K4 Flex Pair of ‘QUAD’ US Box Rear Fins for Windsurfing Board 13cm only.
They come complete with nylon plate and screw for US box fin systems.
“Our original fin for performance wave riding. Lots of flex for amazing grip and looseness on the wave. Raked template for control. A progressive foil for different flows between tip and base. Test winner. Will loosen up a ‘stiff’ board and are particularly sweet as twin fins as they eliminate that unwanted top turn spin out.” – K4 Fins