Single pull macgregor type hatch cover.pdf - Cochin University of ...

Figure 3.1 (a) Rounded deck opening with coaming
(b) Coaming plate following deck opening
Source: Cargo Access Equipment for Merchant Ships.
To avoid high stress concentration in deck plating, the corners of deck
openings should be elliptical or parabolic. As the corners of the hatch covers are
usually rectangular, provision has to be made to accommodate the hatch coaming.
This can be done in two ways: either a square coaming can be built with the rounded
deck plating protruding into the coaming space shown in fig3.1 (a) and (b) or the
coaming can follow the shape of the deck opening with a filling-in plate welded to its
top. In both cases the clear opening for rectangular cargoes like containers will be
appreciably smaller than the maximum dimensions to the inside of the coamings.
3.2.4 Drainage
As mentioned earlier, an important function of access equipment is to prevent water
entering the ship through the openings in its hull. If the hull is flexing in heavy
weather, it is almost inevitable that some water will penetrate the seals of closing
devices, especially if they are worn. Thus there must be a second line of defence, such
as drains, for removing any water before it can damage the cargo.
Drainage facilities must be built into all terms access equipment. Here water
which seeps past the peripheral seal of a hatch cover runs along a channel and is
discharged onto the weather deck through a hole in the coaming. Where the drain is
below the freeboard deck, as in flush weather deck covers, it must be connected to the
bilge, or overboard via a scupper and non return valve.
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