5. Statical Stability Flashcards

Question

Use a diagram to show how an angle of list is affected by the length of GM.

Answer 1

A

Statical Stability (as opposed to Dynamical Stability) is the ability of a vessel to return to her initial position after being forcibly inclined.

It is the measure of the righting moment at a given angle of heel.

Answer 2

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See image.

Answer 3

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Loading a weight high in the ship, above initial CoG
Discharging a weight from low in the ship, below initial CoG
Moving a weight already on board upwards
Lifting a weight with yacht crane (suspended weight)
Ice accretion on superstructure
Water from heavy seas landing on deck
Vessel keel taking to the blocks in dry dock
Grounding
Helicopter landing on deck, above initial CoG
All guests and many crew gathering on upper decks (arrival, departure, sighting of interesting sea life)
Retrofitting (equipment, furniture, pools etc.) above initial CoG
Blocked scuppers and freeing ports on upper decks
Hatches left open leading to flooding and FSE (virtual rise of G)
Free Surface Moments due to flooding or slack tanks (virtual rise of G)
Saturation of timber deck cargo

Answer 4

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Adv:

Returns to upright quicker
More safety in case of flooding, damage or cargo shift
Less tendency for breaking seas on deck
less tendency to synchroniouw rolling.

Diadv

Fast, large, violent rolling
Racking stresses
Discomfort for crew and passengers
Cargo shift possible and cause damage

Answer 5

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Adv:

Slow and more comfortable roll
Less racking
Less chance of cargo shift

Disadv:

Less safety in case of flooding
more tendency for seas to break on deck
More dangerous if synchronius rolling occurs.

Answer 6

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The GM of a vessel is a measure of its transverse stability and the position of G in relation to M, whether below, on or above it, dictates whether the vessel has positive, neutral or negative stability.

“Equilibrium” means that a body is at rest because any forces acting upon it are equal and opposite.

A vessel has heeled over due to an external force.

What happens once that external force has been removed is dependent on the GM and resulting GZ.

In mechanics we say that if, when the vessel is disturbed from her initial position by external forces, she tends to return to that position then she is in STABLE EQUILIBRIUM.

If this disturbance produces a tendency to move further away from the initial position on removal of the disturbing forces then the vessel is said to be UNSTABLE EQUILIBRIUM

Answer 7

A

G and M are equal.

Answer 8

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How does it come about?

Only boxed shaped vessels.

It starts initially in an upright vessel with G very slightly above M, producing a small negative righting lever and creating unstable Equilibrium
If an external force is applied, the vessel will heel over and this small capsizing lever will start to incline the vessel still further to one side.
At some degree of heel, the vessel will leave the realms of initial statical stability and B will no longer be acting through the initial metacentre and M disappears.
The vessel will incline until B moves under G creating equilibrium and this angle of inclination is known as the Angle of Loll.
With any further heeling beyond this point, B will move out further away from the centre line and more importantly, further away from G creating a small righting lever that will only return the vessel to the Angle of Loll
The ship will roll about at this angle of loll.\
If the initial negative GM is too great there is a risk that B will never reach the point where it is under the G and the vessel will capsize, hence this is dangerous and must be corrected immediately.

Answer 9

A

List:

G moves off the centreline, but remains under M
Is a Safe condition and still considered stable.
B is under G acting upwards through M
Will not flop to the other side, but roll about its inclination.
Larger GM means a larger righting leaver.

Loll

G moves slightly above M, creating a small negative righting lever.
M eventually disappears
Positive satbility is must be developed almost immediately
Can flop to the otherside at the same inclination and create possible capsizing moment if the wrong corrective measure is taken.
is very Dangerous
Reactive to sudden movements and shifts in weight.
happens in Box shaped vessels only.
Reducing KG will help the situation, to restore positive GM.

Answer 10

A

Immediately lower G and create stable vessel (do not try to correct inclination, very dangerous as ship could flop to opposite side)
Add weight below G on low side of vessel(ballast double bottom tanks)
Remove weight above G from the high side.
or move weight from above G on the high side to below G on the Low side.
Once Stable Equlibrium achieved, then correct list.

To achieve a reduction in KG under a controlled manner the following can be considered:

Check the state of the tanks and reduce free surface to the narrower width tanks.
Lower weights in the vessel; remove weights from above G taking care to remove those from the high side first;
Add weights below G and add to the low side first.
Ballasting

The reaction to the foregoing will create a greater angle and might
appear to be contradictory but this will not induce any sudden motion and the situation will be kept under control.

Answer 11

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The Righting Moment is the best measure of a ship’s overall stability. It describes the ship’s true tendency to resist inclination and return to equilibrium.

Answer 12

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A

Righting lever (GZ) is defined as the horizontal distance, measured in metres, between the centre of gravity (G) and the vertical line acting upwards of the centre of buoyancy (B1) when the ship is heeled.

§ The perpendicular distance between the weight vector and the buoyancy
vector is known as the righting arm or righting lever
§ Denoted as GZ
§ This is the lever that the vessel uses to return to equilibrium
§ i.e. To return to the upright with the forces of buoyancy and gravity
equal and opposite.
§ The longer the righting arm, the greater is the tendency for the vessel
to return to an upright equilibrium.

Answer 13

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A

Neutral Stability
A Vessel in Neutral stability has G and M in the same position i.e. there is no righting lever.
Vessel will sit at any angle, within the realms of initial statical stability.
However, when this vessel reaches the threshold of initial statical stability (10 degree’s) and beyond, the centre of buoyancy will move outboard sufficiently that a positive righting lever will be created.

Angle of Loll

Answer 14

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A ship’s lightweight or light displacement is the actual weight of the ship with no passengers, cargo, bunkers, lube oil, ballast, fresh water, stores, etc., on board.

Answer 15

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The loaded displacement is the weight of the ship loaded down to its load line marks, that is, loaded to its maximum capacity with passengers, cargo, bunkers, lube oil, ballast, fresh water, stores, etc., on board.

Answer 16

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A

A ship’s deadweight is the difference in metric tons between the loaded displacement tonnage of the ship and the lightweight of the ship.
However, the difference between the loaded displacement tonnage and the lightweight does not reflect the cargo carrying capacity only, as deadweight also includes bunkers, stores, freshwater, etc.