Comparisons are carried out between the wave-induced motions and loads of a trimaran travelling at various forward speeds in regular head waves obtained using three-dimensional fluid-structure interaction models, treating the vessel both as rigid and flexible body. The concept of the trimaran requires estimation pf longitudinal loads, such as vertical bending moments, as well as transverse loads such as prying moment at the connection of the crossbeam to the main hull, in order to evaluate the dynamic response of the structure in waves.