Harken soft ~attach Fly blocks are designed to provide strength without mass, providing sailors with big power in a low ~aero package. Designed specifically for high ~tech line, these efficient blocks have an incredible working load for their small size. Fly blocks are perfect for use on foiling dinghies and sportboats and for vang cascades and backstay systems on Grand ~Prix racers. 18 mm blocks feature an integrated stainless steel inner race and rivet, stainless steel ball bearings, and composite fiber ~reinforced sideplates. Doubles feature composite line guides that are integrated into the block head. They keep the block aligned correctly by pushing the lashing line to the outer edges of the block's head. A length of Spectra® line is included. Spectra® is a registered trademark of Honeywell International, Inc.