Page 163 - Ship Construction.DJ Eyres 6Ed

Ch15-H8070.fm Page 152 Wednesday, October 18, 2006 6:56 AM
152 Ship Construction
LUBRICANT For the ship to start sliding on release of the holding
arrangements it is necessary for the ship to overcome the coefficient of
friction of the launching lubricant. To do this the slope of the ways under
the vessel’s centre of gravity must exceed the lubricant’s coefficient of
friction. An estimate of the frictional resistance of the grease must be
made before building the ship since the declivity of the keel is dependent
to a large extent on the slope of the ways.
--- ใช้เพื่อการศึกษาเท่านั้น---
Formerly melted tallow was applied to the ways, allowed to harden,
then covered with a coat of soft soap. Since the mid 1900’s patent mineral
based greases have been applied to the ground ways, these greases being
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
virtually unaffected by temperature changes and insoluble in water whilst
adhering firmly to the ways. A commercially marketed petroleum-based
launching grease is applied over the mineral grease base coat. This has a
coefficient of friction which is low enough to allow initial starting and the
maintenance of sliding until the initial resistance of the base coat is overcome
by frictional heat. To prevent the petroleum-based grease from soaking into
the sliding ways a base coat may be applied to them. Standing ways which
extend into the water may be dried out at low tide prior to the launch and
the base coat and grease applied.
ึ
RELEASING ARRANGEMENTS Small ships may be released by knock-
ing away a diagonal dog-shore (see Figure 15.3) fitted between the sliding
and standing ways.
In most cases however triggers are used to release the ship. There are
several types available, hydraulic, mechanical, and electrical-mechanical trig-
gers having been used. Electrical-mechanical triggers are commonly used for
rapid simultaneous release in modern practice; the hydraulic trigger is less
easily installed and less safe. The electrical-mechanical trigger illustrated in
Figure 15.3 is generally located near amidships and a small pit is provided
in the berth to accommodate the falling levers. A number of triggers will be
fitted depending on the size of the vessel to be launched; in the case of the
75 000-tonne bulk carrier for which a launching sequence is given below six
triggers were fitted for the launch.
These triggers are in effect a simple system of levers which allow the large
loads acting down the ways to be balanced by a small load on the releasing
gear. The principle is often compared with that of a simple mechanical
reduction gearing. Simultaneous release of the triggers is achieved by means
of catches held by solenoids wired in a common circuit. These are released
immediately the circuit current is reversed.

LAUNCHING SEQUENCE As a guide to the procedure leading up to the
launch, the following example is given for the launch of a 75 000-tonne bulk
carrier. The launch ways have been built up as the ship is erected from aft;
the ways have been greased and the cradle erected.