Stability Flashcards
What is the definition of the forward and after perpendicular?
An imaginary vertical lined drawn to the waterline at the point where the summer load line cuts.
What is the definition of displacement?
The weight of the vessel in tonnes.
What is the definition of load displacement?
The weight of the vessel when loaded down to the summer load line.
What is the definition of light displacement/ lightship?
The weight of the vessel when empty.
What is the definition of deadweight?
The difference between the lightship and present displacement.
What is the definition of gross tonnage?
The total internal volume in tonnes.
What is the definition of net tonnage?
The total weight of the cargo carrying area, does not include accommodation, engine room, etc.
What is the definition of TPC?
(Tonnes per centimetre)
- The amount of tonnage needed to change the vessels draft.
What is the definition of FWA/DWA?
The amount the vessel can submerge her marks to in fresh and dock water.
What is the definition of Free surface effect (FSE)?
Created by the movement of cargo or liquids in a tank?
What is the definition of GZ?
Is a righting leave which allows us to see how the ships act on a GZ curve?
What is the definition of GM?
The vertical distance between the centre of gravity and metacentric height.
What is the definition of KG?
The vertical distance from the keel to the centre of gravity.
What is the definition of TCG?
(Transverse centre of gravity)
The distance from the centre of gravity to the edge.
What is the definition of KM?
The vertical distance from the keel to the metacentric height/ GM.
What is the definition of MCTC?
Moments to change vessels trim by 1cm.
What is the definition of LCG?
The distance from aft perpendicular to the centre of gravity.
What is the definition of LCB?
The distance from aft perpendicular to the centre of buoyancy.
What is the definition of KB?
The vertical distance from the keel to the centre of buoyancy.
What is the definition of list?
the centre of gravity is moved away from the centre line due to uneven loading.
What is the definition of heel?
An external force is pushing the vessel over.
What is meant when GM is negative?
Vessel is in an unstable condition.
What is meant when GM is positive?
Vessel is in a stable condition.
What is meant when GM is the same?
Vessel is in a neutral condition.
What is meant when both G and B are off the centreline?
Vessel is listing.
What is a hydrometer?
An instrument used to work out the density of the water.
Collect the samples from about halfway down the vessel’s draft, wipe the hydrometer and place them in the water twisting as you do so.
Take care when reading, make sure to read the true reading.
What are load lines?
- Forward and Aft draft marks must be placed on the perpendiculars.
- The load line disc/ plimsoll mark shall be placed at a point midway between the perpendiculars.
What are the specifics and dimension of load lines?
- Forward and Aft draft marks must be placed on the perpendiculars.
- The load line disc/ plimsoll mark shall be placed at a point midway between the perpendiculars.
Dimensions: - All lines are 25mm thick.
- The centre of the disk lines up with the summer load line.
- All lines (TF, T, S, W, etc.) are 230mm long.
- Tropical load line is 1/48 above the summer draught line.
- Winter load line is 1/48 below the summer load line.
- The Winter North Atlantic line is 50mm below the winter load line.
- If the vessel is being loaded in dock water or fresh water, DWA and FWA should be considered in calculations, due to different water densities.
Load lines calculated from the top of marks.
Drafts are read from the bottom of the mark.
What is the relative density of salt water?
1.025
HWat is a moment of force?
It is the turning effect produced by a force acting at a distance from a pivot point.
What is the standard unit for measuing force?
Newtons.
When using a hydrometer, why do we not take the measurement where the water touches the stem?
Due to surface tension the water can be higher and give incorrect readings.
Define the metacentre?
The point on the vessels cnetreline where a vertical line crosses through the centre of buoyancy when heeled over at small angles.
What is ‘GM’
It is defined as the vertical distance between the centre of Gravity and the Metacentre.
What is considered a stiff vessel?
A vessle with a large metacentric height and therefore short roll period.
What is a considered tender vessel?
A vessel with a small metacentric height and therfore a long roll period.
Will a vessle have a deeper draft in fresh water than in salt water?
Yes, Fresh water is less desnse than salt water and therefore provides less buoyancy force.
GZ?
The horizontal distance between the centre of gravity (G) and the point Z. THis distance acts like a lever arm.
HWo can you reduce Free Surface Effect on your vessel?
- Pressing tanks up full or emptying them.
- Emptying swimming pools or jacuzzis if fitted.
- Ensuring scuppers of deck are clear of blockage.
How does Free Surface Effect affect the cnetre of gravity (G)?
It leads to a ‘virtural’ rise in the centre of gravity called ‘Gv’.
What is Free Surface Correction (FCS)?
The correction made to satbility caculations to account for the total free surface moment.
What do we call the GM when the Free Surface Correction (FSC) has been applied?
The fluid GM.
IF there is too much Free Surface Effect, what can happen?
The virtual reduction in GM can result in an negative metacentric height.
How would you go about calculating your vessel’s stability by hand?
I would use the information from the Stability Information booklet and then use a loading table to find Final KG and Km of the vessel so I could calculate my GM.
HWat is the final KG?
Is the calculated KG based on the lightship displacement of a vessel, plus all loaded/discharged weights.
What is the formula for calculating GM?
GM = KM - Final KG
Where can you find the KM for a vessel?
In the hydrostatic particulars form the stabilty booklet.
What is the formula for calculating final KG?
Final KG = Final Moment/Final Displacement.
What does TPC mean?
(Tonnes per centimeter)
- The weight required to chnage a vessel’s mean draft (sinkage or rise) by one centimeter.
Why is TPC useful to know?
It can inform you how much cargo, fuel, ballast, etc, can be taken onboard before reaching a desired draught or load line.
Will TPC be higher or lower in salt water compared to fresh water?
HIgher. Salt water has a higher density than fresh water, and therefore provides a greater puoyancy force.
Why does an angle of list occur?
Due to a transverse shift in the caentre of gravity of vessel. This can happen either during loading weights or transferring them onboard or because of a unplanned cargo shift.
What happens to the centre of gravity (G) when a mass is loaded off the centreline?
It moves towards the load.
Why does a vessel stop listing when a mass is loaded off the centreline?
When the centre of buoyancy and the centre of gravity are aligned, the vessel returns to equilibrium.
Would you leave a port with an angle of list?
No.
What is an angle loll?
A vessel will heel over to an angle of loll when she has a negative initial metacentric height. This can be to port or starboard depending on environmental factors.
What can cause an angle of loll?
- Loading weight to high.
- Consumption of Fuel and Water.
- Free-Surface Effect.
- Shift in Bulk Cargo.
- Liquefaction of Bulk Cargo.
- Vessels Transporting Timber Deck Cargoes.
- Heavy Lift Operations.
How do you recover from an angle of loll?
By loading further down the side already heeled over and bringing G down so you regain positive stability and turn the loll into an angle of list.
What are the signs of an angle of loll?
- Sudden heel to one side.
- Unstable Rolling.
- Inconsistent Heel.
What is Deadweight tonnage?
The amount that can be loaded onto a vessel from lightweight to bring her to her summer Loadline. This includes all weights like Provisions, Bunkers, Cargo, Persons and effects.
What are the name of some of the stresses that we measure that act on a vessel?
Bending Moments and Shear Forces.
What is ‘hogging’?
Hogging often occurs when the ship is loaded unevenly, with more weight concentrated towards the bow and stern, leaving the middle section relatively lighter.
What is ‘sagging’?
Sagging often occurs when the ship is heavily loaded in the middle, with less weight at the bow and stern.
How can you determine the exact amount of cargo or bunkers loaded onto the vessel?
By doing a Draft/Bunker Survey.
What is GZ a measure of ?
The righting lever.
What does the “Angle of Conflecture” or “Point of Inflection” show on a GZ curve?
The angle of deck edge immersion (DEI).
What information can you get from a GZ curve?
- Range of statical stability.
- Initial GM.
- Maximum GZ.
- Angle of deck edge immersion.
- Angle of vanishing stability.
Where does an Angle of List GZ curve start?
Below zero on the x axis.
Why does a vessel with a list have negative GZ at zero degrees of heel?
Because the centre of gravity and buoyancy are not in equilibrium, a capsizing lever is generated, causing the vessel to heel over to the angle of list.
Does a vessel with a list have a larger range of stability than a vessel without a list?
No. It is less as the entire GZ curve is reduced.
Describe a Angle of loll GZ curve?
One that starts at 0 before heading downwards below zero then comes back up again to cross zero at the angle of loll before reaching its maximum GZ value earlier than it would have done and its angle of vanishing stability earlier as a result.
Does a vessel with an angle of loll have a larger range of stability than a vessel without a list?
No. It is less as the entire GZ curve is reduced.
Can you still calculate the initial GM from an angle of loll GZ curve?
Yes. However the GM will be a negative value.
What is the minimum GM a Vessel can legally have?
0.15m
What is the requirement for area under a GZ curve from 0º to 40º?
The area under the curve from 0º to 40º shall be at least 0.09 metre-radians.
What is the requirement for area under a GZ curve from 0º to 30º?
The area under the curve shall be at least 0.055 metre-radians.
What is the requirement for area under a GZ curve from 30º to 40º?
The area under the curve shall be at least 0.03 metre-radians.
What are the minimum requirements for GZ?
- GZ shall be at least 0.2m at at angle of heel greater than/equal too 30º.
- Max GZ shall occur at an angle greater than 25º of heel.
What is the minimum GM for a vessel carrying timber deck cargo?
0.1m
What certificate proves you comply with the Load Line Convention?
An International Load Line Certificate.
Will a vessel have more or less displacement when operating in a winter load line zone than in summer?
Less.
The vessel will be at a shallow draft, to ensure a greater freeboard.
What is fresh water allowance?
The distance (mm) which a load line can be submerged when loading in fresh water.
What is dock water allowance?
The distance (mm) which a load line can be submerged when loading in water of density between 1.025 and 1.000.
Will the load line marks be located forward or aft of the load line ring on the starboard side of the vessel?
Forward.
Do you read the load line from the top or bottom of the line?
Top.
What is the thickness (in mm) of a loadline?
25 mm
What is the outer diameter (in mm) of the load line ring?
300mm
What is the length of the deck line mark?
300 mm
What formula is used to calculate the distance between the top of the winter and top of the summer load lines?
Distance = summer draught / 48
If loaded to the summer load line, can you enter a tropical load line zone?
Yes.
As you would have a greater freeboard than the minimum required for this zone.
What document would help you to prepare for a Loadline survey?
The record of conditions of assignment of freeboard
Are fire doors included as part of the conditions of assignment?
No.
What is meant by equipment for protection of the crew?
handrails, guardrails, bulwarks, gangways, underdeck passages, other means of access.
Do you need a copy of the conditions of assignment onboard the vessel?
Yes. It should be available for inspection.
Where can you find the KM value for any given draught of the vessel?
In the hydrostatic tables in the stability information booklet.
What is the purpose of the stability booklet?
To enable the Master to calculate the vessel’s stability under any loading condition.
When is an Inclining experiment done?
For a new build vessel or for one which has a deviation in her expected lightweight condition values to those observed during a lightship survey.
When are lightship surveys conducted on passenger vessels?
- On build completion.
- After any major modification that materially affects the stability of the vessel.
- Every five years.
When are lightship surveys conducted on cargo vessels?
- On build completion.
- After any major modification that materially affects the stability of the vessel.
After a dry dock, a 3% deviation in the lightship displacement was noted during a lightship survey. Is an inclining experiment required?
- Yes.
- The vessel shall be re-inclined whenever a deviation from the lightship displacement exceeding 2%.
Is an inclining experiment required for a 170m bulk carrier which has two sister vessels?
- No.
- Provided that the calculated lightship displacement does not deviate more that 1% from the sister vessels.
It is a force 5 on the day of an inclining experiment, should the operation take place?
No. It should be conducted on a day with little to no wind, waves, or wake that may influence inclination of the vessel or unduly interrupt the reading of draught marks.
When is the critical instant?
The moment just before the whole vessel takes the blocks.
What plans need to be sent to the Shipyard?
- General arrangement plan.
- Docking Plan
- Shell expansion plan.
- Tank capacity plan.
- Fire control plan.
Should a vessel have a large or small trim when dry docking?
Small trim.
How can GM be increased before proceeding to dry dock?
- Take on ballast low down.
- Discharge ballast from high tanks.
- Move weight lower down in the vessel.
- Reduce free surface effect.
What is the best way of reducing Force P?
Have a small trim.
What is parallel sinkage?
The initial increase in draft that uniformly applies along the length of the vessel as a result of the loss in buoyancy in a bilged compartment.
If a vessel bilges a forward compartment, will it sink equally along its length?
- No.
- The loss in buoyancy at the forward end of the vessel will cause a trimming moment by the head.
What effect does bilging have on the stability of the vessel.
- Reduces the GM.
- Reduces the angle of deck edge immersion.
- Reduces the range of positive stability.
- Reduces the maximum GZ.
- Reduces the area under the GZ curve.
What does compartment permeability mean?
The percentage of the compartment which is free to flood.
What is the difference between Probabilistic and Deterministic approaches to damaged stability requirements?
- The Deterministic approach defines a ship must survive a specific amount of damage.
- The Probabilistic approach looks at the probability of damage in each compartment and models the likelihood of losing the vessel if each compartment were lost and compares the probability of survival against the minimum allowable.
Passenger vessel damage stability - What is the minimum GM?
0.05m
Passenger vessel damage stability - What is the angle of list requirements?
No more than 7 degrees for one compartment flooding or 12 degrees for two compartment flooding.
Passenger vessel damage stability - Area under GZ curve requirement?
0.015 meter radians.
Passenger vessel damage stability - What is the range of stability requirement?
15 degrees.
Passenger vessel damage stability - What is the max GZ requirement?
At least 0.1m
Cargo vessel damage stability - What is the minimum GM?
0.05m
Cargo vessel damage stability - What is the angle of list requirements?
Less than 15 degrees (17 degrees if deck edge not immersed).
Cargo vessel damage stability - Area under GZ curve requirement?
0.0175 meter radians.
Cargo vessel damage stability - What is the max GZ requirement?
0.1m
Cargo vessel damage stability - What is the range of stability requirement?
20 degrees
What is SERS?
Lloyd’s Register’s SERS is a 24/7 global emergency support service offering technical expertise and shoreside support for crew dealing with incidents onboard.
What help and advice is available to vessel’s to help deal with a damaged stability condition?
If subscribed, a number of classification societies provide an ‘Emergency Response Service’, which is available 24/7 to assist the crew with specialist knowledge and expertise.
What is the benefit of using SERS?
Enhances the emergency response, by providing additional resources and support from ashore.
What regular checks must be performed on the stability computer?
The master must check the accuracy of the computer calculations annually by applying at least one test condition (other than lightship), typically in the presence of a class surveyor.
