Dorsal Signature Surf SUP Single Center Fin Longboard Surfboard Fins - Blue The DORSAL Signature II (Blue) longboard surfboard fin seamlessly connects turns and allows surfers to move up and down the board. This versatile fin is designed for all conditions and combines drive speed and response for modern longboarding.The Blue Signature Longboard Surfboard Fin is our original fin designed with a slight flex in the first truly transparent material. This fin is the futures answer to longboarding.