Abstract: In purpose to improve the accuracy of predicting the required turning area of a ship and ensure that the turning trajectory operates within a safe area,it is necessary to analyze the factors of ship movement and identify the key factor which determines the turning area. Based on the principles of fluid mechanics,mathematical reasoning,and calculations,this study explores the change in the position of the ship’s pivot point when the ship’s stern undergoes turning motion due to lateral forces,such as rudder usage,in a scenario with no wind nor current. Furthermore,the relationships between various variables during ship turning are derived. The findings indicate that for a ship engaged in turning motion,when the lateral force acts on the ship’s stern,the position of the pivot point remains relatively constant throughout the rotation,located approximately at L/6 forward of the ship’s center of gravity. The required turning area for the ship’s rotation can be determined based on the observation of the turning drift angle β at the observation point.Specifically,when the ship navigates from the observation point to the LOP（line of position）of original pivot point on turning circle,the ship’s turning angle is equal to β. By assessing the magnitude of β as displayed on radar,operator can determine the required turning area and adjust ship’s rate of turn accordingly to keep the ship within a safe area. Practices in ship maneuvering have proved the effectiveness of the research.