Specmar aluminum catamaran hulls are either symmetrical or asymmetrical in design. a symmetrical hull consists of two basic v-bottom hull structures.. Welcome to spira international home-built boat plans. spira international offers a wide variety of different power, rowing, and sailing skiffs, dories, and other boat. Clark craft offers hundreds of boat plans, design patterns and complete boat kits, and a full line of boat building supplies and accessories for the amateur boatbuilder..