Page 96 - Marine Structural Design

72 Part I Structural Design Principles

weight of the liquid displaced by the floating body, and thus the center of buoyancy is the
center of gravity of the displaced liquid.
W
W

L W *

B
B

Figure 4.1 Interaction of Weight and Buoyancy

When a floating body is in equilibrium and is displaced slightly &om its original position,
three conditions may apply. As shown in Figure 4.2 (Pauling, 1988), the body may:
1. return to its original position, a situation known as positive stability;
2. remain in its new position, and this is known as neutral stability;
3. move further from its original position, known as negative stability.

I

(a) (b)
Figure 4.2 Positive and Negative Stability
A ship should be positively stable, so that it can return to its original position without
overturning when displaced from its original position, say by a wave.
The stability of a floating body such as a ship is determined by the interaction between the
forces of weight, W, and buoyancy, B, as seen in Figure 4.1. When in equilibrium, the two
forces acting through the centers of gravity, CG, and buoyancy, CB, are aligned (Figure
4.1(a)). If the body rotates &om WL to WlLI, (Figure 4.l(b) and 4.2(a)), a righting moment is
created by the interaction of the two forces and the body returns to its original equilibrium
state, as shown in Figure 4.l(a). This is a case of positive stability. If the interaction between