A method of manufacturing and assembling an inflatable watercraft, comprising: laying out first and second flexible material panels (18, 20); cutting the first and second flexible material panels to a shape suitable for the particular kind of inflatable watercraft under construction; installing a valve reinforcement patch in the interior side of one of the first and second flexible material panels; cutting a valve hole in the flexible material panel at the valve reinforcement patch; installing a first set of stringer panels (28a, 30a) on the first flexible material panel; installing an opposing second set of stringer panels (28b, 30b) on the second flexible material panel such that when the first and second material panels are approximated in assembly, the first set of stinger panels overlap and engage stringer panels in the opposing second set of stringer panels; connecting the first set of stringers to their respective opposing stringers in the second set of stringer; wherein each of the stringers have an upper portion affixed to the interior side of the first flexible material panel, a lower portion affixed to the interior side of the second flexible material panel, and a medial portion defining a plane generally normal to the interior sides of the first and second flexible material panel, the internal stringers oriented generally parallel to the longitudinal axes of the first and second flexible material panels, and further wherein the stringers have a profile as seen in side view in elevation that have different top and bottom continuous curves along the lengths of the stringers that impart curvature to the shape of the first flexible material panel and the second flexible material panel in the elevation view when the curved stringer upper and lower affixing portions are affixed to the interior sides of the panels in straight longitudinal parallel lines, transferring the independent top and bottom stringer curves to the flexible material panels, when the inflatable watercraft is assembled; folding over a portion of the perimeter of the first flexible material panel and welding the folded portion to a perimeter of the second flexible material panel, leaving an end of the inflatable watercraft open; closing the open end by hand gluing or welding; and installing an air fill valve in the valve hole. laying out first and second flexible material panels (18, 20); cutting the first and second flexible material panels to a shape suitable for the particular kind of inflatable watercraft under construction; installing a valve reinforcement patch in the interior side of one of the first and second flexible material panels; cutting a valve hole in the flexible material panel at the valve reinforcement patch; installing a first set of stringer panels (28a, 30a) on the first flexible material panel; installing an opposing second set of stringer panels (28b, 30b) on the second flexible material panel such that when the first and second material panels are approximated in assembly, the first set of stinger panels overlap and engage stringer panels in the opposing second set of stringer panels; connecting the first set of stringers to their respective opposing stringers in the second set of stringer; wherein each of the stringers have an upper portion affixed to the interior side of the first flexible material panel, a lower portion affixed to the interior side of the second flexible material panel, and a medial portion defining a plane generally normal to the interior sides of the first and second flexible material panel, the internal stringers oriented generally parallel to the longitudinal axes of the first and second flexible material panels, and further wherein the stringers have a profile as seen in side view in elevation that have different top and bottom continuous curves along the lengths of the stringers that impart curvature to the shape of the first flexible material panel and the second flexible material panel in the elevation view when the curved stringer upper and lower affixing portions are affixed to the interior sides of the panels in straight longitudinal parallel lines, transferring the independent top and bottom stringer curves to the flexible material panels, when the inflatable watercraft is assembled; folding over a portion of the perimeter of the first flexible material panel and welding the folded portion to a perimeter of the second flexible material panel, leaving an end of the inflatable watercraft open; closing the open end by hand gluing or welding; and installing an air fill valve in the valve hole.