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. Composite cruiser – the primary purpose of this design was to build a small pram style boat that was ultra-lightweight, easy and inexpensive to build, was unsinkable.