High and Low Pressure AreaBow and Stern Wave System
Forces Acting on the Manoeuvring Ship
Longitudinal, Rotational and Lateral Motions
Pivot Point
The Pivot point (PP) is the point around which the ship rotates: when the ship rotates then at this point there is no transverse velocity.
The Pivot Point is a kinematic object – it tells about the transverse (lateral) velocity distribution of the ship’s centre plane. The Pivot Point moves depending of the instantaneous set of forces acting on the ship.
Position of Pivot Point During Swinging
With no forward speed and at steady rotational motion, when bow and stern swing in opposite directions the Pivot Point is located within the ship (often close to the midship).
When the ship is making headway, the Pivot Point moves forward. When the ship is making sternway, the Pivot Point moves aft.
Position of Pivot Point Wnen the Ship is Making Headway/Sternway
Below picture shows how the Pivot Point position depends on block coefficient (slender ship vs full bodied ship).
Position of Pivot Point Depending on Ship's Shape
Rudder Force & its Effect
Hull Lateral Force Due to Sway
Transverse Trust of the Propeller
Ahead - Higher pressure on the port side transverse thrust causes small swing to port hardly noticeable.
Astern - Higher pressure on the starboard side much more significant transverse thrust causes swing to starboard.
Ahead
Astern
Right Handed Propeller - Ahead and AsternRight Handed vs Left HandedRight Handed Propeller
Wedge Effect While Running Astern
Water is pushed by the propeller working astern in the narrow space between the ship and the jetty causing higher pressure to arise on the ship’s bow area. This results in creation a force F2 that may be larger than the transverse thrust F1. The ship swings then in the opposite direction to which is expected.
The Ship Swings to the Opposite Direction
Effect of Thruster Forces
The effectiveness of thruster depends on whether the ship is moving ahead or astern.
Example: bow thrusters are more effective when the ship is moving astern