The prediction of ultimate ship hull strength using a simplified progressive collapse method critically relies on the evaluation of the load-shortening curve of stiffened panel components. Uncertainty in the characteristics of load-shortening curves causes subsequent uncertainty in the hull girder strength calculation. The load-shortening curve shape is influenced by parameters including imperfection, residual stress, material properties and dimensional variance. To investigate this uncertainty, a sensitivity study on the ultimate strength calculation of a ship-type box girder is completed using parametrically defined load-shortening curve. The curve is formed of linear pre-and post-collapse responses and an arc to represent the nonlinear ultimate strength region. The load-shortening curve is systematically varied in ultimate strength, ultimate strain, elastic stiffness and post-collapse stiffness. A double hull box girder model under vertical bending, combined vertical/horizontal bending and cyclic bending is analysed using this parametric representation. The influence of each parameter is evaluated with a sensitivity index. The study shows that the structural component’s ultimate strength and post-collapse stiffness have the most significant influence.