THE PIVOT POINT
A ship’s pivot point is a point on the center line about which the ship turns when the rudder is put over. The pivot point scribes the ship’s turning circle.
Turning Circle A ship’s pivot point is nearly always located about one-third the ship’s length from her bow when moving ahead, and at or near her stern when moving astern. The location of the pivot point will vary with ship’s speed. An increase in speed will shift the pivot point in the direction of the ship’s movement.
Unless stated otherwise each example assumes a ship on even keel in calm conditions and still water. In this situation no forces are involved and the ship has its centre of gravity approximately amidships.
Each example assumes a ship on even keel in calm conditions and still water. In this situation no forces are involved and the ship has its centre of gravity approximately amidships.
Making Headway
Two forces now come into play, firstly the forward momentum of the ship and secondly, longitudinal resistance to forward momentum, created by the water ahead of the ship. These two forces must ultimately strike a balance and the pivot point moves forward. As a rough guide it can be assumed that 25% of the ships forward momentum, at constant speed, is spent in overcoming longitudinal resistance and the pivot point will be approximately 1/4L from forward.
Making Sternway
The situation is now totally reversed, the momentum of sternway must balance longitudinal resistance, this time created by the water astern of the ship. The pivot point moves aft and establishes itself approximately 1/4L from the stern.
It should be stressed that other factors such as acceleration, the shape of hull and speed may all affect the position of the pivot point. The arbitrary figures quoted here, however, are perfectly adequate for a simple and practical working knowledge of the subject.
