Phase5 - CARBON TWIN FIN SETPhase Five Carbon Twin Fin Set This twin fin set comes stock with all surf style Phase 5 boards. For boards 54 and smaller, it includes two 3. 7 surf fins, while boards larger than 54 come with two 4. 3 surf fins. Both fin sizes are designed for maximum grip and feel while surfing, featuring carbon fiber construction for optimal performance. They are compatible with classic FCS fin boxes and come with a fin bag and an FCS fin key.