GZ curves Flashcards
Increase beam
GM increased
Vanishing stability increased
Range of stability increased WPA increased
Max GZ increased
Deck edge emersion Decreased
Reduced freeboard
GM - no change
Deck edge immersion - decreased
Vanishing stability- decreased
Range of stability- decreased
Max GZ - decreased
Angle of loll GZ information
Initial negative GM
M will rise as the vessel heels up to the angle of DEI
Final resting point is angle of loll
Positive GM at the point of recurve
List condition Curve
2 images -
Healing arm
range of positive stability reduced
Area under the curve reduced
G-G1 + drawing shift of weight
Timber
Freeboard increased
Deck edge imm increased
Righting lever at larger Ang increase
GM reduced
righting levers at small Angle reduced
Preferable to enter dock with small trim
SMALLER STERN TRIM
SMALLER - smaller trim smaller P force at the critical point
STRN - because the frames are stronger at the stern and able to bear the P force better
TRIM - always easier to line up and land on the blocks on an even keel
Improve initial stability if GM at critical point is inadequate
Minimize FSE press up ranks or discharge
Increase the GM by lowering weights or ballasting
Reduce the initial trim by transferring or ballasting to reduce GM
Precautions before inclining:
Positive internal transverse metacentric night
Essential to have positive GM to have a list by moving weights otherwise VSL could go into angle of LOLL due to negative GM
Inclining experiment sufficient under keel clearance
Vessel needs to list freely
Inclining experiment
Conducted in slack waters
Avoid squat
Inclining experiment
Empty bilges
FSE / not accurate calculation
Improve stability at GM critical 3 ways
Minimise FSE- press up tanks
Increase GM by lowering weights or ballasting DB
Reduce initial trim by transferring ballast
Correcting LOLL
Lower COG to restore positive GM
1 top up slack tanks
2 Calc FSM before filling any slack tanks
3 fill empty tanks on low side first - high side could cause roll totally the other way
After a while the weight of water will lower COG
4 fill opposite tank on high side
Fill tanks alternately on low sides until positive GM
Higher GM value ?
The higher the GM value, the better the vessel’s initial stability and the harder it is to get the vessel to heel.
general stability criteria
The area under the righting lever curve (GZ curve) should not be less than 0.055 metre-radians up to 30° angle of heel. 2. The area under the righting lever curve (GZ curve) should not be less than 0.09 metreradians up to 40° angle of heel or the angle of downflooding if this is less than 40°
Inclining experiment - why must there be a positive transverse metacentric hight ?
Must have adequate positive stability for both safety reasons and to ensure accuracy
How dose FSE affect a GZ curve
GM reduces with the introduction of FSA
Reducing the range of positive stability
GZ max reduced
Passenger ship stability criteria
Angle of Heel due to crowding of passengers on one side shall not exceed 10 degrees
Angle of heel for turning shall not exceed 10 degrees
Transverse shift of cargo, what happens
Shift of the centre of gravity off the centre line
Increase beam will ?
Increase the water plane area
Shear force ?
Tendency for parts to be pushed upwards and others downwards
Forces must be equal
Weight / buoyancy
Archimedes
Drawing is lips
Stability requirements
Under the curve
0-30 = 0.055
0-40 = 0.09
30-40 = 0.03
RL occurs not less than 30*
Initial GM occurs not less than 0.15m
GZ not less than 0.2 at 30*
Grain shift resulting in ?
Rise in KG
GM reduced
Change in COG
Shift of G to G1
Vessel will list
