Bow thrusters are type of propellers, which are smaller in size and which help in better maneuverability of the ships at lower speeds. They are generally used for maneuvering the vessel near the coastal waters or while entering or leaving a port. Bow thrusters help in assisting tug boats in berthing the ship without wasting time. This saves a lot of money for the shipping company because of lesser stay of the ships in the ports. Moreover, presence of bow thrusters on a vessel eradicates the need of two tugs while leaving and entering the port, and thus saves more money.
Generally, bow thrusters are transverse thrusters placed at the forward and aft end of the ship. The thruster placed in the forward end is known as the bow thruster and the one placed in the aft is known as the stern thruster. The requirement for the number of thrusters to be installed depends on the length of the ship.
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 ship, 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 water line 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 electric 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.
Bow thruster consists of an electric motor which is mounted directly over the thruster using a worm gear arrangement. The motor runs at a constant speed, and whenever there is a change required in the thrust or direction, the controllable pitch blades are adjusted. These blades are moved and the pitch is changed with the help of hydraulic oil which moves the hub on which the blades are mounted. As the thruster is of controllable pitch type, it can be run continuously, and when no thrust is required the pitch can be made to zero.
The thruster is controlled from the bridge and the directions are given remotely. In case of remote failure, a manual method for changing the pitch is provided in the thruster room and can be operated from there.
1) The insulation needs to be checked regularly and should be kept dry. This is done because bow thrusters are not used frequently and thus there are chances of damages by moisture. Moreover, because of the frequent idle state of the bow thrusters, there can be reduction in the insulation resistance especially in colder regions.
2) The space heater is checked for working condition so that the insulation can be kept dry.
3) The bearings of the motor and the links are to be greased every month.
4) The condition of hydraulic oil is to be checked every month for water in oil and samples should be sent for lab analysis for further checking.
5) The thickness of the contactors is to be checked from time to time.
6) Checks are to be made for any water leakages in the bow thruster room which is indication of seal leaking.
7) The flexible coupling between the motor and thruster should also be checked.
1) Better maneuverability at low speeds of the ship.
2) Safety of the ship increases when berthing in bad weather.
3) Saves money due to reduction of stay in port and less usage of tug boats.
1) A very large induction motor is required, which takes a lot of current and load, and thus large generator capacity is required.
2) Initial investment is high.
3) Maintenance and repairs are costly when there is damage.
You may also like to read Ship’s propeller explained.
Marine Auxiliary Machinery By Mc George