stability

stability is the ability of a ship to return to an upright position after being heeled by an external force, such as a weight being placed aboard or by rough sea conditions. If a ship is inclined at an angle and then naturally returns to the upright, it is known as being stable. Should the ship wish to continue moving from the upright and is heading for capsize, then it is described as unstable.

It is normal for ships to have their centre of gravity higher than the centre of buoyancy (see Fig. 1) creating what appears on first sight to be an unstable situation. However, as can be seen (Fig. 2), the centre of buoyancy travels away from the centre line as the ship is heeled, producing a restoring force.

In Fig. 2 the distance G–M is known as the metacentric height and is a measure of the stiffness, or stability, of a ship. The larger the metacentric height of a ship, the greater the stability. On the down side, very large metacentric heights produce violent rolling motions. So passenger ship operators, in particular, must find a suitable compromise which keeps their ships safe yet creates a slow rolling movement which neither upsets fare-paying passengers, nor damages fragile cargo. To assist in keeping the ship from excessive rolling, fin stabilizers are often fitted.

Naval architects (see naval architecture) calculate the metacentric height of every ship in all conditions, from being in ballast through to part and fully laden. As the safety of the ship is a statutory obligation it is important that the metacentric height be confirmed at full scale. For this reason an inclining experiment is carried out when the ship is afloat in still conditions. The stability can be calculated from the angle of heel adopted by the ship when a known weight is moved across the deck an exact amount.

During a ship's lifetime, it acquires additional topside weight through coats of paint, loose stores, etc. The ship's managers and officers must keep aware of this, and it is recommended that further inclining experiments be carried out every few years.

Fred M. Walker