SUMMARY The objective of the present work is to develop and validate a computational tool for the calculation of large amplitude parametric rolling motions that can be used in conceptual design in a computationally efficient and robust manner. The contribution of this work to the general body of knowledge is in the development of a theory that captures hull interaction effects for a given range of ship speeds, sea states and loading conditions, where interaction effects are likely to materialize in certain non-linear phenomena of dynamic instability known as parametric resonance, while retaining the numerical efficiency of strip theory. The numerical code is based on an hybrid model which transfers frequency domain data to time domain and includes the so called Froude-Krylov nonlinear part of the loading in time domain. The use of a simplified 2½D method showed slight improvement over the classical 2D method with the disadvantage of being more computationally intense than standard strip theory. Furthermore, the implementation of strip theory, with respect to functionality, has been shown to require a more advanced method for including roll damping. The viscous contribution proved far superior in terms of the effect it has on the proneness towards parametric rolling. The problem of viscous roll damping is separated in two models, first when the experimental results are not known with the blending method and second when the roll decrement test are at hand. An extensive experimental program of the Parametric Rolling on a C11 Class Containership was utilized to investigate and explain two consisting inseparable treatments of the damping and seakeeping simulations, respectively. The validation has shown good agreement with the experimental results for roll, both in the experiments where parametric roll resonance occurred and in the experiments where it did not occur. The simulated results, including polychromatic and irregular wave excitation, and the subsequent analysis, provided insight and improved the understanding of the parametric roll resonance mechanism, as well provided the basis for setting a probabilistic methodology proposition.