Scientists Debunk Fin
Claims
... Apollo voted best of seven in performance
from the January,
2004 issue of Undercurrent
Divers today are faced with a dizzying array of fin designs:
split blades, Venturi action, vents, troughs, side rails, etc. Each
has its own performance claims, carefully crafted by advertising copywriters.
Nevertheless, a university study appearing in the spring 2003 issue
of the Undersea & Hyperbaric Medicine Journal explains
why these bells and whistles may be just so much hype.
Researchers at the University at Buffalo in New York purchased seven
different brands of fins, each with at least one unique characteristic:
Mares Plana and Avanti Quattro, Apollo Bio-Fin Pro, US Divers Blades,
Scubapro Twin Jets, and three models that have since been discontinued:
the Mares Plana Attack, Oceanic’s Ocean Pro, and US Divers Compro.
The Apollos were tested in their original split blade design and with
the split blades taped together.
Using recruits from local dive shops, the study measured the energy
cost of swimming (oxygen consumption as a function of velocity, i.e.
aerobic velocity), body drag and efficiency of fins; and kick frequency,
velocity, and maximum thrust in a swimming pool.
Rigid and Flexible Fins Conserve the Most Energy
Interestingly, both rigid (Attack) and flexible (Apollo) fins conserved
the most. The maximum aerobic velocity was significantly higher for
the Attack (6%) and Apollo taped fins (6%) and lower for the Compro
(-10%).
Scubapro Twin Jets, manufactured from the same Nature’s Wing patent
as the Apollo, were stiffer but did not perform as economically as either
the split or taped Apollos. In fact, the Scubapro fins (9%) had a significantly
lower maximal aerobic velocity than all the others.
Apollo Has Less Drag
The drag a diver must overcome has to be offset by the kick thrust,
which comes primarily from the power stroke which was greater in more
aerobically economic fins, like Attack and Apollo. Because a kick’s
recovery phase provides little thrust and adds to body drag, it has
to be compensated for by an increase in kick frequency or thrust. Attack
and Apollo were in the power phase of the kick less time (34%) than
the fins with less power (39 to 41%). The Apollos produced significantly
less drag, because divers kicked deeper with the Attack fins. But with
a narrower kick depth, the frequency of kicking with the Apollo (both
split and taped versions) was significantly higher and the efficiency
was lower (5%) compared with the Attack fin (7%). The two fins with
a higher energy cost also had higher drag and lower efficiencies. This
indicates that kick depth and kick frequency have to be optimized to
minimize the energy cost and accounts for the similar performance of
the Attack and Apollo fins.
Fancy Features Slow Fins Down
The researchers determined that length and width of a fin’s leading
or trailing edges, surface area, weight, and flexibility did not significantly
influence the energy cost of swimming. The type of material and the
use of winglets (flanges), longitudinal splits, vents, or ribs alone
didn’t increase the velocity of water down the fin, and in fact
may decrease it, thus leading to lower thrust and economy.
Stiffness Affects Thrust
Divers generated significantly greater thrust (distance per kick) and
velocity with the Attack. The distance per kick for the Attack fin was
significantly higher (19%), while the Apollo was significantly lower
(16%). The maximum distance per kick increased as a function of stiffness.
The maximum velocity was highest for the Attack (15%). Although the
maximal kick frequency was higher for the Apollo (13%), the distance
per kick was significantly lower (14%).
Flexible fins had to be used at higher frequencies as the distance per
kick was significantly less. The stiffest fin (Attack) had the highest
maximum thrust, and the diver covered the greatest distance per kick
(1.19 m/k).
The Attack’s more flexible sister fin, the Mares Quattro, produced
slightly less velocity and distance per kick. But the Quattro outpaced
both split fin models in distance per kick (that may be why the British
magazine Diver rated the Quattro superior to the Apollo, based
on subjective field tests reported in the August Undercurrent.)
Between the two split fin models, the Scubapro Twin Jet provided significantly
more distance per kick than the Apollo, which scored well below average
in this measure. In fact, the taped Apollos outdistanced the split version.
The researchers say that “The notion that rigid fins develop more
thrust and speed was not supported by this study. The maximal velocity
and thrust were developed by both rigid (Attack) and flexible (Apollo)
fins, with the limitation of the rigid fins being the ability of the
diver to generate a high frequency kick and the limitation of the flexible
fins being the diver’s maximal leg kick frequency.”
More important than any single design feature was the correlation between
fin stiffness and hip angle, reflecting a deeper kick and resulting
in a greater distance of travel per kick. The stiffer Attack, Blades,
Quattro, and Compro fins were kicked deeper and with a lower frequency,
thus achieving a higher thrust per kick.
Conclusion
So what about all those advertising and editorial claims? They sound
good but when compared with the findings from this study, they appear
to be all sizzle, no steak.
The researchers concluded: “Based on the physics of fin swimming
... it is clear that some fins have better performance (Attack, Apollo)
than other fins; however, this cannot be ascribed to a single fin characteristic.
The research also made it clear that Venturis, vents, troughs, and splits
did not improve the performance of the tested fins.” As a result,
they determined, “Further work is needed to develop the optimization
of fin characteristics, by lowering drag (kick depth-rigidity) and maximizing
efficiency (kick frequency-flexibility), to minimize energy requirement
and maximize performance.”
So the ultimate fin isn’t here yet. When shopping around, try
as many models as possible, and consider the type of diving you prefer.
For instance, a more flexible fin worked better for female divers in
this study since women, with lower body weight and density, generally
have less metabolic power and thrust than men, but lower cost of energy.
Some designs help you more when swimming into currents or for short
bursts of speed. Others may prove better when used with a frog kick
or in sculling. All these will be personal judgments, based on your
own leg strength, conditioning, and kicking style.
D.R. Pendergasti, J. Mollendorf, C. Logue, and S. Samimy.
Departments of Physiology and Biophysics and Mechanical and Aerospace
Engineering, Schools of Engineering and Medicine and Biomedical Sciences,
University at Buffalo, Buffalo, NY 14214. “Evaluation of fins
used in underwater swimming.” Undersea Hyperb Med 2003.
30(1): 57-73.
Faulty Fin Claims
Mares Quattro
Claim: Flexing center channels translate leg movement
into directional thrust, while the stiffening side rails increase the
blade’s effectiveness for frog kicks and sculls. (Review in Sport
Diver)
Finding: Channeling of water down the fin by troughs
or rubber channels (Quattro) does not appear to improve thrust or economy.
Scubapro Twin Jets
Claim: Proprietary drag-reducing vents and extended
side plates for increased leverage and power. (ad in Sport Diver)
Finding: The use of Venturis, or vents, either forward
or rearward facing, does not appear to improve thrust or economy. Water
does not pass through the vents, thus they do not relieve the negative
thrust in the recovery phase.
Apollo
Claim: Split fin design allows for more power and less
effort than any other fin. (Apollo’s website: www.apollosportsusa.com/Biofin.html)
Finding: There was no difference in energy cost between
the split fin Apollo and the same model with the split taped closed,
suggesting that the split was ineffective in improving economy. The
longitudinal splits also do not appear to improve its thrust.
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