Ship Stability – Understanding Intact Stability of Ships

The understanding of a surface ship’s stability can be divided into two parts. First, Intact Stability. This field of study deals with the stability of a surface ship when the intactness of its hull is maintained, and no compartment or watertight tank is damaged or freely flooded by seawater.

Secondly, Damaged Stability. The study of damaged stability of a surface ship includes the identification of compartments or tanks that are subjected to damage and flooded by seawater, followed by a prediction of resulting trim and draft conditions.

Damaged stability, however, cannot be understood without a clear understanding of intact stability, and the interesting scenarios related to it. Hence, we will first focus on intact stability from this article onward, leading to a discussion of cases where the application of concepts of intact stability come of use and then move on to damaged stability.

Intact Stability of Surface Ships:

The fundamental concept behind the understanding of intact stability of a floating body is that of Equilibrium. There are three types of equilibrium conditions that can occur, for a floating ship, depending on the relation between the positions of centre of gravity and centre of buoyancy.

1. Stable Equilibrium:

Study the figure below. A stable equilibrium is achieved when the vertical position of G is lower than the position of transverse metacenter (M). So, when the ship heels to an angle (say theta- Ɵ), the center of buoyancy (B) now shifts to B1. The lateral distance or lever between the weight and buoyancy in this condition results in a moment that brings the ship back to its original upright position.

The moment resulting in uprighting of the ship to its original orientation is called Righting Moment. The lever that causes the righting of a ship is the separation between the vertical lines passing through G and B1. This is called the Righting Lever, and abbreviated as GZ (refer to the figure above).

An important relation between metacentric height (GM) and righting lever (GZ) can also be obtained from the figure above.

2. Neutral Equilibrium:

This is the most dangerous situation possible, for any surface ship, and all precautions must be taken to avoid it. It occurs when the vertical position of CG coincides with the transverse metacentre (M). As shown in the figure below, in such a condition, no righting lever is generated at any angle of heel. As a result, any heeling moment would not give rise to a righting moment, and the ship would remain in the heeled position as long as neutral stability prevails. The risk here is, at a larger angle of heel in a neutrally stable shift, an unwanted weight shift due to cargo shifting might give rise to a condition of unstable equilibrium.

3. Unstable Equilibrium:

An unstable equilibrium is caused when the vertical position of G is higher than the position of transverse metacenter (M). So, when the ship heels to an angle (say theta- Ɵ), the center of buoyancy (B) now shifts to B1. But the righting lever is now negative, or in other words, the moment created would result in creating further heel until a condition of stable equilibrium is reached. If the condition of stable equilibrium is not reached by the time the deck is not immersed, the ship is said to capsize.

Remember discussing, in the previous article, that metacentric height is one of the most vital parameters in the study of ship stability? We are now, in a position to appreciate the same. A ship’s stability, as seen above, can be directly commented on, by the value of its metacentric height (GM).

• GM > 0 means the ship is stable.
• GM = 0 means the ship is neutrally stable.
• GM < 0 means the ship is unstable.

Source:www.marineinsight.com