A generally rectangular metallic extrusion (20) for boat hull (21) construction, the extrusion (20) supporting a bottom sheet member (26). The extrusion (20) comprises a number of longitudinal ribs (30) installed upwardly from the bottom sheet, a first longitudinal rib (23) having a generally rounded protruding male joint (22), and a last longitudinal rib (28) having a generally concave female joint (24) mounted to surround the male joint (22) of an adjoinning extrusion (20). The male joint (22) has a recessed neck (39), of the same thickness as the bottom sheet (26), and receives a base (35) of the female joint (24) to form a welding nest. A mounting form is first set and the extrusions laid according to a desired shape. The rotation and adjustment of the protruding male joint (22) in the adjoining concave female joint (24) permits the positioning and meshing of two extrusions (20) at a selected deflection angle, before welding.