Now, since the final CG of the ship has shifted from the centreline, it will create a heeling moment towards the port side.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.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.
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. 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. 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- ), 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 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). 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. 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. A ships stability, as seen above, can be directly commented on, by the value of its metacentric height (GM). The resistance acts as an opposing force on the underwater part of the hull. Now, there are two sets of force couples and corresponding moments generated, in this case. ![]() In this case, a heeling moment is caused by a shift in the center of gravity. How. This also means that once a weight (suppose, a container) is lifted from the berth, the weight of the container acts through the end of the derrick (which is a fixed point with respect to the ship), irrespective of the swinging motion of the container. The ship and the container can now be treated as a two-point mass system. The final centre of gravity of the system (G1) will lie on the line (shown in blue) joining the initial CG of the ship (G) and the centre of gravity of the weight (g).
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