STATIC AND DYNAMIC STABILITY, HIGH GM LESS STABILITY
1) It is defined as the ability of a ship to regain its upright equilibrium position, after the removal of external factor which caused the vessel to heel at an angle.
2) It gives the stability information of a vessel under the condition that the outside water is static.
3) It is expressed in terms of metacentric height. i.e. GM ( for angle of heel up to 10 degree) and righting lever GZ ( for angle of heel above 10 degree)
4) It’s unit is meter
5) Static stability at two different angle of heel can be the same.
1) It is defined as the energy required heeling the ship from upright equilibrium till the angle of heel in question.
2) It gives the stability information of a vessel considering dynamic behavior of the sea.
3) It is expressed in terms of the area under righting moment curve. ( or GZ curve multiplied by displacement of the ship in tons)
4) It’s unit is ton-meter-radian
5) The dynamic stability at two different angle of heel cannot be the same
A value of metacentric height gives accurate measure of stability only for small disturbances i.e. angle not beyond 10 degree. For larger angle of heel, the righting lever GZ is used to measure stability. In any stability analysis, the value of GZ is plotted over the entire range of heel angles for which it is positive or restoring.
So, as a thumb rule we say that vessel stability is decided on its value of GM up to 10 degree of heel. But is it possible that a vessel high on GM can be low on stability?
Yes, a ship high on GM can be low on stability. Let us consider the vessels which are built with high forc’le and low working aft for e.g. offshore supply vessels. These vessels possess a large upright GM value due to generous beam to length ratio. But these boats tend to tolerate less heel angles than narrower boats. Also the vanishing stability of these vessels is relatively low.
The hull form of a vessel is an important factor in determining the characteristics of its stability. Increased beam will result in higher value of GM and righting lever(GZ). However the point of vanishing stability will be less. This is due to free trim effect. i.e. heeling of these vessels produces a trimming moment by astern.
While taking over a new ship in shipyard as chief engineer the following governing factors for having optimum stability in both the static and dynamic categories should be inspected:-
1) The new ship must fulfill six criteria of intact stability as follows –
a) The area under righting lever curve is not to be less than 0.055 m.rad up to 30 degree heel
b) Area under GZ curve not less than 0.09 m-rad up to 40 degree heel
c) Area under GZ curve between 30 degree and 40 degree heel should not be less than 0.03 m-rad
d) The righting lever GZ should be at least 0.2 meter at an angle of heel greater than or equal to 30 degree
e) The angle of heel for max GZ must be at least 25 degree and preferably greater than 30 degree.
f) Initial GM should be minimum 0.15 meter.
2) Elements affecting stability should be taken into account like beam wind of ships with large windage area, icing of top side, water trapped on deck, rolling characteristics, following seas etc.
3) Safe margin should be there for elements which reduce stability during voyage, regarding being given to addition of weight, such as those due to absorption of water and icing and to losses of weights such as those due to store and fuel.
4) For hips carrying passengers, in addition to the general intact stability criteria, the angle of heel on account of turning of the ship through rudder should not exceed 10 degree. Also the angle of heel on account of crowding of passengers to one side should not exceed 10 degree.
5) Apart from above, watertight integrity should also be checked. The reserve buoyancy of a vessel is the measure of her stability.e