Scientists Debunk Fin Claims

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 (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 depthrigidity) 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.