Abstract In this article, the nonlinear motion response of a Trimaran with SWATH-type center hull vessel (TriSWACH) is investigated and compared with the linear response using a hybrid method combining a regression analysis of the viscous damping generated by the hulls with a nonlinear potential flow simulation of rigid body motions. The TriSWACH is a hybrid trimaran hull form, developed using the SWATH (small waterplane area twin hull) technology, from which a single small waterplane area center hull is retained. It is designed to keep the magnitude of its motion response below its equivalent monohull counterpart while also benefiting from large intact transverse stability provided by its side hulls. Since a significant portion of the total damping generated by the submerged hull girder is viscous in nature, an accurate prediction of the steady-state motion response near natural frequency is highly dependent upon the correct modeling of the boundary layer flow. The total damping coefficients obtained experimentally from motion response transfer functions are used to augment the potential flow solution and improve the numerical prediction method at large angles of inclination, characteristic of operation in rough weather near the peak frequency of the motion response. Introduction SWATH1-type vessels exhibit ship motions less pronounced than a monohull of the same displacement. It has been reported that SWATHs have the potential to significantly reduce the impact of harsh environmental conditions on ship operability (Kennell et al. 1985). Although the interest in SWATH technology in ship design continued over the past few decades, it was recognized that this hull form required modifications in order to accommodate high-speed applications. An example of a TriSWACH concept is shown in Fig. 1, with the main distinguishing features compared to the SWATH-type vessel being the presence of the side hulls and one single main hull.