GENERAL
The resistance to longitudinal stress is much reduced in the
way of hatches, on account of the large amount of material which must be cut out of the deck. Stress concentrations cause a tendency for the deck to
fracture at the hatch corners. There is also a loss of resistance to lcads of
water or cargo on the deck, due to the beams being cut at the hatch coaming.
These weaknesses must be compensated for or they might be dangerous. To
give the necessary strength the deck plating must be strengthened or doubled, and the coamings and their connections must be sufficiently strong and rigid.
DECK OPENINGS
With riveted construction, deck openings may have
square corners, with doubling plates fitted at the corners
When the deck is welded, square corners are not allowed. The openings
may be elliptical or parabolic, or the corners may be rounded-off. In the latter case, the radius at the corners must be at least one-twenty fourth of the breadth of the opening; but not, in any case, less than 300 millimetres. Doubling plates are not allowed in welded decks, so insert plates must be fitted at the hatch corners instead.
HEIGHT OF COAMINGS
Coamings on weather decks must extend well above the deck, to ensure that water cannot enter the ship’s hull. The height of coamings on freeboard decks, raised quarter decks, or within one-quarter of the ship’s length from the stem on superstructure decks, is to be 600 millimetres, where they are explosed to the weather. On exposed parts of superstructure decks, abaft one-quarter of the length from the stem, the height is to be 450 millimetres. At other decks, the openings are to be suitablv framed.
COAMINGS
Welded hatch coamings rnay have rounded corners, so as to
fit closely inside the deck opening, but this is not common because it is then
difficult to fit hatch-covers. A common alternative is to fit square-cornered coamings. If this is done within 0’6 of the ship’s length amidships, the ends of the side coamings mustbe extended beyond the hatch ends to form tapered brackets, if possible. The deck plating must be extended inside the coaming so that it can be rounded-off and its edges must be smooth and must not have anything welded to them.
Below the deck, the deck girder may be fitted in line with the side coamings.
If this is not done, and if the side coamings cannot be extended, above the
deck, to beyond the hatch ends; then short pieces of girder must be fitted to
extend the coamings for at least two frame spaces below the deck. In either
case, horizontal gussets are fitted under the hatch corners to strengthen the
connection between the side coamings and the hatch end beams.
Great care must be taken in welding hatch coamings to decks, in order
to make a strong and efficient connection. Full penetration fillet welds are
often used for this purpose.
If coamings are 600 millimetres or more in height, they must be stiffened
by bulb-bars, at least 180 millimetres deep, fitted horizontally near the upper edge of the coaming. Vertical stays or brackets must then be fitted, not more than 3 metres apart, between the stiffener and the deck.
HATCH END BEAMS
The beam at each end of a hatchway is called a’hatch
end beam’. These are usually made stronger than normal and are connected to the frames by heavy, flanged beam knees, because they carry at least part of the weight of the coaming.
HALF BEAMS
Transverse beams in the way of the side coamings are cut
at the latter and are called ‘half beams’. If the side coamings form part of the deck girder, the half beams are attached to them by we’ided bari and by brackets at alternate beams; as for deck girders.
LONGITUDINAL BEAMS AT COAMING
If longitudinal beams are fitted under the Aeclittrey are cut at the hatch and coaming and attached to it by brackets
