Endorfins Slater KS1 Tri FCS II Surfboard Fins - Medium/BlackSlater KS1 medium size tri fin FCS II surfboard fins by Endorfins. Designed by 11 time world champion surfer Kelly Slater. A better build innovative carbon inlays in Endorfins add strength and flex that enable them to be built with a core comprised of 90% pure air. As a result, Endorfins not only provide optimum performance but also literally float on water. FIN BOXES: The FCSII compatible base will require two screws that will come with the fins.