STABILITY & DRY-DOCKING Flashcards
What is stability?
The ability of the vessel to return to the upright when heeled by an external force.
How would you ensure the vessel has sufficient stability?
- 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
What is the archimedes law of flotation?
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.
What are the different tonnages?
TBD
What is the centre of buoyancy and the centre of gravity?
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.
What is the righting lever?
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.
Define the metacentre
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.
What are the Different states of equilibrium.
- 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
What is the difference between stiff and tender vessels in stability?
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.
What is the difference between loll and list?
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.
What are the effects on the centre of gravity of adding, removing and moving weights? Give examples.
Centre of gravity moves towards added weight, away from discharged weight and in the direction of the moves weight.
Describe free surface effect? What could cause it? How to reduce it?
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
What is a GZ curve? What information can be derived from it?
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.
Draw a GZ curve
Draw
Draw a curve with a vessel with an angle of list and loll
Draw
What is the GM on your current vessel?
TBD
What is the minimum criteria for stability?
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.
What is the angle of downflooding?
Angle at which water will be able to ingress the intact hull from above.
What stability information should be provided to the master? What is contained in the stability booklet?
- 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
What is an inclining test?
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.
What is the effect of freeboard and the beam on stability?
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.
What would you find in the damage stability booklet?
TBD
What is the damage stability criteria?
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.
What is the minimum amount of fuel you can have onboard?
Sufficient fuel the intended voyage with a reserve as specified in the SMS, Master´s standing orders, or 10% depending on the circumstances.
