Thrusters

Tunnel thruster

Thrusters are designed to increase the manoeuvring ability of a ship and may make the use of tugs unnecessary. They also be used to impart side thrust to a ship at a berth in order to counteract wind effect thus minimising stress on mooring wires. At slow speeds the effectiveness of a rudder is reduced and so thruster units are very useful, at higher speeds thruster become less and less effective.
Multiple thruster units are fitted when the fitting of a single large unit is impractical. It is necessary to penetrate the hull on order to provide ducting for bow and stern thruster units and arrangements must be made to ensure the integrity of the hull remains. There is a small increase in hull drag and therefore a slight increase in fuel consumption. Grids protect the propeller from debris.
The thruster is mounted as low in the hull as practical to ensure a reasonable head of water. If this is insufficient it is possible that air from the surface might be drawn into the ducting thus reducing the thrust effect and causing cavitation of the blades
Shown above is one arrangement called a tunnel thruster. A prime mover, normally a constant speed electric motor a drive via a bevel gear a rotating element carrying the blade. The pitch of the blades is variable to allow thrust to be controlled in both direction. An alternative to this is to have fixed blades with a variable speed motor. In some cases with gill jet azimuth thrusters a diesel engine is used allowing it to act as emergency propulsion.
An alternative to the tunnel thruster is the gill jet which can take two forms

Gill jet-bottom suction

Water is drawn from the bottom of the ship and directed to either side by means of hydraulically actuated vanes. The angle of the vanes may be varied thereby allowing the water to be directed forward or aft thus providing forward and aft movement as well as sideways. A unidirectional constant speed motor provides the drive. As with the other system described the motor is vertically mounted and so a form of thrust bearing must be incorporated. This will usually be of the roller type and wear down must be checked periodically.

Gill jet-side suction

Thrust is directed via a circular slotted plate on the bottom of the hull. This allows equal thrust in all directions. Power is provided by a variable speed drive either d.c. or a.c. Where the thruster is used as an emergency propulsion unit then it is diesel engine driven