Stability Presentasi

8/2/2019 Stability Presentasi

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SHIP STABILITY

Amiruddin MayaMan Ops Meratus Lines

Surabaya, 05/05/2004


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What

Stability is the tendency of a vessel toreturn to its original position after ithas been inclined due to externalforces.


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Why

To analyze and review commercial andsafety decisions :> The allocation of cargos on specific vessel> Vessel Allocation Plan> Safety and seaworthy of the vessel


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When

`Cargo booking exceeded of ship capacity> Nos of containers. > Weight of cargo.> Cargo type

`Ship Roll over / Change route


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How

Assumptions standardsSimple spreadsheet calculation


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Variables

Light ShipConstant

Ballast WaterFresh Water

FuelCargo

Free Surface


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Main Factors

The condition of the vessel as regardsstability is determined almost wholly bythe location of three points in a vessel :

> Metacenter M > Center of Gravity G > Center of Buoyancy B


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Metacenter M

The Metacentric height of the vessel depends uponthe vertical distance of two points above the keel.Every change of draft, the position of M changes and

effects stability. Theoretically, M start to move ofthe centerline as soon as a vessel inclines butpractically the movement is negligible for inclinations

up to 10 degrees or thereabouts depending uponthe form of the vessel.Therefore, M can safely be used as a point which

each is Vertically above G for small inclination


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Center of Gravity G

Point at which allthe vertically

downward forces ofweight of the vesselcan be considered

to act. Or it is thecenter of the mass ofthe vessel

G


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50

G

100 100

?

10

30

40


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Center of Gravity G

When a vessel is inclined due to someexternal force, that is by the action of

seas. G will remain fixed in its locationin the vessel. Of course if weight arefree to move on the vessel, G will move

too but for the time being, it is assumedthat G does remain in its originalposition


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Center of Buoyancy B Point at which all thevertically upward

forces of support(buoyancy) can beconsidered to act.

Or it is the centerof volume of theimmersed portion of

the vessel.

B


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Center of Buoyancy B

When a vessel is inclined,the B will move since is

it the center of volume ofthe immersed portion ofthe vessel, and the wedgeof buoyancy has beentransferred from one sideof the vessel to otherside

B B


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The Couple

A couple is formed whenever two equalforces are acting on a body in oppositeDirections and along parallel lines.

G

B


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The Couple

G Z

B B

Why doesthe move center of BuoyancyWay from its position directly under center of Gravity cause a Righting Tendency ??


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Righting Moment

The tendency of vessel to return to an erect position(called stability) can be determined for all angles ofinclination, by the value of the righting moment,

w x GZ, or solely by the length of the righting arm,For small angles of inclination, or initial stability, itcan be determined by the distance that G is from M,or metacentric height. In order to find GM it isnecessary to find KG and KM. Where the value ofKM is not available it may be necessary to calculateit by finding KB and BM


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Righting Moment

M

ZG

B B

Q


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Stable Equilibrium

G

BB

Z

M


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Neutral Equilibrium

G M

BB


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Unstable Equilibrim

G

M

B

B


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Free Surface

G

G

M

g g


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Free Surface

Whenever the surface of liquid within avessel is free to move, the condition knownas free surface is present. The effects ofFree Surface on transverse stability sincean excessive amount of free surface caneasily change a vessel with a positive GMinto one with a negative GM.


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> The effects of free surface depend upon thedimensions of the liquid and the volume of displacement of the vessel

> The effect of free surface depend to minor degreeupon the relationship between the specific gravityof the liquid in the tank and the specific gravity of the liquid in which the vessel is floating

> The effect of weight and vertical position of theliquid have an effect on transverse stability whichis not associated with free surface effect

Free Surface


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AbbreviationG : Gravity

B : BuoyancyM : MetacenterVCG : Vertical Center of GravityLCG : Longitudinal Center of Gravity

TPC : Ton per CentimetreMTC : Moment to Change TrimKG : Height of center of GravityKB : Height of center of Buoyancy

KM : Metacentric HeightLCB : Longitudinal Center of BuoyancyLCF : Longitudinal Center of FloatationFSM : Free surface moment


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Abbreviation

Displacement: Total Dead weight plus Light shipLOA : Length Over AllLBP : Length Between PerpendicularDWT : Dead Weight Tons

Draft : Keel to WaterlineGRT : Gross Register TonnageNRT : Netto Register Tonnage

Stability Presentasi

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