Page 80 - Introduction to Naval Architecture

FLOTATION AND STABILITY 67

Figure 4.29 Modified GZ curve

other side. If GGj cos <p is plotted on the stability curve, Figure 4.29, for
the particular condition of loading of the ship, the two curves intersect at
B and C. B gives the new equilibrium position of the ship in still water and
C the new angle of vanishing stability. The range of stability and the
maximum righting arm are gready reduced on the side to which the ship
lists. For heeling to the opposite side the values are increased but it is the
worse case that is of greater concern and must be considered. Clearly
every precaution should be taken to avoid shifts of cargo.

Bulk cargoes
A related situation can occur in the carriage of dry bulk cargoes such
as grain, ore and coal. Bulk cargoes settle down when the ship goes to
sea so that holds which were full initially, have void spaces at the top. All
materials of this type have an angle of repose. If the ship rolls to a greater
angle than this the cargo may move to one side and not move back
later. Consequently there can be a permanent transfer of weight to one
side resulting in a permanent list with a reduction of stability on that
side. In the past many ships have been lost from this cause.
Figure 4.30 shows a section through the hold of a ship carrying a
bulk cargo. When the cargo setdes down at sea its centre of gravity is at
g. If the ship rolls the cargo could take up a new position shown by the
inclined line, causing some weight, iu, to move horizontally by hi and
vertically by h^. As a result the ship's G will move:

The modified righting arm becomes:

where GZ is the righting arm before the cargo shifted.