Chapter2 : Fluid Statics Lecture 5 : Stability of Floating Bodies in Fluid

• When the body undergoes an angular displacement about a horizontal axis, the shape of the immersed volume changes and so the centre of buoyancy moves relative to the body.

•  As a result of above observation stable equlibrium can be achieved, under certain condition, even when G is above B.
Figure 5.9a illustrates a floating body -a boat, for example, in its equilibrium position. Fig 5.9     A Floating body in Stable equilibrium

Important points to note here are

1. The force of buoyancy FB is equal to the weight of the body W

2. Centre of gravity G is above the centre of buoyancy in the same vertical line.

3. Figure 5.9b shows the situation after the body has undergone a small angular displacement q with respect to the vertical axis.

4. The centre of gravity G remains unchanged relative to the body (This is not always true for ships where some of the cargo may shift during an angular displacement).

5. During the movement, the volume immersed on the right hand side increases while that on the left hand side decreases. Therefore the centre of buoyancy moves towards the right to its new position B'.

Let the new line of action of the buoyant force (which is always vertical) through B' intersects the axis BG (the old vertical line containing the centre of gravity G and the old centre of buoyancy B) at M. For small values of q the point M is practically constant in position and is known as metacentre. For the body shown in Fig. 5.9, M is above G, and the couple acting on the body in its displaced position is a restoring couple which tends to turn the body to its original position. If M were below G, the couple would be an overturning couple and the original equilibrium would have been unstable. When M coincides with G, the body will assume its new position without any further movement and thus will be in neutral equilibrium. Therefore, for a floating body, the stability is determined not simply by the relative position of B and G, rather by the relative position of M and G. The distance of metacentre above G along the line BG is known as metacentric height GM which can be written as

GM = BM -BG

Hence the condition of stable equilibrium for a floating body can be expressed in terms of metacentric height as follows:

GM > 0 (M is above G)                                      Stable equilibrium
GM = 0 (M coinciding with G)                          Neutral equilibrium
GM < 0 (M is below G)                                      Unstable equilibrium

The angular displacement of a boat or ship about its longitudinal axis is known as 'rolling' while that about its transverse axis is known as "pitching".

Floating Bodies Containing Liquid

If a floating body carrying liquid with a free surface undergoes an angular displacement, the liquid will also move to keep its free surface horizontal. Thus not only does the centre of buoyancy B move, but also the centre of gravity G of the floating body and its contents move  in the same direction as the movement of B. Hence the stability of the body is reduced. For this reason, liquid which has to be carried in a  ship is put into a number of separate compartments so as to minimize its movement within the ship. 