This paper compares linearization techniques in time-domain seakeeping codes, with focus on differences in predictions of motions, accelerations, and structural loads. The application case is a 100 m frigate hull form sailing at high speed (around Froude number 0.5) in regular head waves. An experimental configuration, used as validation material, is outlined followed by the main assumptions of the seakeeping methods and simulation setup. The effects of constraining the surge motion on motions and accelerations are shown. RAOs for different wave amplitudes and linearization levels are compared, followed by non-linear effects that are not captured by first order RAOs. Time series are compared in a deterministic way, highlighting the relevance of using non-linear methods. Finally, the effects of hydro-elastic responses on motions, accelerations, and structural loads are illustrated.