Page 62 - statbility for masters and mates
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Chapter 7
Effect of free surface
of liquids on stability
When a tank is completely ®lled with a liquid, the liquid cannot move within the tank when the ship heels. For this reason, as far as stability is concerned, the liquid may be considered a static weight having its centre of gravity at the centre of gravity of the liquid within the tank.
Figure 7.1(a) shows a ship with a double-bottom tank ®lled with a liquid having its centre of gravity at g. The effect when the ship is heeled to a small angle y is shown in Figure 7.1(b). No weights have been moved within the ship, therefore the position of G is not affected. The centre of buoyancy will move out to the low side indicated by BB1.
Moment of statical stability W GZ
W GM sin y
Fig. 7.1
Now consider the same ship ¯oating at the same draft and having the same KG, but increase the depth of the tank so that the liquid now only partially ®lls it as shown in Figures 7.1(c) and 7.1(d).
