The purpose of the thruster units is to turn the vessel when is operating at slow speeds or when is not under way, to keep the ship in position in a cross wind and to move the ship towards or away from a mooring position as required. The thrust is produced by rotation of a propeller unit which is housed in a transverse cylindrical ducting, where the propeller unit is rotated by means of a vertical electric motor via bevel gears.
The propeller blade pitch is controllable in order to obtain the desired magnitude and direction of thrust.
The thruster comprises of a number of separate sections:
- The electric motor unit with drive shaft and bevel gearing driving the propeller unit hub;
- The propeller unit with blades mounted in the hub;
- The hydraulic unit which changes the pitch of the propeller blades;
- The control system which regulates the blade pitch in accordance with demand from the bridge.
Power is transmitted from the electric motor through the flexible coupling, input shaft and bevel gears to the propeller shaft, rotating the propeller in a single direction.
The propeller part, usually consists of four propeller blades and a propeller hub. The propeller hub and gear case house a hydraulic servomotor and sliding block mechanism. The propeller blades are connected to blade carriers by blade bolts, and this ensures easy exchange of blades in the thruster tunnel. The gear case, which carries the propeller parts, is connected to the thruster tube by bolts and this ensures easy overhauling of all parts inside the thruster tube.
The power transmission gear is located inside the gear case and consists of the vertical input shaft, the right angle reduction bevel gear and the horizontal propeller shaft, and serves to transfer the power from the prime mover to the propeller. The bevel gear and individual bearings are lubricated by the gravity oil filling the gear case.
The hydraulic power pack unit provides oil under pressure and this is used to change the pitch of the thruster unit blades.
The oil is drawn from the gravity tank, through the suction filter and into the oil service pump. The pressurized oil is pumped to the solenoid valve via the check valve and the flow of oil is controlled by the solenoid valve.
The hydraulically operated solenoid valve is a changeover valve for the distribution of the hydraulic oil to the respective servo cylinders depending on the command entered at the active control panel. When the command is entered on the control panel, the solenoid valve is actuated and pressurized hydraulic oil is supplied to one of the hydraulic circuits down the oil tube, through the feed ring and oil entry tube to the servomotor, causing displacement of the crosshead piston. The reciprocating movement of the piston is converted into a turning movement by the sliding block mechanism and this turns the propeller blades.
The vent side of the servomotor piston drains to the oil bath in the thruster body via a solenoid valve. From this pressurized oil bath, oil returns to the header tank. The main actuator power pack pump takes oil from the header tank and supplies it to the thruster unit via the solenoid control valves.
A shaft sealing mechanism is attached to the gear case in order to prevent leakage of oil out of the system.
When a pitch change command is entered, the propeller will tend to move excessively. The pilot check valve prevents any excessive movement of the propeller whilst changing pitch.
Operation of the bow thrusters requires starting a large induction motor and the power requirement of this electric motor is high, requiring that additional generators are started in order to avoid the risk of a vessel blackout.
It is important to note that, on modern vessels if there is insufficient power capacity available at the switchboard an additional generator is started by the power management system. The thruster drive motor cannot be started until sufficient power is available at the switchboard.
Usually, the main switchboard includes a bow thruster control panel, which includes a control position selection switch, lock-out relay trip reset, motor control and an ammeter. A series of status indicators are also included for monitoring the condition of the VCB, gravity tank, pump pressure etc. If any warning lamp is illuminated, the cause of the fault should be determined and remedied before operation of the bow thruster.
Feeder protection for the bow thrusters is achieved by means of the protection and monitoring panel located on the main switchboard bow thruster panel and offers both measurement and protection for the bow thruster drive motor.
The bevel gear and all the bearings inside the gear case are lubricated by the bath lubricating method. The lubrication oil in the gear case is slightly pressurized by the connection
with the gravity tank which is positioned above the waterline to prevent sea water from leaking into the oil system.
The thruster unit includes a feedback system for transmitting the angle of the propeller blades to the remote control panel located on the bridge. As the oil entry varies, the stroke of the oil entry tube also varies. The movement of the oil entry tube causes movement of the feedback lever. This movement is transmitted via the feedback chain to the blade angle transmitter located outside the thruster gear casing. This mechanical movement is then converted to an electrical signal by the blade angle transmitter and transmitted to the angle indicator on the bridge and local control panels.
To ensure safe, reliable operation of the bow thrusters, limits are imposed on the vessel’s speed and draught. If there is insufficient draught, the thruster will suffer a reduction in performance along with cavitation and the possibility of air drawing. The result of this will be increased vibration which may cause damage. Similarly, at speeds greater than 5 knots there is a risk of drawing air into the thruster, particularly when operating at shallow waters. This will degrade the performance and can cause cavitation damage and it can be detected by hunting of the main motor ammeter and should be avoided. If the vessel’s speed is below 5 knots and air drawing is occurring, reducing the propeller pitch will prevent further air drawing from taking place.
The main motor must only be started when the blades are in the neutral position (zero pitch), or in the allowable zone (blade pitch of ±3°). The system is interlocked to prevent the main motor from starting if the blade pitch is outside of the set limits. The interlock switches also prevent the main motor from starting when:
- The cooling fan is stopped;
- The power pack gravity tank level is low;
- The control oil pressure is low
Under normal circumstances the main power supply is activated by the engineering department and after that the thruster operation and control is undertaken by the deck department from the bridge panels. The main switch at the local thruster control panel should be set at REMOTE in order to allow for this.
Control of the thruster on the bridge is either at the wheelhouse control stand or the control stands on the bridge wings.
It is important to note that, especially on bow thruster, when the hydraulic pump is started the fan is also started and the FAN RUN indicator in the panel will be illuminated. The main motor is interlocked with the fan and oil pump and will not start unless they are running.
Also is important to remember that there are EMERGENCY STOP pushbuttons in the wheelhouse panel, forward mooring station and in the bridge wing panels.
If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.
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