Movements of Ship
AHEAD AND ASTERN MOVEMENTS
Most of the modern merchant ships move by rotation of Propellers. When a Propeller rotates, its blades cause the water to move in astern direction. Its motion is like a screw moving inside a nut as the Propeller blades are designed like threads of a screw. That is the reason it is also called Screw Propeller. A ship can have single Propeller and such ship is called Single Screw Ship. Similarly, a ship with two propellers may be called Twin Screw Ship.
As the water moves astern due to rotation of Propeller, it creates a Thrust Force on the Propeller due to the Newton’s Third Law of Motion. Thrust Force acting through the Thrust Bearing firmly fitted to the ship’s hull causes the ship to move in Ahead direction. Similarly, on changing the direction of rotation of the Propeller, the Thrust Force acting on the Propeller in opposite direction, causes the ship to move in Astern direction.
PORT AND STARBOAD MOVEMENTS
Port (left side) and Starboard (right side) movements of the ship can be affected by turning of Rudder moved by Steering Gear when the ship is motion as the turning by Rudder is effective only when the ship is moving. This is due to the fact that turning of Rudder causes the direction of movement of water by Propeller to change thereby creating a lateral thrust for the ship to move in Port or Starboard direcWhen the ship is not in motion, the lateral thrust can be produced by use of equipment having a Propeller fitted in transverse direction to the motion of the ship. This is called Thruster which may be fitted in the forward part of the ship and may be called Bow Thruster. Thruster fitted in the after part of the ship is called Stern Thruster.
Both the Rudder and the Thrusters are used to maneuver the ship when operating the ship in narrow channels or while berthing the ship alongside a jetty..
Machinery used to turn the Propeller or, to propel the ship is called Propulsion Machinery. This could be an I. C. Engine like Diesel Engine, Steam Turbine or an Electric Motor.
Apart from the screw propellers, other methods of propulsion used are Jet Propulsion, Voith Schneider Propulsion, Kort Nozzle Propulsion etc.