Page 227 - Ship Construction.DJ Eyres 6Ed

Ch19-H8070.fm Page 216 Wednesday, October 18, 2006 6:58 AM
216 Ship Construction
is a draft penalty. This means that most ships are fitted exclusively with the
stronger stiffened self-supporting steel covers.

HATCH COAMINGS Heights of coamings and cover closing arrangements
in some instances depend on the hatch position. The positions differentiate
between regions which are more exposed than others. Position 1 indicates
that the hatch is on the exposed freeboard deck, raised quarter deck, or
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superstructure decks within 25 per cent of the ship’s length from forward.
Position 2 indicates that the hatch is located on exposed superstructure
decks abaft the forward 25 per cent of the ship’s length.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Hatches which are at Position 1 have coamings at least 600 mm high, and
those at Position 2 have coamings at least 450 mm high, the height being
measured above the sheathing. Provision is made for lowering these heights
or omitting the coaming altogether if directly secured steel covers are fitted
and it can be shown that the safety of the ship would not be impaired in any
sea condition. Where the coaming height is 600 mm or more the upper edge
is stiffened by a horizontal bulb flat and supporting stays to the deck are
fitted. Coamings less than 600 mm high are stiffened by a cope or similar
bar at their upper edge. The steel coamings extend down to the lower edge
of the deck beams, which are then effectively attached to the coamings
ึ
(Figure 19.4).

HATCH COVERS A number of patent steel covers, such as those manufac-
tured by MacGregor Group AB and TTS Marine ASA, are available, which
will comply with the requirements outlined by the International Conference
on Load Lines 1966 and are in accordance with the requirements of the
classification societies. The means of securing the hatches and maintaining
their watertightness is tested initially and at periodic surveys. These patent
covers vary in type the principal ones being fore and aft single pull, folding,
piggy back, pontoon and side rolling. These are illustrated in Figures 19.5
and 19.6. Single pull covers may be opened or closed by built in electric
motors in the leading cover panel (first out of stowage) which drive chain
wheels, one on each outboard side of the panel. Each panel wheel is perma-
nently engaged on a fixed chain located along each hatch side coaming. In
operation the leading panel pushes the others into stowage and pulls them
into the closed position. Alternatively single pull covers are opened or
closed by hydraulic or electric motors situated on the hatch end coaming at
the ships centre line driving endless chains running along the full length of
the hatch side coaming port and starboard and connected to the leading
panel. Vertical stowage of panels is at one end of the hatch and covers may
have a nesting characteristic if space is at a premium, also on large hatches
opening may be to both ends with vertical stowage at each end. Folding covers
may be of direct pull type where suitable lifting gear is carried onboard or can
be opened or closed by externally mounted hydraulic cylinders actuating