FINS: 2003 pool tests

by Jim Grier, Dept. of Biological Sciences, North Dakota State Univ., Fargo, ND, USA
(Note: revised to correct originally-miscalculated speeds for unrestricted snorkeling speeds, 28 April 2004.)

I don’t consider pool testing of fins to be very useful or informative because pools are too confining, the conditions don’t really simulate normal use of the fins (except perhaps for snorkeling), and some fins, for reasons that I don’t understand, seem to perform differently in pools than in open water. Nonetheless, pool tests are easy, convenient, and do provide some information. If for no other reason, the differences that I found between pool and open water might help explain some of the differences among fins that other persons have reported (and sometimes debated), as their tests may have been done entirely in either pools or open water, not both.

I have access to a large, full-sized pool at our university, frequently use it for workouts, and decided that I might as well get exercise and data at the same time.

The pool results for 2002 are currently reported elsewhere (for that, click here). I ran several new series of fin tests in the pool during 2003 and the results of those are reported here, including pointing out the relative differences among fins that I got between the pool and open water tests for the same fins.

All tests were randomized and balanced, using Latin Square experimental designs, so all fins in each test were run in all sequence positions and at random.

In brief, here is a summary of the findings:

• There are measurable, significant differences among fins tested in the pool.

• The results from pool tests are not always the same as for open-water tests. Pool tests of the various Apollo open heel bio-fins (black, blue, XT), along with the Cressi Maxirondine for comparison, yielded the same relative outcomes as open water scuba tests with the same fins. However, pool tests of a variety of full foot fins produced differences among some of the fins than for their comparative open water tests. I prefer the open water tests as the most useful for fin comparisons.

• Speed generally improves with practice and conditioning, over a period of days, but only up to a point. Thus, series of tests early in the season or when using different kicks (e.g., backward finning) showed marked increases over the days that the tests were run. But series involving routine flutter kicks later in the season were already under conditions of previous training and maximum physical conditioning, and did not show further, significant day to day improvement.

• Backward finning is difficult and, as might be expected, MUCH slower than forward flutter-kick finning. Actual speeds achieved for backward finning ranged from 0.41 to 0.73 mph while unrestricted on-the-surface-snorkeling fast forward speeds ranged from 3.32 to 3.84 mph. Speeds for the impeded pool tests, using a board held perpendicular to the direction of movement for induced drag to simulate the presence of scuba gear, were faster than backward finning but slower than unrestricted snorkeling speeds. The speeds with increased drag were mostly in the 2+ mph range. (Note: Most scientific measurements and most countries of the world use the metric system. However, since I anticipate that most of the readers of these reports will be divers from the U.S., I am using the English system of measurement. I considered using both, but it would be too messy. If you want metric, you can easily convert from my reported values [including doing it in the Excel spreadsheets that I have made available throughout the various reports].)

• At least for this set of full-foot fins tested in both forward and backward finning in the pool, there appears to be NO correlation between how well a particular fin does in backward finning compared to normal forward flutter kicks. I have been unable to predict how well any given fin might do for any given characteristic. Each fin simply has to be tested empirically for performance for each characteristic separately to determine how good it does in that aspect.
  

The remainder of this report shows the details. I am only providing the graphs and brief comments (above their respective graphs), including whether the differences are statistically significant (by ANOVA or correlation coefficients; tables not included in this report). (For readers who are not familiar with standard statistical testing, a smaller P value means greater signficance, with the usual rule of thumb for significance being "P < [less than] 0.05".) If you want a copy of the raw data (which includes all of the pool results for both 2002 and 2003), click here.

Note that the design of this testing, measurement techniques, and subsequent analyses and graphs provide very high resolution for differences in the factors being considered. Most divers, including myself, doing routine diving would not be able to detect these differences in speed. It takes careful, objective tests and accurate instruments, such as used in these studies, to reveal the differences. Probably the only differences among fins that are easily sensed by the diver involve the feeling of strain when kicking the fins or a feeling of "weight" of the fins under water (which doesn't necessarily correlate with their actual mass or weight out of the water, as reported in the 2002 report).  Occasionally one becomes aware of differences in speed or air consumption between fins if a buddy is using a different kind and the two of you are swimming together. But because of a large amount of variation among divers themselves, differences caused by the fins they are wearing are usually not easy to detect by subjective comparisons.

OPEN HEEL APOLLO BIO-FIN POOL TESTS

The open heel Apollo bio-fins consistently perform the best among all of the open heel fins in my tests as well as in most other recent tests, such as by ScubaLab. I was interested in seeing if I could measure any differences among the different versions of the bio-fins. I tested them using the same techniques as during 2002 pool tests (for details, see the 2002 report), including a board in the water held perpendicular to the direction of travel to increase drag, to simulate wearing scuba gear.

For all practical purposes, all three of the Apollo bio-fins performed similarly, although I found measurable and statistically significant (P < 0.01) differences. The relative differences among the three bio-fins (and the Cressi Maxirondines that I used for further comparison) were the same as I also got in open water tests with scuba gear.

Over the period of days that the tests were run, there appeared to be a trend for increased speed, but it was only very slight, not statistically significant (P = 0.16), and not likely real. I had recently been doing many strenuous pool workouts and other previous tests and was, thus, probably in more or less maximum condition at the time.

The sequence of runs during a day, except for the first run when my legs were freshest, did not appear to affect the speed. Note that each fin was tested once during the first run of a day at some point in the series, thus, even if there was a sequence affect, it would be balanced out and not bias the results for the different fins. (Note that the graph does not show the runs in complete proper sequence! It puts runs 10-16 before 2-9. That is an artifact created by the software I used to do the analysis and graphing. I could have found a way around the problem and fixed it but, particularly since the sequence didn't make any difference anyway, I wasn't in the mood to waste time fighting with the computer.)

FULL FOOT FINS

... tests with induced drag (to simulate wearing scuba gear)

By later in the year during 2003, by the time I did the open water tests, I had obtained more full foot fins to test. When I did these pool tests, however, I had only a limited number of kinds to work with. The relative comparisons among the fins by these pool tests were different than for the same fins compared under open water conditions. For the best, most useful comparisons among fins, I believe the open water results should be used. The pool results are shown here only to show what I got and to emphasize that different outcomes can occur between pool and open water tests. Thus, for what they are worth, here are the pool results. Note that the actual speeds obtained with the full foot fins are considerably higher than for the open heel fins above, which is consistent with observed differences between open heel and full foot fins in general.

In this series of tests there was a significant (P < 0.01) day effect. I had been doing other exercise (including diving in Cozumel!) but not the strenuous pool workouts for nearly three weeks. My conditioning had dropped slightly, and the differences probably reflected my getting back in shape.

The sequence of testing, again and aside from the first run of the day being noticably higher, was not significant. (The sequence order in the graph has the same problem, for the same reason, as described above with the Apollo fins.)

... tests at unrestricted snorkeling speeds, without added resistance

After doing the tests with increased drag, I became curious about how fast I could go flat out snorkeling with no increased resistance. So I ran an entirely new series of tests with the board held flat on the surface, as only an object to hold onto and stabilize my runs and timing, using a snorkel and remaining in a flat, face-down position on the surface, with minimum drag. I also decided to experiment further with backward finning, and threw my old Contempro full foot fins into the mix for good measure. I alternated between backward finning and fast forward, which also provided some relief for my leg muscles, by alternating which muscles were being used. I made two runs each day with each set of fins and averaged the two. The design, continuing to use randomization and balance, was a 5 (fins) by 5 (day) Latin Square. The results were interesting, including higher speeds for forward flutter kicks and new differences among fins than what I had gotten with higher drag! The differences among fins were still significant (P < 0.01) but the fins changed positions somewhat in their relative performances.

forward flutter kicks --

I had been getting lots of leg exercise during this period, including much diving, which, perhaps in conjunction with the muscle relief that occurred by alternating between backward and forward tests, evidently resulted in no significant differences among days.

There was also no significant effect resulting from sequence of the run during the day.

backward finning --

The differences among fins were barely significant (P = 0.047) and their relative positions were different than with the forward performances.

Although the day of testing was not important for forward movement, as seen above, it was significant for backward finning (P = 0.01). I believe that resulted not from a muscle conditioning effect but, rather, practice and becoming better at backward finning and coordination, that is, my technique was improving. I could feel that I was improving as I practiced it during the course of the tests. I'll have more to say about being able to fin backwards at the end of this report.

Sequence of the run during the day, however, still showed no significant effect. There appears to be a very slight trend, perhaps also from getting better from run to run throughout the whole series of tests, but the sequence effect is not statistically significant and, therefore, probably not real.

How do performances of given fins compare between the different kicks, forward flutter and backward finning? Is there a correlation between the two, either positive or negative? That is, if a fin is better at one, is it also better with the other, or worse? Interestingly, I found NO correlation, in either direction, between the two performance characteristics!!! The statistical correlation was about as close to zero (-0.02) as one can get! The correlations between the first and second runs were high (as they should be, or else all of these tests would be meaningless!), with r = 0.67 for forward and 0.93 for backward, respectively. But there was virtually no correlation between forward and backward. Different fins evidently have their own performance characteristics, which can vary from characteristic to characteristic!

Here are plots of the correlations:

Additionally, here are the speed graphs, forward and backward, repeated side by side for comparing the relative performances of the different fins tested:

What about paddle versus split fins?

Overall, all other factors being equal, I still believe that split fins are better and more efficient than paddle fins because they direct more of the water out the end of the fin, as thrust, and have less wasted turbulence off the sides. Split fins definitely create less strain on the legs as a result. However, all other factors in fins -- including the specific formulas of the materials that go into the fins -- are NOT equal. Unlike the situation when split fins first appeared, there are now a tremendous number and variety of fins in all categories. As I have noted repeatedly, there is even variation among fins of the same model or under the same name.

Because of the variation among fins within each category, I now find it increasingly difficult to generalize, or even attempt to generalize, about paddle versus split (or other) fins. It simply depends on which particular fins are being considered. A different mix of fins in any category can change the averages and comparative statistics for that category.

In the present small, nonrandom sample that I tested in the pool and am reporting here, for example, the differences between paddles and splits were not significant for either backward (P = 0.51) or forward (P = 0.24) speeds. If different or additional fins were to be included in each of the categories, I would expect the average outcomes and statistics to also change as well.

For what it's worth (not much), here are the comparisons by paddle (p) versus split (s) categories for this particular, small group of fins that I tested. Backward speeds are shown on the left and forward on the right, offset because the actual speeds are so different between backward and forward. As indicated above, the differences between paddle and split are not statistically significant for this group of fins for either backward or forward direction.

 Additional comments on backward finning

I find backward finning to be an intriguing but difficult skill. Speaking for myself I have found it, so far, NOT to be like riding a bicycle (that is, once you've learned it, you never lose it)! After I did my preliminary learning in the pool, I tried to fin backwards several times under different conditions and with different fins in open water dives. Sometimes I could do it and sometimes I couldn't, as I found when I asked other divers (who told me they could do it) to demonstrate to me -- sometimes they could and sometimes they couldn't! Furthermore, when I went back to the pool (in the tests reported above), it was almost as if I had to learn backward finning all over again! Backward finning, at least for me, is difficult, variable, and seems not to be an easily retained skill. Although I have now done it many times, such as for the pool tests above, maybe I still haven't done it enough to get it completely fixed in my neuromuscular coordination.

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James W Grier
North Dakota State University              

1 December 2003
-- revised to correct originally-miscalculated speeds for unrestricted snorkeling speeds, 28 April 2004 --