This 4.6 inch River Flex Fin (fin screw included) is an essential addition to your Atoll inflatable stand up paddle board, and the perfect shallow waters solution! Dont get stuck on the river with a proprietary fin that won’™t fit your paddle board. Atoll Boards uses the US or ‘œUniversal’ fin box on all their stand up paddle boards which will accommodate this sleek 4.6’ river fin. Do not risk hitting rocks and damaging your fin box and board if you are using a hard non-river fin. This paddle board fin is designed to flex and be used in shallow water conditions. Shallow water conditions may exist in lakes and rivers, so be prepared and add this accessory to complete your Atoll Boards iSUP package. This fin will fit any stand up paddle board that uses the Universal Fin Box. Beware of boards that limit you to having to use their fin box and fin systems. With these paddle boards you are limited to their fin design system which does not accommodate iSUP river fins. Inflatable stand up paddle boards are meant to be enjoyed in a multitude of environments. Rivers, lakes, harbors or big wave surfing. Make sure you have the correct fin for your next adventure.