This study is aiming at quantitative evaluation of crew's judgement for ship handling in waves. This paper, as the first report of the study, explains the outline of the onboard monitoring equipment named Ship Motion Analyzing Computer System (SMACS) developed by the authors, being capable for recording simultaneously ship motion responses in waves, encounter waves and the reasons for ship's course and/or speed changes. Some of the data obtained from three patrol boats of Maritime Safety Agency of Japan with SMACS are also shown in the paper.