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Bow thruster is a type of propeller-shaped system fitted either on the bow (forward part) and stern part (known as a stern thruster) of the ship. They are smaller in size as compared to the ship’s propeller and help in better manoeuvrability of the vessel at a lower speed.
Bow thrusters are generally used for manoeuvring the ship near the coastal waters, channels or when entering / leaving a port while experiencing bad currents or adverse winds.
Bow thrusters help in assisting tugboats in berthing the ship to avoid unnecessary wastage of time and eventually money because of lesser stay of the vessel in the ports. The presence of bow thruster on a vessel eradicates the need of two tugs while leaving/entering the port.
Nowadays ships have both bow and stern thruster, which makes them independent of the tugboats for manoeuvring in the port limits (if the port regulation does not make it compulsory to use tug boat).
Installation Of Bow Thruster
Generally, side thrusters are transverse thrusters placed in a duct located at the forward and aft end of the ship. The thruster set in the forward end is known as the bow thruster and the one placed in the aft end is known as the stern thruster.
The requirement for the number of thrusters to be installed depends on the length and the cargo capacity of the ship. The route of the vessel also plays an important factor as many countries have local regulations of compulsory use of tugboats to enter or leave their port limits
For the installation of the side thrusters, the following things are important:
-The thruster compartment, also known as bow thruster room, should be easily accessible from the open deck by the screws.
-As most of the seagoing vessels use an electric motor for the thruster, which is heat-generating machinery and must, therefore, be positioned in a dry and well-ventilated area
-The bow thruster room should be fitted with a high-level bilge alarm and the indication to be provided in engine control room and bridge
-The room should be provided with at least one light supplied from the emergency source
-In the case of installation of more than one panel, make sure to operate the thruster from only one panel at the time.
-The thruster room should not be used to store flammable products in the area of the electric motor.
-The installation of the tunnel or conduit containing the propeller must be positioned perpendicular to the axis of the ship, in all the directions.
-The propeller should not protrude out of the conduit
-Grid bars may or may not be fitted at both ends of the tunnel (taking into account of how much debris the ship bottom will experience in its voyage). The number of bars for them to be kept at a minimum as they tend to reduce the thrust force and overall performance of the bow / Stern thruster.
-Sharp edges on the grid bars to be avoided. Trapezoidal shape with no sharpness is a good choice of design for grid bars installed perpendicularly to the direction of the bow wave.
-The design and position of the thruster tunnel should not interfere with the water flow under hull or should not add to hull resistance
-Ensure that the material used for the installed thruster does not foul existing equipment inside the ship such as steering links etc.
Construction and Working of Bow Thrusters
The bow and stern thrusters are placed in the through-and-through tunnels which open at both sides of the ship. There are two such tunnels – at forward and aft ends of the ship.
The thruster takes suction from one side and throws it out at the other side of the vessel, thus moving the ship in the opposite direction. This can be operated in both the directions, i.e. port to starboard and starboard to port.
The bow thrusters are placed below the waterline of the ship. For this reason, the bow thruster room should be checked for water accumulation at regular intervals of time.
The bow and the stern thrusters can be electrically driven or hydraulic driven or diesel driven. However, the most commonly used are electric driven, as in hydraulic driven thrusters there occur many leakage problems.
Also, with diesel-driven bow thrusters, the amount of maintenance required is more and every time before starting someone needs to go to the thruster room to check the thrusters.
The thruster used are usually of CPP type, i.e. the blades on the propeller boss can be moved to change the direction of the thrust.
The boss which carries the blades is internally provided with a movable shaft (operated by hydraulic oil) also know and Hydraulic Pod Motor-driven Thrusters.
Once the signal is given to change the pitch, the hydraulic oil will be supplied to operate the internal shaft (within the boss) to change the blade angle of the thruster .
The motor shaft drives the shaft of the thruster via pinion gear arrangement. The sealing gasket is provided in the motor casing which holds the water which is in the tunnel.
The Thruster assembly consists of the following components:
The electric motor with safety relays
The flexible coupling between motor and thruster
Mounting and casing for the electric motor
The connecting flange and shaft
Motor casing seal
The tailpiece with shaft seal
The propeller shaft
The zinc anodes
Grid with bars at both ends of the tunnel