Untiring Resilience of Monprene TPE Helps Put the 'Jet' in Scubapro's Hydrofoil-Design Twin Jet Swim Fin

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EL CAJON, CALIFORNIA, U.S.A., June 19, 2000: An innovative swim fin with the same propulsive action as an aircraft wing or hydrofoil enables scuba divers to achieve high forward thrust using an effortless, relaxed kick, according to Scubapro, a leading manufacturer of diving gear and equipment.

"The key to the high efficiency of our Twin Jet (Trade Mark) fin is its dual-blade design, which converts much more of the diver's kicking energy into forward motion than other swim fins," said Dr. Sergio Angelini, Director of Engineering for Scubapro. "With a conventional paddle-shaped fin, each kick creates turbulence that wastes energy. Water displaced by the Twin Jet fin, however, is redirected over the top and bottom of each blade, creating a pressure differential like that which generates lift in an aircraft wing, but in a forward motion."

To ensure that the fin flexes smoothly as the diver's foot cycles from downstroke to upstroke and back, Scubapro required a material with toughness and elastic memory. These properties would enable the blades to snap back quickly to their original positions at the end of each stroke and resist tearing after constant flexing. The company evaluated thermoset rubber but selected Monprene (Registered Trade Mark) thermoplastic elastomer (TPE) from QST Inc., which is part of the Specialty Elastomers Group of Teknor Apex, to injection mold each fin. For each of two models of the Twin Jet fin, QST supplies a different custom formulation of the Monprene TPE compound.

Besides twin blades, other features of the Twin Jet fin include: 1) vents through which water flows after passing over the blades; and 2) an overall angular configuration (when
the fin is viewed in lengthwise profile), that contributes to a nearly effortless glide through the water, Angelini said, by maintaining symmetry between upstroke and downstroke.

These combined features enable divers to reach higher speeds than with conventional fins, maintain greater control when changing direction or speed, and produce more power when pulling, towing, or swimming against currents, all with less exertion. "As a result, Twin Jet fins reduce diver fatigue and cut air consumption by up to 40% compared to conventional swim fins," said Angelini.

Diving is even more effortless with a special version of the fin called the Twin Jet Graphite. While the standard Twin Jet model is black in color, is slightly heavier than water, and has the feel of a conventional paddle fin, the gray Graphite model is molded of a softer, more flexible Monprene formulation that is lighter than water. "With greater recoil than any other fin on the market, the Twin Jet Graphite achieves the highest kick efficiency with the least resistance," said Angelini. "It feels as if nothing is on your feet."

In both Twin Jet models, the interplay between Monprene TPE performance and fin design is most apparent in the unique rib or rail along the outer side of each blade. "With each upstroke or downstroke, the rail must bend within a narrow range of degrees, no more or less, so that the thinner blade remains perpendicular to the diver's direction of motion," said Angelini. "Then it must rapidly recover its original shape in time for the next stroke."

With a thickness of close to an inch, this rib presented a processing challenge, according to James McGee, President of Pacific Molding Inc., which molds many components for Scubapro. "We needed to find a way to cool a part that thick without refrigerated water, which would have compromised its surface appearance," McGee said. QST consulted with Pacific Molding on the solution to the problem, which included modifying the Monprene compound to optimize processing. "Wayne Thornton [Market Manager for QST] was here almost every day until we found exactly the right way to produce this part," said McGee.

In conventional swim fins, the key property of "snap" (i.e., rapid elastic recovery or resilience) is typically provided by natural rubber formulations, according to McGee, but these materials provide insufficient outdoor weathering resistance. "We looked at EPDM synthetic rubber because of its outstanding weatherability," McGee said, "but it does not provide enough 'snap'. The same is true for most types of TPE on the market. Monprene TPEs, however, deliver a high degree of resilience combined with excellent weathering resistance."

Elastic recovery can be measured by tests for compression set, QST's Wayne Thornton pointed out, but elastomers with similar compression set values may differ widely in speed of recovery, or 'snap,' as demonstrated in tests for the modulus of elasticity. "Our Monprene compounds provide not only the elasticity and resilience required for the Scubapro fin but also high levels of tensile and tear strength, which often do not go hand in hand with elasticity," Thornton said. "In addition, Monprene TPEs have a low coefficient of friction, which contributes to low drag in water. In fact, these fins virtually repel water."

QST Inc., which is part of the Specialty Elastomers Group of Teknor Apex, is located at 300 Industrial Park Road, St. Albans, VT 05478. Tel: 1-802-524-7704. Fax: 1-802-524-7860.