STABILITY & DRY-DOCKING

Question 1

What is stability?

Answer 1

The ability of the vessel to return to the upright when heeled by an external force.

Question 2

How would you ensure the vessel has sufficient stability?

Answer 2

• Check the loadline marks are not submerged
• Ensure watertight integrity of the ship. All shell doors and hatches are closed.
• Ensure freeing ports and scuppers are unobstructed
• Ensure tank levels are distributed properly.
• Reduce any free surface effect. Empty swimming pool, jacuzzi, ensure tanks are pressed or empty.
• Ensure all loose equipment is secure.
• Weight is distributed equally. The vessel is not listed.
• Consult stability booklet

Question 3

What is the archimedes law of flotation?

Answer 3

The upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces
Therefore, when a vessel is floating freely, the mass of the vessel is equal to the mass of liquid displaced by the vessel.

Question 4

What are the different tonnages?

Answer 4

TBD

Question 5

What is the centre of buoyancy and the centre of gravity?

Answer 5

Centre of buoyancy. The point through which the buoyancy force is said to act vertically upwards. Positioned at the centroid of the underwater volume of the vessel
The centre of gravity of a body may be defined as that point at which the total weight (i.e. gravitational force) is considered to act on the body and its position will depend upon the distribution of weights.

Question 6

What is the righting lever?

Answer 6

Righting lever. This perpendicular distance between the Gravity force and the Buoyancy force is called the ‘Righting Lever’. Ability of the vessel to withstand external heeling forces and maintain the vessel in the upright.

Question 7

Define the metacentre

Answer 7

Metacentre. A fictitious point on the centreline of the vessel. If the vessel is heeled to a small angle and a vertical line is projected up from the centre of buoyancy, where this projected line cuts the centre line of the vessel is the called the Metacentre. Calculated by the architect and dependant on the shape of the hull.
Metacentric height. This is the height of the Metacentre above the ship’s centre of gravity (G), measured along the ship’s centre line.

Question 8

What are the Different states of equilibrium.

Answer 8

• Stable. KM>KG. GM positive. If an external force is applied a righting lever will develop and return the vessel to the upright.
• Neutral. KM=KG. GM neutral. The vessel will heel over with very small external force. No righting lever will develop.
• Unstable. KM

Question 9

What is the difference between stiff and tender vessels in stability?

Answer 9

A stiff vessel has a large GM and, thus, a large righting moment at all angles of heel. This results in a fast, snappy return to the upright position for any angle of heel. This motion can be uncomfortable for the crew and passengers, could cause unsecured items to shift, can also cause stresses in the structure of the vessel. However, it is more stable.
A tender vessel. In this case the GM is small and the righting moments are correspondingly small, so the boat responds to an angle of heel by returning to the upright position slowly. The danger here is that the boat may ‘hang’ at an angle of heel and be further pushed over by a gust of wind or a beam sea, which may lead to a possible capsize. However, there is little strain on the fabric of the hull and the motion is likely to be more comfortable for passengers and crew.

Question 10

What is the difference between loll and list?

Answer 10

Loll. The angle at which the vessels comes to rest at when heeled by an external force while the vessel is in unstable equilibrium. Capsizing levers develop which heels the vessel over until gravity and buoyancy come into the same vertical plane. The angle of loll can be identified as the vessel will be tender and will flop from one angle of loll to the other, without staying in the upright. To correct an angle of loll, we would add weight to the lower part of the lower side of the vessel, in order to lower the centre of gravity. This will restore positive stability. The vessel will be listed instead of in an angle of loll. The list can then be corrected by adding weight on the other side of the lower part of the vessel.
List. The angle of list is an angle at which the vessel will rest caused by asymmetric disposition of internal weights about the centreline. It can be identified because the vessel will remain listed to one side. A list may be corrected by moving masses from the low side to the high side, adding weights to the high side, or removing weights from the low side.

Question 11

What are the effects on the centre of gravity of adding, removing and moving weights? Give examples.

Answer 11

Centre of gravity moves towards added weight, away from discharged weight and in the direction of the moves weight.

Question 12

Describe free surface effect? What could cause it? How to reduce it?

Answer 12

The virtual rise in the centre of gravity and therefore a reduction in metacentric height caused by the movement of a liquid with a free surface. This causes a reduction in the righting lever.
Examples
• Fuel and water consumption
• Water on deck from Ice accretion which then melts, rain or spray
• Pools and Jacuzzis
• Helicopter operations
• Launch or recovery of a tender

How to reduce

• Tank divisions and Tank baffles
• Freeing ports and scuppers
• Pressed or empty tanks
• Empty pools and jacuzzis

Question 13

What is a GZ curve? What information can be derived from it?

Answer 13

Represents the Righting lever at different angles of heel.
Contains the following items:
• Maximum righting lever.
• Angle of vanishing stability
• Angle of Deck edge immersion. Maximum waterplane area.
• Initial GM
• Range of positive stability
• Area under the curve. Ability to withstand external heeling forces.
GZ curves of list and loll.

Question 14

Draw a GZ curve

Answer 14

Draw

Question 15

Draw a curve with a vessel with an angle of list and loll

Answer 15

Draw

Question 16

What is the GM on your current vessel?

Answer 16

TBD

Question 17

What is the minimum criteria for stability?

Answer 17

Area under the GZ curve
- Up to 30 degrees must be 0.055 m/rad area under the curve.
- Up to 40 degrees must be 0.09 m/rad area under the curve.
- Between 30 and 40 degrees must be 0.03 m/rad. Area under the curve.
Maximum GZ must occur preferably over 30 degrees. Never less than 25 degrees.
Minimum GZ at 30 degrees must be 0.2 metres.
Minimum GM 0.15m. after correction for fse.

Question 18

What is the angle of downflooding?

Answer 18

Angle at which water will be able to ingress the intact hull from above.

Question 19

What stability information should be provided to the master? What is contained in the stability booklet?

Answer 19

• The ship’s particulars and dimensions, tonnages, draughts
• A profile view and plan views of the ship drawn to scale showing all compartments
• A diagram or scale showing the load line mark and load lines
• The capacity and the longitudinal and vertical centre of gravity of every compartment
• The estimated total weight of crew and passengers and their effects
• The effect on stability of free surface in each tank in the ship
• A diagram showing the curve of righting levers (GZ) in different conditions.
• A diagram or tabular statement showing the hydrostatic particulars of the ship
• A statement of instructions on procedures to maintain adequate stability.
• The report on the inclining test

Question 20

What is an inclining test?

Answer 20

An inclining test is a test performed on a ship to determine its stability, lightship weight and the coordinates of its center of gravity. The test is applied to newly constructed ships greater than 24m in length, and to ships altered in ways that could affect stability.

Question 21

What is the effect of freeboard and the beam on stability?

Answer 21

If the freeboard of a vessel is increased it can be seen that the angle of deck edge immersion will also increase and hence the curve remains concave for longer. At smaller angles of heel little change is likely, but at larger angles the centre of buoyancy moves further to the low side, resulting in an increased maximum GZ at a larger angle with a greater angle of vanishing stability, increased range of positive stability and increased area under the curve

If the beam of a vessel is increased then the centre of buoyancy will move further outboard for a given angle of heel, resulting in an increased horizontal distance between the buoyancy and gravity forces, hence increased GZ. GM is also increased and so the righting levers will be larger for lesser angles. The angle of deck edge immersion will occur at a smaller angle. The stability at larger angles will be less affected.

Question 22

What would you find in the damage stability booklet?

Answer 22

TBD

Question 23

What is the damage stability criteria?

Answer 23

Minor hull damage that results in free flooding of any one compartment, shall cause the vessel to float at a waterline which at any point is not less that 75mm below the freeboard deck, the residual stability shall be such that:
• any angle of equilibrium does not exceed 7º from the upright,
• the resulting righting lever curve has a range to the downflooding angle of at least 15º beyond any angle of equilibrium,
• the maximum righting lever within that range is not less than 100 millimetres
• the area under the curve is not less than 0.015 metre radians.

Question 24

What is the minimum amount of fuel you can have onboard?

Answer 24

Sufficient fuel the intended voyage with a reserve as specified in the SMS, Master´s standing orders, or 10% depending on the circumstances.

Question 25

What to do in the event of non-routine dry-docking?

Answer 25

Seek specialist advice from the classification society

Question 26

Describe what precautions you would take prior to drydocking

Answer 26

Coordination with the dry-dock
• Engage with docking master.
• Drydock plans submitted in advance to ensure Drydock is arranged for the vessel. (Docking plan, shell expansion plan, detailed keel plan, underwater fittings plan)
• Ensure underwater hull fittings and equipment are taken into account, including propellors, rudders, sounders, intakes, anodes, tank plugs, thrusters, azipods.
Onboard preparations
• Sufficient stability. Sufficient GM.
• Ensure minimum deadweight. Drydock in the lightest condition.
• Trim vessel by the stern
• Minimise free surface effect
• Ensure all loose items are secured
• Ensure anchors are secured
• Note all tank soundings, weights and draught marks. (For subsequent re-floating)
• Inform relevant parties (Flag, class, P&I)

Question 27

Describe the actual process of drydocking

Answer 27

• The vessel is trimmed by the Stern in order that the stern comes in contact with the blocks first
• The vessel is lowers as the dry-dock is emptied. The contact in between the stern and the blocks causes an upthrust from the block known as the p-force, which causes a virtual discharge of weight and therefore a virtual rise in the centre of gravity. This is know as the commencement of the critical period.
• As the water level falls, the metacentric height reduces and therefore the GM.
• Once the bow has touched the blocks, the p-force is distributed along the length of the vessel. Once the entire keel is resting on the blocks, G and M can be considered to be in the same place. The stability of the vessel is provided by the blocks and not the dynamic stability

Question 28

What action would you take once secure?

Answer 28

• Ensure underwater fittings are accounted for and isolated for divers
• Set up Fire mains, sewage, Water and shorepower.
• Ensure appropriate safeguards are in place to prevent persons from falling into the dry-dock
• Provide safe access to crew
• Ensure there are at least 2 means of escaping the vessel
• Ensure shoreside security measures are in place
• Garbage disposal arrangements
• Carry out a crew briefing highlighting the above and any other inherent dangers of being in a dry-dock.
• Remove anchors and chain for maintenance

Question 29

Chief mate is walking out the anchors in the dry-dock. What would you ensure?

Answer 29

• Risk assessment
• Make sure the chief officer is accompanied
• Make sure there is someone from the yard present

Question 30

What precautions would you take when Leaving the dry-dock?

Answer 30

• Ensure all the work is completed
• Ensure stability condition is the same as when the vessel was dry-docked.
• Watertight integrity, Seaworthiness and compliance.
• Dry-dock is clear of tools and materials
• Inform relevant authorities such as class, flag, DPA, Insurance.

Question 31

What do you do if the vessel gains an angle of loll when flooding process?

Answer 31

Continue to flood.