Explain Bow thrusters and their use?

A Bow thruster is a transversal propulsion device built into, or maounted to, either the bow or stern of a ship to make it more maneuvrable.  

Bow thrusters are most effective when a ship has neither headway nor sternway. They create a turning effect by providing a side force at their location. Their effectiveness will depend upon the distance between the thruster  and ship’s pivot point. When berthing a ship that has a single bow thruster, and no stern thruster, it is important  not to become too focused on the bow, stern alongside as a priority. Remember that pure rotation can only be induced by two lateral thrusters,  one forward  and one aft, opposing each other, and that a tug may be needed to control the stern of a large Ship. Bow thrusters  are used when it is required to ‘breast’ on to or off a berth or to move the ship’s head from a jetty. Modern ships fitted with a bow thruster  will often berth without  tug assistance.
 However, a bow thruster  will lose its effectiveness  as a ship’s speed increases.  Depending on the hull and thrust tunnel design, thrust  effectiveness  can be lost at between 2 and 5 knots. The reason for this is the merging of the slipstream  from the  thruster  with the general flow around a forward  moving hull  When speed increases above two knots, local loss of pressure over the hull, downstream  from the thruster, creates a turning  moment  opposite to the moment produced by the’ thruster . The thruster may become ineffective.

Thrusting when stopped – When stopped and thrusting, a ship’s pivot point is likely to be aft. If a bow thruster is put to starboard on a stopped ship, the ship will turn to starboard.

Thrusting with headway – The pivot point will be forward, so thrusting will not be very effective, especially at high speeds.

Thrusting with sternway – The pivot point is aft and when the bow  thruster  is put to starboard, the ship’s bow will swing to starboard. The thruster will be effective, and will act as a form of ‘rudder’.

Rudder response –  The time it takes  for the rudder to respond to a helm order will determine  how rapidly a ship gets into a turn. The quicker the rudder responds, the sooner the ship will begin to turn.