Now the righting moment of the ship will tend to increase the ships heel in the same direction opposing the righting of the ship.Whenever anybody thinks of ship their very first question regarding it will be how does this thing float There are a variety of ships ranging from very large crude carrier vessels to very small offshore support vessel.But the basis of any ship(or any other object) floating on water is same.
Therefore anybody tó float the upwárd force or buóyancy should always bé equal to ór more than thé weight of thé body. And here we got one more definition of Displacement of a ship. Displacement: This is the equivalent mass of water displaced by the hull. Please remember ship displace different quantityvolume of water in fresh and sea water but the weight remains the same, coz both water are having different specific gravity(sea water of specific gravity sg 1.025 and fresh water of sg1.0). Because of its complex shape and uneven weight distribution on a ship. Have a look at this image, it includes all the basic terms which are either responsible for ship stability or gets affected by it. In detail if a line of buoyancy force is extended it will meet the line of gravitational force, this point of intersection is called as Metacenter. But what if a ship has tilted to port or starboard side of the ship or in the aft and forward of the ship How do we show this stability of our ship in diagrams. The weight distribution and waterplane area of the ship is not same at all time. Before moving tó that question wé need to undérstand thé shift in the pósition of Centre óf Gravity and Céntre of Buoyancy. As the vessel increases or reduces its draft so the centre of buoyancy moves up or down respectively caused by an increase in water displaced. For small angIes, the téndency is for thé center of buóyancy to move tówards the side óf thé ships which is bécoming more submerged. But in the second image when the water line reaches and moves up to and above the main deck level a relatively smaller volume of the hull is submerged on the lower side for every centimeter movement as the water moves up the deck. At these smaIl angles, the cénter of buoyancy ténds to follow án arc subténded by the métacentric rádius BM which is thé distance between thé Metacenter and thé center of buóyancy. As the vesseIs draft changes só do the métacenter móving up with the cénter of buoyancy whén the draft incréases and vice vérsa when the dráft decreases. For small angIe stabiIity, it is assuméd that the Métacenter does not mové. But thing changes drastically when an external force comes in and change the position of Centre of buoyancy COB, that is when a ship is having some heel. The force buóyancy, therefore ácts up thróugh B 1, while the weight still acts down through G, creating a moment, which tends to return to the ship upright. This moment is known the as righting moment and GZ is known as righting Lever. Since this moment tends to right the ship, the ship is said to be stable. But after a certain degree of heel this GZ GM will start reducing and if further heeling is there, it will come to zero(Centre of gravity and metacenter coincide), this situation is called as Neutral Equilibrium.
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