STRESSES ON A SHIP

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upwards which tends to bulge the sides outwards, hence the necessity
for shoring the bilges and sides to assist the hull to keep its shape,

Local Stresses.—Stresses are also set up when weights are unequally
distributed in the ship as in Fig. 12, which is intended to represent a
vessel divided into watertight compartments some of which are empty
and others laden with cargo. Unequal vertical stresses are thus created,
a downward pressure in the laden compartments and an upward pressure
in the empty ones, as, of course, the ship floats at a draught which
corresponds to the mean weight of the hull and its contents. Suppose it
were possible to disconnect the several compartments and that each
one had sufficient buoyancy within itself to float upright, then the
loaded compartments At € and E would come to rest at a deeper
draught than the mean draught, and the lighter compartments B
and D would float at a draught lighter than the mean. 4

Fig. 12.—Vertical Stresses,
Localised stresses are also set up in the way of deck machinery
such as windlass, cargo winches, derricks, steering gear, etc., and the
structure in their vicinity is stiffened by thickening the plating, putting
in additional or stronger beams, and arranging the material so that the
local stress may be distributed to adjacent parts and not centralised
too much at one place.
This very general and brief reference to the stresses thrown upon the
ship's hull when considered as a compound girder may direct the mind
of the student to the fact that the longitudinal and transverse frame-
work of the ship is designed to enable the shell plating to keep its form
and to resist any distortion and strain which might produce a leaking
hull. The structure must, therefore, be made rigid enough longitudin-