Page 289 - Ship Construction.DJ Eyres 6Ed

Ch24-H8070.fm Page 278 Wednesday, October 18, 2006 7:00 AM
278 Ship Construction
MAST CONSTRUCTION AND STIFFENING Tubular steel sections are
commonly used in mast and post construction, the sections being rolled in
short lengths and welded in the shipyard. The short lengths may be
tapered and are of different plate thickness to allow for the greater
stresses experienced at the base of the mast. Where connections are made
for fittings such as the gooseneck and a masthead span swivel, doubling or
welded reinforcing pads may be provided. To obtain the necessary mast
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scantlings, excessive doubling or internal stiffeners are rarely found in mod-
ern practice, except where a heavier derrick than that for which the mast was
originally designed is carried. Higher tensile steels are often used to advan-
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tage in mast construction, giving less weight high up in the ship and dispens-
ing with the need for any form of support, without excessive scantlings.
Cross-trees, mast tables, etc., may be fabricated from welded steel plates
and sections.
Derrick booms are as a rule welded lengths of seamless tubular steel. The
middle length may have a greater diameter to allow for the bending
moment, to which the boom is subject in addition to the axial thrust.
At the base of the mast adequate rigidity must be provided, the amount
of additional structural stiffening increasing with the size of derricks carried
by the mast. Many cargo ships have mast houses into which the masts are
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built, the house being suitably strengthened. These houses need not be
designed to support the mast, the structure being of light scantlings, and the
support provided by stiffening in the tweens. Where the house is strength-
ened the masts or posts generally land on the upper deck, but where heavy
derricks are installed the mast may then land on the upper tween deck.
Since the derricks and mast are as a rule midway between holds they land
over the hold transverse bulkheads which lend further support.
Heavy derrick masts will require extensive stiffening arrangements in the
mast house, and also in the tweens, with support for the transverse bulkhead
so that the loads are transmitted through the structure to the ship’s bot-
tom. Partial longitudinal and transverse bulkheads with deck girders may
provide the mast house stiffening. Stiffened plate webs at the ship’s centre
line in the tweens, and heavier stiffeners on the transverse bulkhead in the
hold then provide the additional strengthening below decks (see Figure 24.1).
Heavy insert plates are fitted in way of the mast at the various decks.

Derrick Rigs

Various forms of derrick rig may be used aboard the cargo ship, the com-
monest use of the single derrick being as a ‘single swinging derrick’ (see
Figure 24.2(a)). Adjacent derrick booms may be used in ‘union purchase’
(Figure 24.2(b)) the booms being fixed in the overboard and inboard posi-
tions. Cargo is lifted from the hatch and swung outboard by the operator