Page 232 - Ship Construction.DJ Eyres 6Ed

Ch19-H8070.fm Page 221 Wednesday, October 18, 2006 6:58 AM
Decks, Hatches, and Superstructures 221
Superstructures and Deckhouses

Superstructures might be defined as those erections above the freeboard
deck which extend to the ship’s side or almost to the side. Deckhouses are
those erections on deck which are well within the line of the ship’s side.
Both structures are of importance in the assignment of the load line as
they provide protection for the openings through the freeboard deck. Of
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particular importance in this respect are the end bulkheads of the super-
structures, particularly the bridge front which is to withstand the force
of any seas shipped. The bridge structure amidships or the poop aft are,
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in accordance with statutory regulations, provided as protection for the
machinery openings. It is possible however to dispense with these houses or
superstructures and increase considerably the scantlings of the exposed
machinery casing; but in other than very small vessels it is unlikely that such
an arrangement would be adopted. Unless an excessive sheer is provided on
the uppermost deck it is necessary to fit a forecastle forward to give added
protection in a seaway. Each structure is utilized to the full, the after struc-
ture carrying virtually all the accommodation in modern ships. The crew
may be located all aft in the poop structure or partly housed in any bridge
structure with the navigating spaces. Passenger liners have considerable
areas of superstructures covering tiers of decks and these will house the
majority of passengers and some of the crew. ึ
Of great structural importance is the strength of the vessel where super-
structures and deckhouses terminate and are non-continuous. At these dis-
continuities, discussed in Chapter 8, large stresses may arise and additional
strengthening will be required locally as indicated in the following notes on
the construction. Long superstructures exceeding 15 per cent of the ship’s
length and extending within 50 per cent of the vessel’s length amidships
receive special consideration as they contribute to the longitudinal strength
of the ship, and as such must have scantlings consistent with the main hull
strength members.

FORECASTLE Sea-going ships must be fitted with a forecastle which
extends at least 7 per cent of the ship’s length aft of the stem, and a mini-
mum height of the bow at the forecastle deck above the summer load line is
stipulated. By increasing the upper deck sheer at the forward end to obtain
the same height of bow, the forecastle might be dispensed with, but in practice
this construction is seldom found. The side and end plating of the fore-
castle has a thickness which is dependent on the ship’s length and the frame
and stiffener spacing adopted, the side plating being somewhat heavier than
the aft end plating. If a long forecastle is fitted such that its end bulkhead
comes within 50 per cent of the ship’s length amidships, additional stiffening
is required.