ALKALI FINS BIGSKY high aspect twin Japan limited Brand new, unused Retail price: ¥26,180 Base: 140mm Depth: 135mm Foil: 50/50 center foil System: Future compatible The Bigsky high aspect twin is smaller and more refined than most modern twins. Initially, Andrew Kidman used a slightly larger version of this fin with Larry Gephart. Over the years, they kept making the fin smaller, considering how small they could go before the fin became too small. Eventually, Andrew settled on this size. The smaller the fin, the smaller the wetted surface area, and the faster the surfer can go. The double foil helps control this speed and the cavitation that occurs when turning at high speeds. Why double foil? Both twin keels and high aspect twins use double foils. Basically, this allows water to wrap around the fin as it moves through the water, just like the center fin of a thruster or single fin. The double foil helps prevent cavitation during turns, and the water clings to the fin rather than splitting at the edge of the flat inside surface. This gives the surfer more control and prevents the fin from becoming free during turns. It provides a smoother feel.