Abstract: Twin-hulled boats such as SWATH (Small Waterplane Area Twin Hull) boats are designed and built to be so dimensioned as to fit transportably and removably atop the sidewalls of the open bodies or beds of various vehicles such as pickup trucks, trailers and the like. The horizontal deck and substantially solid hulls or struts of the boat provide a cover, closure or "cap" for the truck bed while installed. Such boats can be easily removed from the truck bed for use or storage. In one embodiment, the twin hulls have a cross section which is a hybrid of the relatively wide, shallow hulls of a catamaran and the narrow, deep struts attached to the lower flotation hulls of a SWATH boat. In the SWATH embodiment, the flotation hulls can be removed for separate transport, the struts themselves being secured atop the sidewalls, or the hulls can be left installed and themselves secured to said sidewalls.