Abstract: A double-wall structural vessel includes an inner tubular liner and an outer tubular shell surrounding the liner in spaced relation, both the liner and the shell comprising helically curved strips of material, and a continuous and helically curved spacer/stiffener therebetween. The liner and shell are relatively shifted axially such that the shell edges are out of alignment with the liner edges, and the spacer spans contiguous edges at successive turns of the liner as well as at successive turns of the shell, and is secured to the strips as well as to the liner edges and to the shell edges.

Abstract: A structural vessel includes helically curved inner and outer spaced strips of material each having a plurality of helical turns, with contiguous edges of each two successive turns of the inner and outer strips being seamed together, and a helically curved structural beam disposed between the strips and secured thereto along their turns. A floatable work housing is provided for vessel fabrication, the housing having a sealed opening through which the completed portion of the vessel progressively extends and is floated on a body of water to avoid a need for external supports and bearings.