Slamming-induced whipping is traditionally computed as the response of a linear-elastic structural model. However, in order to investigate the consequence of whipping on the hull girder’s collapse, the hydro-structure interaction must be performed in a fully-coupled approach where the nonlinearities of both domains are considered. Therefore, this paper presents a new approach developed for solving the fully-coupled hydroelastoplastic problem. The structural part is modeled as two non-uniform Timoshenko beams, connected with a nonlinear hinge, described by the nonlinear relation between the internal bending moment and the relative rotation angle. The hydrodynamic problem is solved using the 3D boundary element method, and the exact coupling between the structural and the 3D hydrodynamic models is achieved by constructing the hydrodynamic boundary value problem for each shape function of the finite element beam model. The hydroelastoplastic response is calculated using a hybrid nonlinear time-domain approach, allowing for very fast computation of the nonlinear whipping response. Finally, the nonlinear whipping response is calculated on a broad range of ships and it is compared to the linear whipping response in order to derive the whipping effectiveness coefficients. It is shown that the nonlinear structural behavior has a very small influence on the whipping response, and thus, the effectiveness of whipping should not be reduced.