Ship stability

Question 1

What is the formula for TPC?

Answer 1

TPC = WPA/100 x water density

Question 2

What is the definition of transverse statical stability?

Answer 2

The ability of a ship to return to the upright when has been forcibly heeled by an external force and is momentarily at rest when floating in still water

Question 3

What is the righting lever?

Answer 3

The horizontal distance between the centre of gravity and the vertical line of buoyancy acting through the centre of buoyancy when the ship is heeled

Question 4

How do you calculate the righting moment?

Answer 4

Righting moment = GZ x displacement

Question 5

How can you calculate the ships GM using hydrostatic tables?

Answer 5

Taking the KM values for the draft in the hydrostatic table and calculating the current KG, GM is found by KM-KG.

Question 6

How do you describe a stable ship?

Answer 6

When heeled to a small angle, it returns to the upright

Question 7

How do you describe a neutral ship?

Answer 7

When heeled at a small angle, it comes to rest at an indeterminate angle

Question 8

How do you describe an unstable ship?

Answer 8

When heeled to small angle and the external force is removed, it continues to heel until the centre of gravity and centre of buoyancy are in vertical alignment where it rests at an angle of loll.

Question 9

Describe the free surface effect

Answer 9

As liquid is free to move and the ship heels, it is free to move to the low side of the ship. The centre of gravity is therefore moved closer to the vertical line of buoyancy, reducing the GZ.
This reduction in GZ gives a virtual vertical rise in G to an equivalent GZ when the ship was upright, giving a virtual reduction in GM.

Question 10

What information can be taken from the GZ curve?

Answer 10

1. The GZ value for any angle of heel
2. Maximum GZ and at what angle of heel
3. Range of positive stability
4. Angle of vanishing stability
   Approximate deck edge immersion (point of inflexion)

Question 11

What is the ideal GM value?

Answer 11

Between 4 and 8 % of the ships breadth

Question 12

Who does the intact stability code apply to?

Answer 12

All ships 24m in length and over

Question 13

What is the definition of dynamical stability?

Answer 13

The work required by the external forces to heel the ship to that angle of heel

Question 14

What is the KN value in hydrostatic tables?

Answer 14

KN is the righting lever if the centre of gravity is assumed to be on the keel. This allows the production of hydrostatic data without knowing the ever changing KG value.
KG can then be used to determine the actual GZ for the corresponding angle of heel using the formula

GZ = KN - (KG x Sin heel) angle

Question 15

What are the actions to correct an angle of loll?

Answer 15

1. Put the prevailing weather on the bow to maintain the angle of loll. Do not let the ship roll the other side.
2. Verify soundings of tanks and check for any cargo shift which could have listed the vessel
3. Recalculate the ships KG to verify the GM, using the KM values in the vessel hydrostatic tables
4. Check for any slack tanks giving free surface moments
5. Either remove ballast from high tanks on the high side or add ballast to low tanks on the low side to give maximum downward movement of G.
   Heel angle will increase but GM will also increase.
   Take action in small tanks where possible to limit FSE when tanks are slack
   Remove free surface from tanks.

Question 16

What is bilging?

Answer 16

A breach of the hull which allows water ingress into the hull

Question 17

What are the two methods of calculating stability when compartments are flooded?

Answer 17

1. Constant displacement or lost buoyancy method (Most commonly used as worst case scenario)
2. Added weight method

Question 18

Explain the constant displacement method of bilging calculation?

Answer 18

It is assumed the displacement remains the same but the vessel sinks lower in the water to regain the buoyancy lost by the flooded compartment and therefore draft increases

Question 19

Explain the Added weight method of bilging calculation?

Answer 19

The compartment flooded with water is assumed to be an added weight to the vessel and so G moves towards the added weight, changing KG and displacement increases

Question 20

What is permeability?

Answer 20

The amount of space that a compartment is capable of being filled with floodwater

Question 21

What is the purpose of an inclining test?

Answer 21

To determine the ships light KG and displacement

Question 22

According to the Intact Stability code, which ships must have an inclining test?

Answer 22

All passengers ships and every cargo ship of 24m and over must be inclined on completion

Question 23

What is the process of carrying out an inclining test?

Answer 23

By hanging a pendulum of known length on the ship and inclining the vessel using known weights, the deflection of the pendulum can be measured. The angle this makes with the vertical is the same as the angle that the force acting through the new centre of gravity makes with the vertical in upright condition.

Therefore, the two triangles can be used to find the vessel GM. Using the drafts taken around the ship and the hydrostatic tables, displacement and KM can be found and KM-GM = KG

Question 24

What should be prepared for an inclining test?

Answer 24

1. Adequate UKC for the vessel to incline
2. Moorings slack, gangways removed and any shore connections removed or slack
3. All non essential personnel ashore and any additional weights removed
4. Equipment such as cranes or lifeboats in their normal stowed position
5. Tanks either full or empty to remove free surface moments
6. Any water, snow or ice removed from the decks

Question 25

When must an inclining experiment be conducted?

Answer 25

1. For passenger ships and cargo ships over 24m, on completion of build
2. When the ship structure has been modified which will affect its stability
3. For passenger ships, after the 5 yearly lightship survey when the displacement is found to have changed by more than 2% or the LCG has changed by more than 1%

Question 26

What is the definition of damage stability?

Answer 26

The act of maintaining the boundaries and effective watertight subdivision to prevent progressive flooding