, in the opposite direction, have improved my runs with the sp-tj-gs. I don't think so, in either case, but it remains a possibility.
Consistency was assessed on the basis of the overall variation in performance by each fin. Although the box plot of the sp-tj-g is the smallest, it also had an outlier value that, if considered with the rest of the values, would reduce that fin's consistency slightly, making it more similar to the consistency of the ap-blu. The sp-tj-blk and sp-ts-oh showed more variability and were similar in that respect to each other.
Perhaps one of the most important outcomes of this experiment is that the results for these four particular fins were entirely consistent with their performance in the other, ad libitum, non-randomized and non-blind tests, which provides increased confidence in the validity of the other results.
(If interested in the original data, click on: Speed test data -- web page or Speed test data -- Excel file and scroll
down to the bottom section [Part B]. For the ANOVA results, click here.)
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Ad libitum measures: Maximum speed
Speed tests as seen from above: bubbles rising to the surface from the
moving diver (toward right side of photo).
The results (averages of 3 to 4 replicates) are plotted in the following figures. Maximum speeds for various fins were mostly in the 2 to 3 mph range. The maximum that I (diver "g") reached so far was 3.05; diver "t" hit 3.10; and Mike (diver "c") got up to 3.20. The maximum speed for the majority of the tests in Honduras also ranged from around 2 to 3 mph; with maximums for one exceptionally strong diver being 3.60.
As can be seen in the second figure, or by comparing fin by fin in the first one, and as with previous tests by other persons and in our Honduras tests, split fins generally gave higher maximum speeds than paddle fins, and the full foot fins were faster than open heel fins. For a given fin, there is usually some variability from test to test, among different divers, on different days, etc. (Note: the box plots indicate variability. Fins that are represented by single lines did not have enough replicates to measure variability.) However, while variable, most of the fins were also remarkably consistent among divers, days, and so forth, so that the variability was not excessive for most of them. (Regarding the Contempro fins, a full foot paddle fin, see the separate section devoted to it in the discussion.)
An outcome that I did not anticipate involved surprising differences among similar types of fin, even among fins of the same brand and model but made of different materials. At first I thought we were just encountering variability in measurements among the same fin, but the differences held up with repeated testing and among different divers! Examples include the sp-tj-g vs sp-tj-blk, which are the same fin except for the material that they're made from, and, similarly, the ap-blu vs ap-xt as well as the full foot versions of the sp-tj's. I was quite surprised at the difference between the sp-ts-ff vs sp-ts-oh. Although they are full foot vs open heel, the difference in performance was even greater than I expected, with the sp-ts-ff being the top fin tested and the sp-ts-oh being one of the bottom ones! The two pivot fins in these tests, ma-volo and da-tigers, were similar (and better than I remember the performance of the panthers, which we tested in Honduras), but there weren't enough replicates yet to really separate them. The ff-orig vs ff-td were different, but that difference was previously known, as described in advertising from the company (see links at bottom of report, go to Force Fins and compare specs for Tan Delta vs Orig). The differences in the ff's, as well as the others, are at least partially if not mostly, related to the stiffness of the material and ability to rebound; a subject (or subjects) that I hope to delve into more in future tests. I suspect that for a given design there is an optimal stiffness, with performance dropping for either less or more stiffness.
The results are shown below. The first graph will require some concentration and studying to compare among the many fins included. (For symbols and acronyms, see the table of fins that were tested.)
Full foot (ff) vs open heel (oh) and split (s) fins vs the other types (paddles and others).
(If interested in the original data, click on: Speed test data -- web page or Speed test data -- Excel file.)
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Note that with air consumption, unlike speed and static thrust, low values = good performance. The results of the air consumption tests (aside from having values in the opposite direction, that is, low air consumed or high speed) were very similar to the results for maximum speed on a fin by fin comparison. As with the speed results, you can study the graph for specific comparisons.
Only a few fins had enough replications for air-consumption measurements to indicate variability via the box plots. The others are represented by single lines at the measured value.
(If interested in the original data, click on: Air consumption data -- web page or Air consumption data -- Excel file.)
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Because the static thrust tests required more time to set up, could be done at fewer sites (which provide relatively deep water immediately next to a dock or pier), the other tests were receiving most of the initial attention, and several testing periods were lost to bad weather, the static thrust tests had the fewest number of replications. Thus, the results only show single points (represented by lines), without the box plots that otherwise would indicate variability among replications. Also, since we had no previous experience with this type of test, it took more time to work out the initial stages and some of the data are less reliable. I plotted here only the results that I was most familiar with and believed to be the most valid, that is, the tests that I ran myself.
The following graphs are set at the same scale, with the diagram of peak speeds shifted higher to reflect the higher values. The two diagrams are equal horizontally.
(If interested in the original data, click on: Static thrust data -- web page or Static thrust data -- Excel file.)
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Correlation between the different measures
How well do the different measures correlate with each other? Remarkably well and the correlations are statistically significant.
Before conducting these tests, I did not know what to anticipate for comparisons between the tests, particularly involving the static thrust tests. Just as there is a difference in power vs speed among vehicles of different sizes and shapes, I thought there might also be such a contrast among fins, where those doing well at maximum speed would not necessarily do as well in a static situation. However, the outcomes were basically the same. One qualification, however, is that the same kicking style (shallow and fast) appeared to produce peak static thrust just as it did with maximum speed. Perhaps at slower moving (but not static) speeds with heavier loads and different, slower kicks or pushing kicks (vs normal finning), differences might be seen between the different tests. I hope to devise and run such tests in the future, including with varying loads.
In the meantime, based on the results so far, the different tests seem to be giving very similar results. In other words, fins that did well on maximum speed also did well on air consumption and static thrust (only peak thrust shown here, but same applies also to plateau thrust).
The three fins among these tested that performed consistently the best in all categories of tests were: Apollo bio-fin blue among the open heel fins, the ScubaPro Twin Speed full foot, and the Contempro -- an old full foot paddle fin. (See separate section below under the discussion concerning the Contrempro.) Next in line on an overall basis were the ScubaPro full foot Twin Jet black and the ScubaPro Twin Jet open heel black model. I suspect the Force Fins Tan Delta would also do well if tested further, based on the limited preliminary tests with them here.
For the statisticians among the readers, the details are shown below, for both parametric and rank correlations. Also, since there is a potential family error rate (three different tests of the same hypothesis) here, I also included the family error rate correction, i.e, using the 0.02 alpha level to achieve an overall 0.05 alpha for the set (0.95 to the cube root = 0.9830, thus 1.00 - 0.9830 = 0.017, or via Bonferroni correction, 0.05/3 = 0.0167 [for references on Bonferroni correction: 1, 2]).
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Physical meaurements -- putting fins behind the 8 ball!
In an attempt to find out what factors might be responsible for the differences between performances of various fins (in addition to whether they are full foot, open heel/adjustable, paddle, split, pivot, etc), I measured several physical characteristics of the fins, including length, width, weight, stiffness, and rebound, then attempted to relate those to measured speed during diving, from earlier ad libitum tests of the same fins.
Because I no longer had all of the fins that I had measured for speed, efficiency, and static thrust (some were borrowed and I had already returned them, for example) plus I had acquired additional fins that had not yet been tested for speed etc., my measurements at this point have some gaps in them. Included in the new acquisitions are a full set of the open heel/adjustable Apollo Biofins in all of the current versions, blue, black, and the XTs; I have focused on them since all of the previous tests of the blue ones, the only ones I had to begin with, showed them consistently to be the best performing fins in all aspects. (The Biofins, incidentally, are also now available in a full foot version [e.g., click here] but I haven't gotten them yet.) I wanted to try other versions of the Biofins and attempt to figure out what makes them superior performers. I got the open heel ones to fit the boots I wear for both wet and dry suits (Apollo sizes M and LL respectively). (In future tests I plan to investigate such factors as different sizes of the same fins and performance under different diving conditions, such as with dry suits, heavier drag and loads with double tanks, tech diving, and other aspects.)
Stiffness of various materials is related to hardness, which is measured by instruments called Durometers. The units of measure are also called "durometers." There are several different scales depending on the type of material involved. For links with further descriptions and information on durometers, click here, here, and/or here. However, I wanted something that represented the stiffness of the fin blade on a larger scale, such as when the surface is pushing against water, and which also took into account whether the fin was split or not. So I developed my own techniques for fins, which I call the SF (scuba fin) durometer using a rounded stainless steel bowl to push against the fin by a standard amount and measuring the difference in pressure with a spring weighing scale.
I built a frame to hold the fins using 2"x2" boards with steel rods for mounting standard laboratory clamps. The rods permitted the clamps to be adjusted along their length. Two clamps held the fin at the sides, at the front edge of the foot pocket, and one clamp braced against the back of the foot pocket to keep the fin solid in the structure, so it wouldnt rotate against the pressure. The frame was built in such a way that it could hold the fins in either a horizontal position (for measuring stiffness) or vertically (for measuring rebound, see below).
Photos of fin stiffness measuring technique. Left: The fin is positioned so it pushes lightly, by a standardized amount (4 oz or 0.25 lb), against the scale. Right: The scale has been raised against the fin by a standard amount (4 inches or 10 cm) by placing it on a wood platform.
Rebound of the fin. This one was tricky to measure! But I solved it by mounting the fin vertically in the frame, pulling it back a standard distance (4 inches or 10 cm), and seeing how far its snap-back would kick a billiard ball, as measured in inches with a steel tape along a carpeted track.
Photos of fin rebound measuring technique:
Examples of two fins (sp-tj-blk to left and cr-max to right) mounted vertically in the frame. As can be seen in all of these three photos, but particularly in the closeup at the bottom right, a piece of lightweight, flexible plastic ruler was held in place at the edge of the fin with binder clips, to accomodate the differences in the shape of the fin tip and type of fin blade (paddle-vs-split), for pushing against the billiard ball in a standardized manner.
Left: The fin is positioned against a meter stick which is set along marks on the frame, then clamped in place. Once in position, the fin is pulled back 4 inches (10 cm) to the back edge of the frame by hand and held in place by the edges; the billiard ball is placed next to its front edge; and the fin is released to kick the ball down the carpeted runway. Right photo: The distance that the ball travels is measured with a steel tape along the carpet, starting at the back edge of the frame.
View of the lab with the frame and carpet on a central table and other fins ready for testing laid out in the fore- and backgrounds. Note: The carpet is flat and level along the runway where the ball normally travels. It appears wavy or irregular in this photo, but that is only along the edges where the carpet is raised by chairs placed in position to keep the ball from accidentally going off the edge.
Table of measurements for fins that also had speed measurements from previous open water scuba diving tests (see other parts of this report). (If interested in the raw data, in an Excel file, for the physical measurements including fins that don't [yet] have accompanying speed data, click here.)
Units: Speed = average maximum speed in mph. Length, in cm, from front edge of foot pocket to farthest point at front edge of the fin blade. Width, in cm, at widest point on fin (note: widest point on many fins is not at the end of the fin). Weight, in lb. Stiffness = difference, in lbs of pressure, between the two standard positions using the SF durometer. Rebound = distance in inches that the billiard ball rolled after being kicked by the rebound of the fin. (I apologize for mixing metric and English measures in the same table. It depended on what I was using for measuring [e.g., meter stick in cm, scales in lb, and shop tape in inches]. I'll convert everything and clean it up in a future revision.)
I expected some or most of the physical characteristics to correlate to some degree with the performance of the fins. But, to the extent that I've analyzed these data so far, there seem to be few clear trends! I've tried several types of regression (both linear and curvilinear), but with few good relationships appearing. Most of the graphs, for example, show very broad, shotgun scatters of points, as can be seen in the following diagrams. I also looked at the data by fin style and category, rather than with everything lumped or collapsed together, but little or nothing showed up with that either, except a hint of possible relationships between speed and three of the measures: speed and weight (open heel fins only, not splits, see below), fin length, and stiffness (see further below).
Here are the correlation statistics for all fins combined, in a table and graphically. The only correlations that are statistically signficant for these 15 fins (at the p<0.05 level, 14 d.f., for the statisticians in the audience) are those with values larger than 0.497 (i.e., greater than +0.497 or less than -0.497). The only significant ones are weight and rebound, stiffness and rebound, and a borderline relationship between width and weight. Speed does not correlate significantly with any of the other measures!
Here's the linear plot for rebound as a function of stiffness, which makes sense.
Speed can be compared to the other various measures by category of fin (split vs paddle; there were only two pivot fins [Volos and Tigers], which aren't included because two points aren't enough), as mentioned above. However, most of the plots are still meaningless. The only two that had even a hint of meaning (but still weren't significant) when broken down by category of fin involved curvilinear plots of speed against length and stiffness, as shown below.
There is a suggestion of optimum fin length, for example, with performance dropping off for too short or too long. Caution: the math (and computer software) will plot curves as instructed, whether valid or not (you know, "garbage in ...") and, if the lines are removed, these are still not very good (or significant) relationships. For similar lengths, there also is a huge spread among performances. But the possible relationship does make sense. The optimum length may depend on whether the fin is split or paddle, or it may just be a coincidence of the particular splits and paddles that were tested.
In the case of speed against stiffness, if these plots are valid (rather than just a coincidence involving the particular fins that were included), it appears that there may be an optimum stiffness for best performance, in the "2 to 3 lb-pressure/4 inch" (SF durometer) range for both splits and paddles. If so, many of the splits are too soft (as many divers have remarked), with one (ap-xt) possibly being too stiff. Most of the paddles, on the other hand, may be too stiff for the best performance. Note that the best paddle fin for speed (the full foot contemp) is similar in stiffness to some of the split fins in the same speed range. Also note, however, that there is a wide range of speed performance among split fins of similar stiffness. A final word of caution involves the possible influence of the diver factor: Since I myself have done the most speed runs in my data set and have the most confidence in them, the speed results have a large bias toward my measures of them. Perhaps these results would shift or relationships among fins change if the bias changed to weaker or stronger divers, or if a better cross section representing all possible divers was used!
The bottom line thus far is that relationships between fin performance and basic physical characteristics are far from clear and consistent!
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Pool tests-- for what they're worth
While doing routine noon workouts in our university swimming pool, I combined the exercise with tests on different fins. I used random sampling and Latin-square designs to properly compare different fins in different series, with each series testing 4 fins 4 times each per day (i.e., 16 runs per day) over 4 days. So far I have completed tests on 16 fins, in 4 series, and 256 individual runs of fins. I used surface swims with a kickboard held perpendicular to the water, to deliberately increase drag and simulate the presence of scuba gear. To balance the use of muscles, I did half a lap on my back and the return leg on my front. The pool's length is 25 yd, thus, each lap equals 150 ft. Between laps I exchanged fins and rested for a total of approximately 90 seconds. I used a pool timing clock to time the runs, then converted that to mph.
Photos ... (With my skin of northern winter faded pale -- I need to get back to some warm sun!)
Results and graphs.
Pool tests, in my opinion, are not as useful or meaningful as actual open water tests. There are too many constraints (too short of distances, too shallow for good runs underwater with scuba gear, boxed in with walls) and additional sources of variation that are not related to real scuba diving (such as turns at the end of the pool, surface effects). Pools do not provide as good of resolution of differences among fins as open water tests and, because of the constraints and different factors, may even shift the direction of ranked relationships among some of the fins. That is, fins that do best in actual scuba diving tests don't necessarily perform better in pool tests, and vice versa. Although not as good and with a few minor differences in outcomes, however, I have found that pool tests are mostly in agreement with the open water scuba tests. They also permit a greater number of tests in a given amount of time, tests when one might not otherwise be able to do open water tests as easily (e.g., during the winter in northern latitudes), better control of conditions and scheduling, and because it is easier to do more tests under controlled conditions, they shed light on other factors -- such as changes over time like swimmer fatigue and improvements from practice and muscle condition.
For examples at different scales, I looked at average performance for all tests by order of test during the same day's tests, over days within the same set of tests, and over longer periods of several days between series of tests. As seen in the following figures, in the one on the left, the first tests of the day, when I was freshest, were slightly faster on average, but there was so much variability from among the runs during the different parts of the sequence that the differences were not significant.
From day to day (middle figure), there was a slight (and of borderline signficance) improvement in speed. However, over many days from series to series (right figure), there was a marked improvement in speed which I believe comes from increased muscular conditioning over a long period of exercise. Note that the first set in the series was higher than the following three. That was because the first set were all full foot fins, whereas the next three sets were all open heel, adjustable fins. (I was randomly assigning fins within any given series, but different series were just different sets and choices were not randomized between series.) I believe that if all four sets had been open heel fins, the first set would have been lowest and in line with the other three.
The results for specific fins are shown in the following graphs. Keep in mind that various fins don't seem to perform under pool conditions exactly as they do with scuba tests in open water. The differences among fins within each series are statistically siginficant and speak for themselves. The full foot fins, although showing statistically significant differences, were nonetheless more similar to each other than were the fins in the other series. In my opinion the pool testing didn't separate between the full foot fins very well and, what slight differences there were, were different than in open water scuba tests (see earlier results in sections above). Note that I removed the scales and units of speed for the vertical axes because of the conditioning differences from series to series, which would make it difficult to compare the actual speeds between series. The different fins can only be compared at this point to others within their own series. I have other eventual tests planned for run-offs among the best fins from each of these individual series.
Other tests that I am currently running or plan to run include comparisons of the same fins for wide vs narrow kicks (I'm currently in the process of running these tests; the incomplete results thus far, however, clearly show narrow kicks winning for all fins, regardless of whether paddle or split). I also want to run pool tests on other fins that I haven't previously tested at all yet, if and when I can beg, borrow, or buy them.
(If interested in the raw data for the pool tests, in Excel, click here.)
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The basic generalizations that can be derived from these tests are as follow:
- There are measurable differences among fins.
- Similar (or in some cases even the same brand and model) fins may perform differently when made of different materials or with other subtle differences in design or construction. (Examples include the different types of ScubaPro Twin Jets and Twin Speeds; the Apollo bio-fins made of different materials [only two tested so far; other tests of the third type planned for the near future]; and the different pivot fins [e.g., Volos vs Tigers]).
- Performance on any of the three aspects measured (speed, air consumption, static thrust) correlates well with performance in other aspects tested so far.
- These results are somewhat tentative, limited, and should be treated skeptically, not as gospel truth. Inferences are subject to change with future testing.
Leg/muscle strain ...
We did not attempt to measure or evaluate leg strain per se as caused by the different fins, partly because the difference between most paddles and splits is so obvious to begin with. Paddle fins generally cause much more strain and resistance in the water, partly from overall differences in fin stiffness and partly from less efficiency of water movement across the fins, with paddle fins creating more turbulence off the sides. The strain of paddle fins and less strain of most split fins is most noticeable when one starts into a dive with one or the other. Then, as the diver shifts attention to other things, the strain of paddle fins becomes less noticeable, unless or until cramps start to occur. Most split fins are much less likely to cause cramps. Some fins, such as the pivot types and really stiff split fins (e.g., ap-xt) were intermediate in causing strain and cramping.
Feel versus real ...
On a somewhat subjective note, one of the interesting aspects of fin performance that Ive stumbled across is that perceptions can be misleading. Ive not only heard about it from others but experienced it first-hand myself many times. Two of the main aspects with differences between feel and real involve speed and "weight" in the water. A third aspect, involving the feeling of strain, appears to be more real. Ill discuss all three. Except for a couple of examples where differences between the good and not-as-good performances involve the same brand, Ill just speak in general terms to avoid focusing on particular names.
Speed. For some weird reason, many fins that feel really fast are not in fact fast when actually measured. And, to the contrary, some that feel slow (until you compare them and pay closer attention) are actually very fast. Also, using wide kicks with some fins gives a feeling that one is going faster than with narrow kicks, but it may not actually be the case. Stop watches and UW speedometers dont care what fins you are wearing or how you are using them, and the instruments dont lie; they report simply how long it takes to get from one point to another or how fast you are actually moving through the water. These measurements often contrast with ones sensation of movement! As an example involving the same fin but different materials and different speed performances (at least for me), with the Apollo Biofin XT I feel like Im really moving -- but the speed simply isnt there on the speedometer, whereas the Apollo Biofin blue and black versions have a feeling that is neither fast nor slow (the best word I can come up with for how they feel to me is "smooth"), but they both produce excellent speed. The bottom line is that you cant always trust how it feels. This problem of perception occurs both in relatively calm water and in strong current. If in doubt, check it out and go with the instruments or other side-by-side comparisons.
"Weight" in the water. Another misleading feeling underwater is the "weight" of the fins. This feeling has little to do with actual mass of the fins or their weight in the air (also see section below about physical measurements). Rather, the sensation of UW weight seems to be related more to resistance to movement, that is, drag in the water, or perhaps the difference between thrust and drag. Some of the lightest fins feel the heaviest in the water and vice versa: some that are actually heavy, feel the lightest in use. However, some fins that are actually light also feel light and the same can be said for heavy. A good example for the contrast between feel and real weight involves the ScubaPro Twin Speeds. The Twin Speeds come in both full foot and open heel versions. Both are light in actual weight (for their respective full foot or open heel categories) but only the full foot version feels light. The open heel one feels heavy in the water, "heavier" than some of the other open heel fins that are actually heavier. The bottom line is that there seems to be little relationship between actual mass and perceived "weight." Actual weight is probably only a factor when one is carrying them above water, on the way to a dive site or in luggage when traveling.
Strain. As described in its own section above, the perception of strain in the water is something that I havent yet been able to measure or categorize well (or correlate with other measures such as speed or efficiency), although all divers know what it is and I think the differences are there and very real. Strain basically involves resistance to the movement of the fins in the water, probably involves or is closely related to drag, and there seems to be a high correlation between strain and muscle cramping. "Strain" is perhaps the single biggest, and most easily perceived difference between paddle fins and split fins.
Further discussions on several additional, specific topics are provided in the following sections.
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Familiarity and previous experience (or lack thereof) with particular fins?
Familiarity and previous experience with particular fins is a potentially confounding issue for tests of fins. Familiarity, for example, could pose a bias favoring the familiar fin and against others. On the other hand, perhaps one should be familiar to some degree with all fins being tested to avoid lack of familiarity becoming an issue. That is, one might need practice with fins in order to use them properly. Some people have argued, for example, that split fins require some practice and experience with their use before a diver can realize their full potential.
I agree but I disagree, depending somewhat on the fins. The fins that I believe this applies to the most are the Force Fins, to which I've devoted a separate section. Regarding other fins, whether paddles, splits, pivots, or others, I honestly don't think it's that big a deal. I believe that practice, and paying attention to how one kicks with any fins can potentially make a person a better user of fins in general ... assuming that one is not developing bad habits (such as "bicycling") that become ingrained.
I have seen many experienced divers, and even some inexperienced divers, do well with a new type of fin such as some of the split fins right from the start. And as long as (experienced) divers in the tests reported here were given instructions on how to properly do the tests (rather than having to learn for themselves by trial and error), they had no problems and were able to achieve useable measurements from the very beginning, including with fins that they had never seen or tried before (again, with the possible exception of the Force Fins).
In my own case, for one example, I own and have used sp-tj-g for two years. I've made lots of dives, probably over a hundred, with them. And I like them (and still do), so I don't believe I have any bias against them. But there is nothing I have been able to do so far to get high speed or efficiency out of them. Concerning my old Contempros, I have used them for years for workouts in the swimming pool, so maybe my legs and muscles are into some kind of a groove with them. But I don't think so; and I had not previously used them in open water for scuba diving. Also, I have grown to where I don't particularly like paddle fins any longer. But those Contempros did well in virtually every test I tried; I don't think it was my familiarity with them. Furthermore, the highest speed reached with any fin by any of the divers in all of these tests was 3.20 with the Contempros, by Mike (diver "c" in the tables) during one of his first speed runs, when he was new to both doing the speed runs and had never worn the Contempros before (see speed test data under 20830). He clearly was not previously familiar with those fins. (I can't explain why those fins performed so well. Also see the separate section on the Contempros.)
This issue is complex, not entirely clear-cut, and I remain alert to possible complications from familiarity. However, at present I'm not overly concerned about it; I don't think it diminished the validity of these tests.
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My old Contempro full foot fins?
For some strange reason (better fin performance???!), these fins somehow managed to perform the best in virtually all of the tests! There was some variability, and they didn't always do best; but they nonetheless stand out on most of the tests. Sometimes when I got the readings, I didn't trust them and double-checked myself. But sure enough, the gauges and computer didn't lie ... psi are psi and the computer had the numbers right there in front of me. Furthermore, it happened independently with all three types of tests. It wasn't as if there was something wrong on any particular testing day with the speedometer, air-integrated computer, or digital scale. The good performance kept cropping up time and again. It was like when the some of the full foot fins in Honduras performed so well that we all had to stop and scratch our heads. I myself still find it difficult to take these Contempros seriously; but the results beg for attention.
I honestly do not know what is going on with these fins. I even considered taking them out of the report at one point, particularly since they are no longer in production and available. But the results are the results. I tested them and in all good faith have to report them. To me they are a humbling lesson to all of us that there are still some unknowns and unpredictables out there. And don't take anything for granted (e.g., that just because something is old and cheap it must not be all that good or worthy of attention).
What about Force Fins?
When we just put Force Fins on and tested them alongside the many other types, they didn't particularly stand out as having better performance. They weren't bad, but other fins consistently did better among various divers on the average. Divers who had not used them before generally reacted negatively to them. I myself, in fact, didn't like them in several aspects: hard to get the strap on and off; pressure was placed on different parts of the foot than what I'm used to; they seemed to "wobble" in the water during kicks; and I found sculling and other fine maneuvering to be more difficult with them compared to what I am used to.
However, I have some diving friends who love Force Fins and claim they are the best.. Also, the advertising, as well as some of the previous tests of them, certainly tout them as high performance fins. So I figured that I might be missing something. Not getting the best performance out of them might be a result of I and the other test divers not kicking them properly or something we were doing (or not doing), rather than a result of the characteristics of the fins themselves.
So I deliberately sought other divers who liked Force Fins and who were very active divers and had lots of dives and experience with them. I figured that if anyone knew how to use them, it would be those divers. I wanted them to test Force Fins along side other fins using my standardized tests, as well as to permit me to observe how they used their Force Fins.
I tracked down two such divers, but one was traveling in Alaska and unavailable at the time. Thus, I was only able to get one (identified as diver "z" in the speed data, an instructor with over 2,700 dives, mostly with FFs) to run the tests to date. He did excellent with the FFs (using the Tan Delta model). When moving at normal diving speeds, he used a kick style, sort of a modified frog kick with a twist at the end (which I've also observed among a couple of dive masters in the tropics) that appeared very efficient and required only about one kick to five or six of my normal flutter kicks. I was unable to duplicate that kick even after watching and trying it for much of one of our dives together.
When moving faster, for maximum speed, he switched to a regular flutter kick. He was able to get better speed from the FFs than from the other fins that he tried, better speeds than the other divers got with the FFs (original model), and right up there with the top speeds (but not the very top) that anyone got with any of the fins. I was able to get good speed myself with his ff-td, but not quite as fast as I could get with the ap-blu. I have to conclude that I'm missing something in Force Fins and still don't understand them. They might have a learning curve not found with most other fins. Force Fins may be very fast and efficient if used properly by some people. I'm hoping to also run further tests by other experienced users of Force Fins.
While discussing the Force Fins, incidentally, as a good indication of how the speed tests use a lot of air without really going very far (regardless of which fins are used), the diver with the Force Fins accompanied me on some of the speed runs that I did with other fins. He did not try to keep up with my top speeds but let me get ahead and just followed along at normal speed. After I had speeded up and made the run, he soon caught up to where I was, then just hung out with normal, slow breathing while I needed a few minutes to recover my breath.
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The human (diver) factor!
Much of how a diver does with a pair of fins depends on the diver, not just the fins! And, although some fins seem to be generally better than others, just because a particular fin performs well for one diver does not necessarily mean it will also do the same for another diver. Depending on a diver's weight and physical condition, some fins may be better for some divers (e.g., softer/more flexible fins for divers who are not in as good of physical condition, or with injuries, weak knees, etc) than others (e.g., stiffer fins for stronger divers).
Humans in general, and divers in particular, are extremely diverse themselves, both in terms of what they can do (such as when swimming with different fins) and in terms of what they want and how they think! Some go for function; others go for looks; others want both function and looks; and some could care less. Many divers are not interested in fin performance per se and not worried about efficiency, or how they kick. As one diver told me, "I just kick."
Although, I didn't record it, I was alert to what I call the "smile index," that is, the facial and body language of a diver when trying out new equipment, along with their verbal comments. Sometimes they emerge from the water smiling and saying, "Yes!;" other times it's a scowl and thumbs down; and sometimes it's a reserved look or perhaps a gradual change over time (from no opinion to gradually warming up to a particular set of fins and liking them better, or perhaps disliking them at first but then changing over time to a better opinion of them).
From the set of tests reported here, I got several strong smiles: at least six (I lost count) after first using the ap-blu fins, one with the sp-ts-ff (the diver was so excited about them that he couldn't wait to order a pair, even though he was an instructor and staff member of a dive shop where they didn't carry that brand), one with the sp-tj-mblks fins, and one with Force Fins. On the other hand, several divers, including myself, did not like their initial encounter with Force Fins, although, speaking for myself, I gradually warmed up to them during the course of further testing. (Also see the separate section on Force Fins.) Responses to the other fins were mixed, with several of the divers saying that they would never go back to paddle fins -- although a couple of others said they still liked their paddle fins and intended to stay with them. One diver, the one who liked the Force Fins right from the start, also was attracted to the looks (primarily) of the ma-tigers, and didn't mind the semi-paddle feel of them while diving, but preferred the Force Fins; the ma-tigers were his second choice (and above all the other fins he tried or looked at).
The biggest problem with the matter of full foot vs open heel fins is the limited use of full foots (restricted to warmer water and generally with boat diving, where foot protection is not an issue). In cold water or with rough shorelines or risky shallow water (corals, urchins, poisonous organisms), divers simply need to wear booties and, hence, open heel fins. Or, if a person is only going to have one set of fins, which is the case with most divers, they want the one that is most generally useful in the widest variety of situations, that is, the open heel type. Several divers who tested these fins just shrugged their shoulders and said, "Well, the full foots do better, but what are you going to do; I can't use them."
In relation to factors beyond fin performance, there seems to be much that goes into a diver's evaluation of fins that has little to do with function per se. One dive shop owner and instructor who sells Scubapro told me that, although he already knew that the black open heel Twin Jets do better than the grey, graphite ones, and he carried them all in his shop so he could use anything he wanted, he personally still preferred the graphites and intended to continue using them. Speaking for myself, the ap-blu and ap-blk bio-fins perform the best among the open heel/adjustable fins for me in all aspects; and I anticipate using them most of the time in the future. The sp-tj-blk fins also work great for me. However, I still also like the sp-tj-g in some settings such as when working with new divers, where my speed or efficiency are not important anyway, and in some other situations while working with other divers and helping them gear up or checking them out it is handy to have a pair of fins that float and I can just lay them on the water where they will stay until I'm ready to put them on. We all, including myself, have our individual quirks and preferences.
The bottom line on the human element is that divers are not all alike, just as the fins are not all alike! That is a major factor in the equation and cannot be ignored.
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Other elements (comfort, fit, buckles, appearance, cost, etc.)
There is much more to a given set of fins than just performance in terms of speed, air-consumption, and thrust per se. They also vary widely in terms of overall feel, foot-pocket fit, attractiveness of appearance, cost, weight, shape, convenience for packing in luggage, straps, buckles, easy of donning and removing, how they carry in the hand, and on and on. I was not particularly interested in evaluating these other aspects, as I was mostly focused on performance. However, in the course of testing performance, the other characteristics were certainly noticed.
Regarding strap buckles, as we found in the Honduras tests, the Mares ABS buckles are in need of improvement. I and at least one other diver hurt our fingers (and even tore fingernails) with those buckles. The edge of the plastic where they open up is fairly narrow and sharp and the buckle does not release smoothly, so that as you increase the pull on it, it suddlenly comes loose and catches the finger. As a result, I and most of the other volunteer divers simply pushed in on the buttons to lengthen the straps, then slipped them off around the heel in standard fashion. They were put on with the buckles closed but the straps extended and loose. The straps were then cinched up tight as when tightening the straps on most fins. The ABS buckles might have a lot of patents, but I and others have not been impressed with them. I prefer standard quick-release buckles, which were adequate to excellent on most of the other fins (except for the Force Fins -- which have a strap system that may be indestructable but, for me, is a hassle).
Foot pocket fit and comfort are important. Improperly fitting fins are almost worthless. For me, the most comfortable foot pockets were my old Cressi's, with most of the others being good to excellent. The only ones I dislike somewhat (not a major problem) are the sp-tj open heel fins and the Force Fins, in both of which the toes tend to stick out the front of the pocket too much. As far as the full foots, the sp-ts-ff was perfect and easy to put on and off (yet staying on with no problem while in the water) while the sp-tj full foots were more difficult to get on and off because the pocket is higher up the foot.
Cost is an important factor but needs little discussion; divers can only decide for themselves where they stand on that issue.
Attractiveness of the appearance of a fin is in the eye of the beholder. Fins that look cool to one person might seem really ugly to someone else; and among a group of divers (such as by age, location, or where they get their training), fin styles and what's "in" or "out" can vary from group to group or change over time just as with any style or fad.
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Kicking into the future
I myself will probably continue off and on with additional fin testing; and I'll incorporate future results in this online report (with notices on D2D so people will know when updates have been made). I want to look at performance of fins when using more gear and weight, such as with dry suits, double tanks, and under different conditions such as colder water than has been previously tested. I've given some thought to tests along other points of the spectrum from slow to fast, including "push tests" where divers who are otherwise closely matched using different fins and push against each other (the opposite of a tractor pull or tug-of-war).
For faster tests but below maximum speed, there are a couple of testing methodologies that might work, as suggested by the fact that at, say 1.2 mph, some fins can maintain that speed but others can't. One might use a stop watch to time how long one can maintain a given speed, then increase it at faster and faster increments and measure those times, or find the highest speed that one can maintain for a fixed time or distance. For example, one could try going a full time or distance at 2.0 mph, then, if that can't be done, halving it each direction depending on the outcome (e.g., if 2 doesn't work, try 1.5, if that works go to 1.75 or, if it doesn't, 1.25, etc.) until you find the top speeds that works for particular fins.
There are MANY other fins that have not yet been thoroughly tested; better sample sizes (of divers and replications) are needed even for those that have been investigated; a wider diversity of divers needs to be covered; and more properly-designed experiments, preferably with blind measurements, need to be conducted. Most of the past tests, including these, have used mostly experienced, active divers in good physical condition. I think it would be useful to perhaps try the same tests on several categories of divers, say, three groups: (1) active-experienced, good-condition divers as in previous tests; (2) really exceptional, strong athletes, for whom some of the stiffer fins might perform better; and (3) average infrequent divers in relatively poorer condition and for whom softer fins of different designs might work better. Or, perhaps we would find that some particular fins perform best for divers in all categories; at present I think that is an open question.
I also encourage others to try some of this if you're interested. My work only represents a limited number of tests and I'd like to see my results either confirmed or refuted by others. I have only so much time (and interest) myself; and I have lots of other diving to do. (I'm not making a career of fin testing or interested in doing it for the rest of my life!) Perhaps others can improve further on some of the techniques that have been developed so far, or come up with even newer, better ways of measuring fin characteristics.
Whatever may happen in the future, I consider fins to be an important component of one's dive gear. I've found out a lot about them, including a few things that I didn't expect. And I had fun doing it.
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I and several volunteer divers evaluated the performance of 22 different past and present scuba diving fins, some more than others (because of logistical constraints and limited availability of some of them). Although that is a large number of fins, it is nonetheless only a small sample and many of the fins currently available on the market were not tested. Thus, it is very important to note that the conclusions from this study only apply to the particular fins that were studied (which seems obvious; but it is easy to forget that there are also other fins out there). Other fins that were not tested might perform better or worse than these. This study was also limited in terms of replications, balance, and the fact that most divers knew which fins they were wearing. We did, however, conduct one experiment that was completely randomized, balanced, and where the diver did not know which fins he was wearing during the tests. The results from that experiment were revealing and also consistent with the remaining ad libitum testing. Divers were mostly male, in good physical condition, and active divers. Results might be different for a different set of diver types and a wider range of divers needs to be tested in future studies.
Fins were measured for performance at three points along a continuum: maximum speed that could be achieved, as measured with an underwater speedometer; static thrust by pushing against a stationary bar with a line connected to a digital scale; and, in the middle of the spectrum, air consumption under a standardized set of sustainable swimming conditions. This combination of testing conditions provided excellent resolution to discriminate the differences among various fins.
The results showed the same trends that have been reported in previous tests by others, namely that in general split fins are faster and more efficient (and generate more static thrust) than the other types of fins and in general full foot fins perform better than open heel/adjustable strap fins. However, not all split fins do better than all paddle fins and not all full foot fins do better than all open heel fins. I also discovered measurable differences among the various types, including even among fins of the SAME brand and model but made of different materials or with other slight differences in design or construction. How a given, specific fin performs apparently depends on a very subtle combination of materials and design! The differences might not be entirely predictable and only show up once the fin is made and rigorously tested!
Furthermore, the differences do not depend entirely on the fin itself! There is also an apparent, very important, human element in the story. Different divers do better or worse with different fins than other divers, and performance in terms of speed, efficiency, and thrust are not the only factors in why some divers prefer one fin over another.
Additional testing would be useful, perhaps by other persons.
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For the original data in spreadsheet form (available either as web-readable pages or Microsoft Excel files), click on the indicated links:
Miscellaneous, preliminary data -- web page Miscellaneous, preliminary data -- Excel file Speed test data -- web page Speed test data -- Excel file Air consumption data -- web page Air consumption data -- Excel file Static thrust data -- web page Static thrust data -- Excel file
Fin brands tested (so far), in alphabetical order:
- Apollo (USA, with links for other countries; note the new bio-fin XT is available but not on website yet)
- Aqua Lung
- Atomic Aquatics (SplitFin)
- Cressi-Sub (main page [via Italy]; choose country then click on fin menus)
- Force Fins
- ScubaPro ... USA, Italy (web site works best with latest browser vesions, not well with some old ones)
Some of the previously reported fin tests:
- Rodale's Scuba Diving online fin reviews (a series of reviews from 1999 to 2001, covering many fins).
Also see RSD Scuba Diving, July 2002 issue, for the Honduras fin testing results ...
(article per se not posted on the web, click here for: table of subjective characteristics and ranks)
- Diver Magazine's Divernet fin tests (1999, static thrust tests), fin review (2002, speed tests)
- Navy sponsored tests (1991-92) including Force Fins (via Force Fins' website)
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First of all, I want to thank all of the various fin manufacturers and distributors, regardless of how their fins performed. Without them, we wouldn't have any fins for diving or to test!
I thank the following individuals and businesses (in alphabetical order) for help as divers, loaning fins or other equipment, and/or providing insights and useful discussions on the subject: Scott Blessum, Gary Bombadier, John Brumm, Steve Cole, John Francis, Dave Fraser, Karlene Froehling, Shawn Hawks, Jim Hegdahl, Josh Hegdahl, Duane Kashmark, Mike Knorr, Matt Lommel, Mark Lutman, Mick's Scuba, Northwest Divers, Brian Nylander, Laureen Parsons, Paradise Divers, David Rogowski, ScubaLab, Ross Stadsklev, Sunset Diving, Ron Teitelbaum, Gary Thompson, Tri-state Diving, Charlie, Pat, and Rick vanRaden, Peter Van Scoik, and the Guanaja fin-testing team (not listed individually here). In addition I thank 6 persons who either preferred not to have their names listed here or, for various reasons, I chose to keep them anonymous (including two divers who came along and helped one day but we didn't get their names); most of you know who you are and I thank you very much.
Mike Collyer, who wrote one of the sections of this report, was especially involved in this project and report and I thank him very much for all of the many discussions, analyses, and conducting many of the test runs himself. He was also the guinea pig to see if we could do measurements blindly (which we did).
I particularly thank my wife for helping record some of the data and putting up with this project. I was gone diving much of the time or, even when around home, often lost in thought when she would rather be doing something else. Fins and related measuring and diving paraphenalia frequently littered the house. And, in addition to selling some of my non-diving stuff on eBay and using most of my monthly diving allotment for a few months, I also occasionally spent into the family general account to help finance this project; the general fund expenditures are to be eventually reimbursed, of course. :)
Finally, I thank Rodale's Scuba Diving for providing this online message board which makes the many D2D interactions and posting of reports like this possible.
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James W Grier
North Dakota State University