Page 207 - Introduction to Marine Engineering

Deck machinery and hull equipment 191

must be adjusted according to the ship's loaded condition. Also there is a
free surface effect resulting from the moving water which effectively
reduces the stability of the ship. The tank system does, however, stabilise
at zero speed and is a much less complex installation than a fin stabiliser.

Watertight doors

Watertight doors are provided where an opening in a watertight
bulkhead is essential. On cargo ships with a shaft tunnel, the entrance
would have a watertight door fitted. All doors fitted below the waterline
must be of the sliding type, arranged horizontally or vertically.
A horizontal sliding watertight door is shown in Figure 10.9. The
robust frame fits into the bulkhead and provides the trackway along
which the door slides. The door is moved by a hydraulic cylinder which
may be power or manually operated. The door must be arranged for
local opening and closing as well as operation from a point above the
bulkhead. deck. The power unit situated above the bulkhead deck
provides either powered or hand operation of the door.
Watertight doors should be tested for operation by closing and
opening during fire drill. The hydraulic system should be occasionally
checked for leaks and to ensure sufficient oil is present in the system.
The bottom trackway of the door should be checked for cleanliness and
freedom from obstructing matter.

Bow thruster

The bow thruster is a propulsion device fitted to certain types of ships to
improve manoeuvrability. The thrust unit consists of a propeller
mounted in an athwartships tunnel and provided with some auxiliary
drive such as an electric or hydraulic motor. During operation water is
forced through the tunnel to push the ship sideways either to port or
starboard as required. The unit is normally bridge controlled and is
most effective when the vessel is stationary.
A controllable-pitch type thruster unit is shown in Figure 10.10. A
servo motor located in the gear housing enables the propeller blade
pitch to be altered, to provide water flow in either direction. With this
arrangement any non-reversing prime mover, like a single-speed electric
motor, may be used. The prime mover need not be stopped during
manosuvring operations since the blades can be placed at zero pitch
when no thrust is desired. The drive is obtained through a flexible drive
shaft, couplings and bevel gears. Special seals prevent any sea water
leakage into the unit. The complete assembly includes part of the