The FCS VS Knubster was developed by Sean Mattison for Von Sol surfboards and is a quality centre fin that can be added to any quad set-up. :: Men > Surf Accessories. Glass Flex Construction: Formulated to replicate the flex and memory properties\n of traditional hand laid fibreglassDrive: Low drive ratingPivot Tight arc ratingHold: Low hold ratingDepth: 1.71''Base: 3.82''Area: 4.40''Sweep: 32 degrees Size: one size.