Abstract: A high performance portable knockdown boat structure having one or more unique inflatable hull structures in the form of a relatively low pressure envelope which tapers from its mid-length to its opposite ends, and is provided with a sectionalized light weight reinforcing tubular member which can be inserted endwise into a longitudinally extending surface sleeve. The reinforcing member is easily inserted into the sleeve when the envelope is deflated, and upon inflation of the envelope will be retained and anchored against removal. The unique envelope may be readily embodied in boats of the single hull type with or without an outrigger, as well as in boats having more than one hull, and particularly to provide a unique, lightweight, high performance catamaran.