First lets go with basic meanings of both: Note that metacenter exists for floating Center of buoyancy: consider a ship, half of the protion is. The Buoyancy Force act through the Centre of Gravity of the Displaced Fluid and is called The Centre of. Metacentre M (initial transverse metacentre). When a ship floating at rest in still water is inclined by an external force to a small angle fi, the centre of buoyancy.

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The force of buoyancy F B is equal to the weight of the body W Centre of gravity G is above the centre of buoyancy in the same vertical line. Metacentre is determined by the ratio between the inertia resistance of the boat and the volume of the boat.

T herefore, for a floating body, the stability is determined not simply by the relative position of B and G, rather by the relati ve position of M and G. Learn More in these related Britannica articles: The inertia resistance is a quantified description of how the waterline width of the boat resists overturning.

It can be calculated by theoretical formulas based on the shape of the structure. The distance of metacentre above G along the line BG is known as metacentric height GM which can be written as. When setting a common reference for the centres, the molded within the plate or planking line of the keel K is generally chosen; thus, the reference heights are:. A ship with low GM is less safe if damaged and partially flooded because the lower metacentric height leaves less safety margin.

Help us improve this article! For the body shown in Fig. Similarly, the downflooding angle is the angle at which water will be able to flood deeper into the vessel. The centre of gravity is the point in a body about which all parts of the body balance each other.

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Differentiate Centre Of Gravity, Centre Of Buoyancy And Metacentre?

The period of roll can be estimated from the following equation [2]. This point is referred to as B in naval architecture. The centre of buoyancy of a floating body is the point about which all the body parts exactly buoy each other—in other words, the effective centre of the displaced water. When the ship is vertical, the metacentre lies bkoyancy the centre of gravity and so moves in the opposite direction of heel as the metaventre rolls.


When the body undergoes an angular displacement about a horizontal axis, the shape of the immersed volume changes and so the centre of buoyancy moves relative to the body.

Metacentre | fluid mechanics |

In contrast, a “tender” ship lags behind the motion of the waves and tends to roll at lesser amplitudes. Hence the stability of the body is reduced. Coast Guard Technical computer program support accessed 20 December A larger metacentric height implies greater initial stability against overturning. Stable floating objects have a natural rolling frequencyjust metacentrd a weight on a spring, where the frequency is increased as the spring gets stiffer.

It might also move up or down with respect to the water line. Society of Naval Architects and Marine Engineers.

Meyacentre the vessel is inclined, the fluid in the flooded volume will move to the lower side, shifting its centre of gravity toward the list, further extending the heeling force.

In metacenfre boat, the equivalent of the spring stiffness is the distance called “GM” or “metacentric height”, being the distance between two points: At the bottom of the article, feel free to list any sources that support your changes, so that we can fully understand their context.

As the hull rights, work is done either by its centre of buoynacy falling, or by water falling to accommodate a rising centre of buoyancy, or both. The point, about which the body starts oscillating, is called Metacentre. This page was last edited on 22 Novemberat Metacentrealso spelled metacenterin fluid mechanicsthe theoretical point buoyanc which an imaginary vertical line passing through the centre of buoyancy and centre of gravity intersects the imaginary vertical line through a new centre of buoyancy created when the body is displaced, or tipped, in the water, however little.

Stability of Floating Bodies in Fluid. For example, when a perfectly cylindrical hull rolls, the centre of buoyancy stays on the axis of the cylinder at the same depth.


Wide and shallow or narrow and deep hulls have high transverse metacenters relative to the keeland the opposite have low metacenters; the extreme opposite is shaped like a log or round bottomed boat. In tanks or spaces that are partially filled with a fluid or semi-fluid fish, ice, or grain for example as the tank is inclined the surface of the liquid, or semi-fluid, stays level.

KM is the distance from the keel to the metacentre. The concept is sometimes useful in designing static structures e.

This increases the risk of damage to the ship and to cargo and may cause excessive roll in special circumstances where eigenperiod of wave coincide with eigenperiod of ship roll. Views Read Edit View history. Let the new line of action of the buoyant force which is always vertical through B’ intersects the axis BG the old vertical line containing the centre of gravity G and the old centre of buoyancy B at M.

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The centre of buoyancy is at the centre of mass of the volume of water that the hull displaces. The metacentre is considered to be fixed for small angles of heel; however, at larger angles of heel, the metacentre can no longer be considered fixed, and its actual location must be found to calculate the ship’s stability.

Desirable and Undesirable Characteristics of Offshore Yachts. Very tender boats with very slow roll periods are at risk of overturning, but are comfortable for passengers.

The metacentric height is an approximation for the vessel stability at a small angle degrees of heel. Because the vessel displacement is constant, common practice is to simply graph the righting arm vs the angle of heel. The righting couple on the ship is proportional to the horizontal distance between two equal forces. The stability increases with the distance between metacentre and centre of gravity, called the metacentric height.

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