Page 158 - Ship Construction.DJ Eyres 6Ed

Ch15-H8070.fm Page 147 Wednesday, October 18, 2006 6:56 AM

15
Launching

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งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Whilst many modern shipyards now construct ships in building docks and
float them out and some utilise ship lifts, a good number of long established
shipyards are still launching ships in the traditional manner. A significant
advance has been provided by computer programs which allow prior assess-
ment of the performance of the ship and the loading it may experience dur-
ing the launch, both on the ways and in the water.
Launching involves the transference of the weight of the ship from the
keel blocks, shores, etc., on which it was supported during construction, to a
cradle on which it is allowed to slide into the water. Normally the vessel is
launched end on, stern first, but a number of shipyards located on rivers or
ึ
other narrow channels are obliged to launch the vessel sideways. Vessels
have been launched bow first, but this was a rare occurrence as the buoyancy
and weight moments, also the braking force, are generally more favourable
when the vessel is launched stern first.

End Launches
On release of a holding mechanism the launching cradle with the ship slides
down the ground ways under the action of gravity. When the stern has
entered the water the vessel is partly supported by buoyancy and partly by
the ground ways. If this buoyancy is inadequate after the centre of gravity of
the ship has passed the way ends, the ship may tip about the way ends causing
large pressures on the bottom shell and on the ends of the ground ways. To
avoid this the greatest depth of water over the way ends should be utilized,
and the ground ways extended into the water if necessary. Where this proves
impossible it becomes necessary to strengthen the way ends and provide
shoring in the bottom shell region which is likely to be damaged. These
remedies are often expensive.
As the vessel travels further into the water the buoyancy becomes suffi-
cient to lift the stern. The vessel then pivots about the forward poppets, i.e.
the fore end of the launching cradle. These are designed to take the load
thrown on them by the pivoting action, the load being widely distributed in
order not to squeeze out the lubricant between the sliding surfaces. Shoring