1. Hjj
Ship Stability
RONALD M H
Ronald M H

2. Defenition of Stability
The statical stability of a ship is the tendency the vessel has to return the upright when inclined from that position (initial)
wind
wave
The statical stability comprises of
1. Stable Equilibrium 2. Unstable Equilibrium 3. Neutral Equilibrium CL

3. Recapitulation
The centre of gravity of a body is the point through which the force of gravity is considered to act vertically downwards with a force equal to the weight of the body. The weight acts downwards through the center of gravity (G).
The center of buoyancy is the point through which the force of buoyancy is considered to act vertically upwards with a force equal to the weight of water displace. It is the center of gravity underwater volume. The buoyancy acts upwards through the center of buoyancy (B).
To float at rest in still water, a vessel must displace her own weight of water, and the center of gravity must be in the same vertical line as the center of buoyancy
The center of gravity when inclined is considered as Z. G and Z must be in the same horizontal line, and buoyancy when inclined is considered as B1. Z and B1 must be in vertical line L W C W L C G G L L B B L

4. The Metacenter L C B G M
The Metacenter is the intersection of middle line by the vertical through B1 at small angle, .  Z L W W 1 L B1

5. A ship Stable Equilibrium is said to be in stable equilibrium if when inclined, she tends to return to the initial position. For this occur the center of gravity must be bellow the metacenter L C B G M  Z W L W 1 L
Moment of Statical Stability make the ship in stable condition and return to initial condition
Moment of Statical =  x GZ [ton. m]
GZ is righting lever B1
Stable Equilibrium

6. Unstable Equilibrium
Moment of Statical Stability make the ship in unstable condition Moment of Statical =  x GZ [ton. m] Z G M K  B B1
The moment of statical stability is clearly a capsizing moment CL
When a ship inclined to a small angle she will tend to heel over still further, she is said to be unstable equilibrium. For this to occur the ship must have a negatif GM

7. Neutral Equilibrium G B
Buoyancy and Gravity in a vertical line, no moment will be created, GZ = 0 Moment of statical: W x GZ = 0 M G  B1
B1`
When a ship inclined to a small angle she will tend to remain at that angle of heel until another external force is applied. The ship has zero GM
Neutral Equilibrium is known when G coincides with M