States of Stability and GZ Curve

Question 1

What the labels for the X and Y axies for a GZ Curve?

Answer 1

X-axis = degrees of heel
Y-axis = GZ and GM in metres

Question 2

What are the 5 values you can take off a GZ curve?

Answer 2

Range of Stability
- Range of positive GM - expressed in degrees

Point of Vanishing Stability
- Where line cuts x-axis

Max GZ (righting lever)
- Peak of the bell (in metres and degrees)

Angle of Deck Edge Immersion
- Before the peak of the bell, where the down curve meets the up curve (expressed in degrees)

GM
- Mark 57.3° on the x-axis (draw a vertical line up), draw a tangent off the curve at 0. Where the tangent intersects the 57.3° line is the vessels GM

Question 3

What is a GZ Curve?

Answer 3

• A tool for analysing a ships surface stability

Question 4

Define and describe the three states of stability?

Answer 4

Stable

• A ship in a stable condition, when heeled by an external force will return to upright when the force is removed
• G is below M (Positive GM)

Neutral

• A ship in a neutral condition, when heeled by an external force will come to rest at an undetermined angle of heel
• G is in line with M (Neutral GM)

Unstable

• A ship in a unstable condition, when heeled by an external force even when the force is removed, the ship will continue to heel eventually leading to capsize
• G is above M (Negative GM)

Question 5

What is an angle of loll?

Answer 5

• The angle at which a vessel with an initial negative GM will come to rest at
• To fix an angle of loll you create a list by moving weight down which will bring G below M

Signs of Angle of Loll
- Speed of vessel righting will begin to slow

Question 6

What is the free surface effect?

Answer 6

• The free flow of liquid on the deck or a partially filled tank which creates a VIRTUAL LOSS IN GM

To Reduce Free Surface Effect
- FIll tanks or empty tanks
- Longitudinal bulkheads
On Deck in heavy weather
- Scuppers open
- Slow down
- Run with the sea