Longitudinal stringers...

It's far cheaper in labor and materials to install a few well placed stringers in a 'glass hull then to engineer and install increased laminate thickness to absorb the loads. This can be clearly seen in craft from the 50's thru the 70's, where the laminate is quite thick, but unnecessarily heavy and costly to manufacture. Now hulls have progressively thinner laminates from the centerline to the rails, saving considerable material and labor. A structural grid, bonded liner and panel stiffness stringers complete the project, with a lighter, usually stronger boat, having a higher margin for the manufacture.