Page 230 - Ship Construction.DJ Eyres 6Ed

Ch19-H8070.fm Page 219 Wednesday, October 18, 2006 6:58 AM
Decks, Hatches, and Superstructures 219
principles to the single pull cover except that they remain in the horizontal
stowed position when the hatch is open. Various other forms of cover are
marketed, and tween deck steel covers are available to be fitted flush with
the deck, which is essential nowadays when stowing cargoes in the tweens.
To obtain weathertightness the patent covers have mating boundaries fitted
with rubber gaskets; likewise at the hatch coamings, gaskets are fitted and
hand or automatically operated cleats are provided to close the covers
--- ใช้เพื่อการศึกษาเท่านั้น---
(see Figure 19.6). The gasket and cleat arrangements will vary with the
type of cover.
Pontoon covers of steel with internal stiffening may be fitted, these being
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
constructed to provide their own support without the use of portable beams.
Each pontoon section may span the full hatch width, and cover perhaps
one-quarter of the hatch length. They are strong enough to permit Type B
freeboards to be assigned to the ship, but to satisfy the weathertightness
requirements they are covered with tarpaulins and battening devices.
Where portable beams are fitted wood or stiffened steel plate covers
may be used. These and the stiffened beams have the required statutory
scantlings but an increase in the freeboard is the penalty for fitting such
covers. The beams sit in sockets at the hatch coamings and the covers are
protected by at least two tarpaulins. At the coaming the tarpaulins are
ึ
made fast by battens and wedges fitted in cleats and the sections of the
cover are held down by locked bars or other securing arrangements
(see Figure 19.7).

Bulwarks

Bulwarks fitted on weather decks are provided as protection for personnel
and are not intended as a major structural feature. They are therefore of
light scantlings, and their connections to the adjacent structures are of
some importance if high stresses in the bulkwarks are to be avoided. Free-
ing ports are cut in bulwarks forming wells on decks in order that water may
quickly drain away. The required area of freeing ports is in accordance with
the Load Line Convention requirement (see Chapter 31).

CONSTRUCTION OF BULWARKS Bulwarks should be at least 1 m high
on the exposed freeboard and superstructure decks, but a reduced height
may be permitted if this interferes with the working of the ship. The bulwark
consists of a vertical plate stiffened at its top by a strong rail section (often a
bulb angle or plate) and is supported by stays from the deck (Figure 19.7).
On the forecastle of ships assigned B-100 and B-60 freeboards the stays are
more closely spaced. Where the bulwark is cut for any reason, the corners
are well rounded and compensation is also provided. No openings are
permitted in bulwarks near the end of superstructures.