( Twin-Fin Set ) Futures Compatible,Concave Inside Foil:The Wayne Rich Nightmare keels have been carefully crafted with close attention to detail in order to harness thefull potential of the fins’ design. The swept-back, hatchet style profile allows for the ability to reach maximum drivewhile still allowing for release and maneuverability without tracking. The inside of the fins feature a concave ‘football shape’ foil in the top portion of the template which provides extra lift and reduced drag. If you’re looking for a hybrid-keel alternative to spice up your twin fin, these fins might be just the ticket.