The critical speed of a high speed container ship in irregular waves is investigated theoretically by strip method and statistical calculations. In this paper, “deliberate loss of speed” is mainly discussed in details. The purpose of this study is the followings : (1) To find the best way of shiphandling in rough sea. (2) To inquire into the ship forms which have better seakeeping qualities. Seven factors of seaworthiness are discussed : (1) Vertical Acceleration at F. P. (2) Deck Wetness at F. P. (3) Slamming (4) Vertical Wave Bending Moment (5) Propeller Racing (6) Lateral Acceleration at S. S. 8¹/2 (7) Rolling The critical values and probabilities of these factors are checked, and the maximum ship speed, . below which a ship safely navigates across the rough sea, is determined. These analyses are possible for all kinds of ships, and they will be usefull not only for ship-handlers but also for ship designers who are concerned about seakeeping qualities of ships.