Page 254 - Ship Construction.DJ Eyres 6Ed

Ch21-H8070.fm Page 243 Wednesday, October 18, 2006 6:59 AM
Aft End Structure 243
The main steering gear for oil tankers, chemical tankers or gas carriers of
10 000 gross tonnage or more and every other ship of 70 000 gross tonnage
or more is to consist of two or more identical power units which are capable
of operating the rudder as indicated for the main steering gear above and
whilst operating with all power units. If a passenger ship, this requirement
is to be met when any one of the power units is inoperable.
Steering gear control for power operated main and auxiliary steering
--- ใช้เพื่อการศึกษาเท่านั้น---
gears is from the bridge and steering gear compartment, the auxiliary
steering gear control being independent of the main steering gear control
(but not duplication of the wheel or steering lever).
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Steering gear on ocean-going ships is generally of the electro-hydraulic type.
Where the rudder stock is greater than 230 mm an alternative power
supply is to be provided automatically from the ship’s emergency power
supply or from an independent source of power located in the steering gear
compartment.

Sterntube

A sterntube forms the after bearing for the propeller shaft, and incorp-
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orates the watertight gland where the shaft passes through the intact hull.
Two forms of sterntube are in use, that most commonly fitted having water-
lubricated bearings with the after end open to the sea. The other type is
closed at both ends and has metal bearing surfaces lubricated by oil. In the
former type the bearings were traditionally lignum vitae strips and the tail shaft
(aft section of propeller shaft) was fitted with a brass liner, today composites
are commonly used for water lubricated sterntube bearings. The latter form
of sterntube is preferred in many ships with machinery aft, where the short
shaft is to be relatively stiff and only small deflections are tolerated. Where
this patent oil lubricated sterntube is fitted, glands are provided at both
ends to retain the oil and prevent the ingress of water, white metal (high
lead content) bearing surfaces being provided and the oil supplied from a
reservoir. Both types of sterntube are illustrated in Figure 21.6.

Shaft Bossing and ‘A’ Brackets

Twin-screw or multi-screw vessels have propeller shafts which leave the line
of shell at some distance forward of the stern. To support the shaft over-
hang, bossings or ‘A’ brackets may be fitted. Bossings are a common feature
on the larger multiple-screw passenger ships and are in effect a moulding of
the shell which takes in the line of shaft for some distance. Access from
inside the hull is thus provided to the shaft over a great proportion of its
length, and it is afforded greater protection. Many large liners having high