Marine screws bite and hold fast in fiberglass, plywood and wood substitute materials. Use for projects exposed to harsh marine or many chemically caustic conditions. Ideal for applying trim, rub rail and molding to fiberglass boat hulls. Extra-sharp points speed screw starts during assembly. Specially engineered, high/low thread form incorporates indentations (teeth) that cut easily and quickly through high-density materials. Back-out resistance: once driven home, Sharx ; are locked in place thanks to their teeth (the thread indentations will resist back-out even after prolonged vibration). Holding power: Pull-out values of Sharx ; ;screws exceed those of ordinary tapping screws in fiberglass and wood. Screws up to 1" in length are fully threaded. Screws more than 1" long are threaded 2/3; of the shank length. Phillips drive.Simpson Strong Tie Co., Inc.