Stability of Immersed and Floating Bodies

Stability becomes an important consideration when floating bodies such as a boat or ferry is designed. It is an obvious requirement that a floating body such as a boat does not topple when slightly disturbed. We say that a body is in stable equilibrium if it is able to return to its position when slightly disturbed. Failure to do so denotes unstable equilibrium. What equilibrium a body enjoys is decided by the couple formed by the weight of the body and the buoyancy force. Consider the immersed body shown in Fig.2.19. In general, if the center of gravity of the body lies below the center of buoyancy stable equilibrium prevails. An overturning couple leading to unstable equilibrium results if the center of gravity is above the center of buoyancy (Fig.2.20).

Figure 2.19 : Stability of an immersed body

Figure 2.20: Instability of an immersed body

It becomes more complicated when floating bodies are considered. Now as the body rotates responding to any disturbance the center of buoyancy can shift. This could render the body stable even though the center of gravity is above the center of buoyancy. This is particularly true of the bodies with a broader base such as a barge (Fig. 2.21). A slender body as shown in Fig. 2.22 is very susceptible for instability.

Figure 2.21 : Stability of a floating body

 

Figure 2.22 : Instability of a floating body

 

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney