Page 159 - Ship Construction.DJ Eyres 6Ed

Ch15-H8070.fm Page 148 Wednesday, October 18, 2006 6:56 AM
148 Ship Construction
may also be found necessary forward in the ship to prevent structural damage
at the time the stern lifts.

BUILDING SLIP Conventional slipways or berths are relatively solid and
reinforced with piles to allow them to sustain the weights of ships built upon
them. During building the keel blocks take the greater part of the weight, the
remainder being carried by shores, and where used bilge blocks. Foundations
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under the probable positions of ground ways should also be substantial since
during a launch the ways are subject to large pressures.
Keel blocks are arranged so that the height of keel above the ground is
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1.25 to 1.5m, giving reasonable access, but not too high so that a large amount
of packing is required (see Figure 15.1). At the bow the height of the keel
must be sufficient to allow the ship’s forefoot to dip the required amount
without striking the ground during pivoting when the stern lifts at launch.
To suit the declivity of the launching ways determined beforehand, the keel
is also inclined to the horizontal at about 1 in 20, or more where the ship-
yard berths have a larger slope.
To transfer the ship from the building blocks to the launching cradle, the
commonest practice is to drive wedges into the launching cradle. This lifts
the ship and permits the removal of the keel and bilge blocks together with the
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shores. In large ships it becomes necessary to split the blocks to remove
them, but several types of collapsible blocks have been used to overcome
this difficulty. One type is the sand box which contains sand to a depth of 80
to 100 mm held in a steel frame located between two of the wooden blocks.
This steel frame may be removed and the sand allowed to run out. Another
type is a wooden block sawn diagonally, the two halves being bolted so that
they collapse on removal of the bolts.

LAUNCHING WAYS AND CRADLE The fixed ground ways or standing
ways on which the cradle and ship slide may be straight or have a fore and
aft camber. Transversely the ground ways are normally laid straight but can
be canted inwards to suit the ship’s rise of floor. Usually the ground ways
have a small uniform fore and aft camber say 1 in 400, the ways being the
arc of a circle of large radius. This means that the lower part of the ways has
a greater declivity (say 1 in 16) than the upper part of the ways (say 1 in 25).
As a result a greater buoyancy for the same travel of the ship beyond the
way ends is obtained, which will reduce the way end pressures. Additional
advantages are increased water resistance slowing the vessel, and a bow
height which is not excessive. The slope of ground ways must be adequate
to allow the vessel to start sliding; and if too steep, a large amount of
shoring will be required to support the bow; also the loads on the releasing
arrangements will be high. Straight sliding ways have declivities of the order
of 1 in 25 to 1 in 16.