Basic Terms In Ship Stability

Q. What is stability?
Ans. Stability is the ability of vessel to return to the up-right equilibrium. In case of a listed vessel or the vessel at angle of loll it would be the ability to return to the respective angles of equilibrium. The stability of a ship thus is GZ_T rather than GM_T. The GZ at various angles of heel i.e. GZ curve represents the true stability status or picture of the ship. One glance at the GZ curve would provide a clear picture of ship’s stability.

Q. As per Solas, what stability information must be provided to ships?
Ans. According to the SOLAS and also the MSA, the stability information provided to ship, must be such that the Master is able to make an accurate assessment of ship’s stability by rapid and simple means, in all service conditions including in the impaired conditions. This objective of SOLAS can be met only if the stability information provides the GZ curve for all the conditions in which Master may find his vessel. A comparison of GZ curve with relevant stability criteria (Intact or Damage) can help Master making an accurate assessment of stability in any condition of stability.

Q. What is the importance of beam-draft ratio?
Ans. Ratio of beam to draft has an important bearing on the height and movement of transverse metacentre with respect to centre of buoyancy. BM_T of a box shaped vessel is given by $\dfrac {beam^{2}}{12draft}$ .

Q. What is center of flotation? What is its significance?
Ans. The center of flotation is the centroid of the ship’s (or for that matter any floating body’s) water plane area. It is the point about which a vessel trims longitudinally or rolls transversely. Thus, rolling as well as pitching axes pass through it. Hydrostatic draft is measured abreast the COF.

Q. What is displacement?
Ans. Displacement of the vessel (∆) is actually the mass of the vessel. For the normal shipboard calculations the displacement is referred to as the weight of the ship. The term displacement in respect of a ship is derived from the fact that a vessel displaces the equivalent mass of water in which she floats. The unit used is metric tonnes.

Q. What do you understand by centre of buoyancy?
Ans. Centre of Buoyancy is the centroid of underwater portion of the vessel. At COB, the entire buoyancy force acting vertically upwards is assumed to act.

Q. Explain the term ‘transverse metacentre’?
Ans. While the ship rolls on either side of upright equilibrium, the COB would go ‘to and fro’ about this mean position. Uprights drawn through these COB’s to the respective water lines would meet at the transverse metacentre of the ship.

If we assume that the COB goes ‘to and fro’ along a swing, then the imaginary suspended point of the swing can be understood as transverse metacentre. In fact, in a swing of about 7^o on either side, there can be infinite centre of buoyancies and respective water lines. The uprights drawn from each of the centre of buoyancy points, on respective waterlines would meet at the transverse metacentre. This can be true for any floating object, rolling about the equilibrium position. The vertical distance BM_Tfor any equilibrium position is found by following formula:

Q. What is metacentric height?
Ans. It is the vertical distance from the metacenter to the center of gravity of a ship. If the center of gravity is below the metacentre the vessel is said to be in stable equilibrium whereas if the metacentre is vertically below the COG, the vessel is said to be in unstable equilibrium.

Q. What is longitudinal metacentre?
Ans.While the ship pitches slightly to either side of normal trimmed equilibrium, the COB would go longitudinally ‘to and fro’ of this mean position. Uprights drawn through these COB’s on respective water lines would meet at the longitudinal metacentre of the ship.

If we assume that the COB goes longitudinally ‘to and fro’ along a swing, then the imaginary suspended point of the swing can be understood as longitudinal metacentre. The vertical distance BM_Lfor any equilibrium position is found by following formula:

Q. What is equilibrium?
Ans. A ship not making any movement whatsoever & is absolutely standstill is said to be at equilibrium. This can be established by even viewing from outside. Equilibrium means the state of rest, which is achieved when the following two conditions are satisfied:

The vector sum of all external forces is zero.
The sum of the moments of all external forces about any point is zero.

Thus, if a body does not move or turn then, it can be said that net forces equal up to zero. Also, the total moments acting on the body result in zero moment about any point on it.

In case of a ship, the major forces are the weight and the buoyancy. The other external factors (wind, swell etc) can cause movement of ship about rolling axis, pitching axis and heaving axes or along an axis formed by combination of these basic axes. We may normally say that a ship is standstill or stationary or is at rest when she is not rolling pitching or heaving. Thus, a listed ship, a ship at angle of loll, a ship that is trimmed or even grounded can be considered to be at equilibrium, if at rest. A rolling or a pitching vessel is not in equilibrium but is in the process of attaining equilibrium.

In case of a ship at equilibrium:

1. The weight force (W) is equal to buoyancy force (B).

2. COB and COG are in the same vertical line

3. The righting lever GZ_T, GZ_Lare both zero.

4. On the GZ curve the heel angles at which the ship can be at equilibrium are indicated by the points at which GZ curve touches x-axis.

Q. What is heel and how is it different from List?
Ans. The inclination of a ship to either side of upright, responding to any external cause is called heel. Normally, heel is understood as inclination due to external forces as against list, which is the inclination due to the weight distribution within the ship. The term heel, often is also used in place of list. E.g. ‘grain heeling moment’ is the term normally used to mean grain listing moment.

Q. Compare heel and list.

Q. Is COG in line of COB in case of a trimmed ship.
Ans. LCB has to be equal to LCG when the ship is trimmed and is still. In the formula,

LCB is tabulated even keel LCB, whereas LCG is from the result of loading. The trim is actually change in trim from even keel trim to present trim. After the vessel is settled to new trim the LCB_EK becomes equal to LCG

Q. What is statical stability?
Ans. Statical stability is stability of ship with water level being calm (static) as the ship heels (rolls).

Q. Why is this curve also called curve of Statical Stability?
Ans. The values of GZ given for different angles of heel, is for static water condition. This means that the righting lever values are true, provided a vessel is rolling in absolute calm sea conditions. This means, even if the heel in question is large say 50⁰, the GZ obtained from the yard data is assuming that the ship is heeled to this angle in static sea conditions.

Q. What information is provided in Hydrostatic Curves?
Ans. Hydrostatic Curves are based on the form of the immersed portions of a vessel. They include: various coefficients, TPC, displacement, MCTC, wet area, height of B and M_T, M_L above the keel, etc for different drafts.

Q. What is opposite of statical stability?
Ans. Opposite of statical stability is stability in dynamical condition. Stability of ship with water level being dynamic as the ship heels (rolls). The GZ data will vary in various sea conditions and will be different from the values provided by yards. The vessel will behave in varying ways in dynamic conditions.