The required minimum output of main engine for large fat ship from the view point of safety navigation is evaluated. The manoeuverable sea conditions are evaluated by estimation of ship speeds, drifting angles and check helms in various seas. In this estimation, influences of added resistance, steady lateral force and steady yaw moment in weves are accounted. The characteristics of main engine is also accounted. The evaluated manoeuverable sea conditions are in good agreement with ones by log data. Assuming the several cases of output of main engine, the relation between the manoeuverable sea condition and the output of main engine is evaluated. Using this result, the relation between the unmanoeuverable probability and the required minimum output of main engine is estimated. The probability that the ship meets unmanoeuverable condition will be reduced by the increasing of main engine output, however, it shall be limitted up to mechanical availability of main engine. Thus the criteria for the required minimum output of main engine shall be set up.