Stability Flashcards
Intro to Forces, Moments and Density
What is relative density?
What is the relative density of Salt Water?
The density of a substance compared to fresh water (1000kg/m3)
1.025
Buoyancy , Gravity and the Metacentre
What is Metacentric Height?
What is the formula?
GM is the simplest measure of the stability of a vessel. It is the vertical distance between the Centre of Gravity and the Metacentre.
GM = KM (Keel to Metacenter) - KG (Keel to Center of Gravity)
Buoyancy , Gravity and the Metacentre
Define Stiff and Tender vessels…
The metacentric height has an effect on the roll period of a vessel. (A larger metacentric height = a greater righting force when the vessel is heeled over by a gust of wind or wave)
Stiff condition - With a greater righting force, the vessel will tend to return upright more quickly (short roll period) once the external heeling force is removed.
Negative characteristics, more violent, uncomfortable.
Tender condition - A lesser GM with its smaller righting moment will come back upright slower (slower roll period). Negative characteristic of loss of stability in heavy conditions.
Buoyancy , Gravity and the Metacentre
Define Heel and List
Heel - When a ship is inclined by an external force
List - When a ship is inclined by an internal force
Free Surface Effect
How can you reduce free surface effect of your vessel?
Press tanks up full or empty them.
Final KG and Loading Tables
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 the Final KG and Km of the vessel so I could calculate my GM.
TPC
What does TPC mean?
Tonnes Per Centimeter.
The weight required to change a vessel’s mean draft (sinkage or rise) by one centimeter.
Angle of List
Why does an Angle of List occur?
Due to the transverse shift in the center of gravity of a vessel. This can happen either during loading weights, transferring them onboard, or because of an unplanned cargo shift.
Angle of Loll
What is Angle of 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 stb depending on environmental factors.
Angle of Loll
How can you correct an angle of loll?
G must be bought below M by moving or adding weight further down into the vessel toward the keel (shifting cargo, ballasting in double bottom tanks, remove weight from the vessel located about G). Care should be taken to account for FSE.
When adding or transferring weight to correct AoL, it must be place on the low side - it will heel futher, but turn the AoLoll into AoList to regain a positive GM (adding to high side could result in capsize)
Deadweight and Deadweight Scale
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.
Bending Moments and Sheer Forces
What are the names of some of the stresses that we measure that act on a vessel?
Bending Moments (hogging and sagging)
and
Sheer Forces (localized stresses ex: full next to empty cargo hold)
Draught and Bunker Surveys
How can you determine the exact amount of cargo or bunkers loaded onto the vessel?
By doing a Draft/Bunker survey
Intro to GZ Curve
What is GZ a measure of?
The Righting Lever
Angle of List GZ Curve
Where does an Angle of Lost GZ curve start?
Below Zero on the X Axis
Angle of Loll GZ Curve
Describe an Angle of Loll GZ curve.
One that starts at 0 before heading downward below 0 then back up to cross 0 at the Angle of Loll: reaching its maximum GZ value earlier than normal and reaching its angle of vanishing stability earlier, as a result.
International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements
What is the minimum GM a vessel can legally have?
0.15m
International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements
What is the requirement for area under a GZ curve from 0º to 40º?
and a GZ curve from 0º to 30º?
and a GZ curve between 30º and 40º?
The area under the GZ curve (righting lever) from 0º to 40º (angle of heel) shall be at least 0.09 meter radians.
The area under the GZ curve from 0º to 30º shall be at least 0.055 meter-radians
at least 0.03 meter-radians
International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements
Where can you find the IS Code?
IMO Resolution MSC.267(85)
International Code on Intact Stability (IS Code) and Area under the GZ Curve Requirements
Fill in the blanks…
The righting lever shall be at least ___ at an angle of heel equal to or greater than than 30º
The max righting lever shall occur at an angle of heel not less than ___
0.2m
25º
Load Line Regulations
What certificate proves you comply with the Load Line Convention?
International Load Line Certificate
Conditions of Assignment and Freeboard
What document would help you prepare for a Load Line survey?
The record of conditions of assignment of freeboard
Stability Information Booklet
Where can you find the KM value for any given draught of the vessel?
In the hydrostatic tables in the stability information booklet
Inclining Experiment and Lightship Surveys
When is the inclining experiment done?
For a new build vessel or one that has deviation in her expected lightweight condition values compared to those observed during a lightship survey.
Dry Docking Stability
When is the critical instant?
The moment just before the whole vessel takes the blocks
Bilging
What is parallel sinkage?
The initial increase in the draft that uniformly applies along the length of the vessel as a result of the loss in buoyancy in the bilged compartment
Damage Stability Requirements
What is the difference between Probabilistic and Deterministic approaches to damaged stability requirements?
The deterministic approach defines a specific amount of damage a ship must survive
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
Ship Emergency Response Services (SERS)
What help and advice is available to vessel’s to help deal with a damaged stability condition?
If subscribed, a number of Class Societies provide a 24/7 Emergency Response Service which are available to help in those situations.
Stability Computers and Damage Stability
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 surveyor.
Heavy List Operations Stability
What effect can lifting a heavy object off the quayside with a ship’s crane have on your stability?
The weight of the object will act through the crane head, greatly reducing the stability of the vessel as the CoG of the vessel will move toward the crane head. I would expect this to induce an Angle of List on the vessel as well, due to the transverse shift in the CoG of the vessel.
