Stability

Question 1

Why does a vessel return to the upright?

Answer 1

Because the buoyancy force (B1) acts through the metacentre (M) above G, creating a positive righting moment (GZ).

Question 2

What is Free Surface Effect (FSE)?

Answer 2

Shift of liquid in a tank moves G upward/outward → reduces GM and stability (apparent rise in G).

Question 3

Effect of discharging a heavy weight with ship’s gear?

Answer 3

G shifts to crane head → moves vessel’s G outwards → list develops → final G shifts away from weight discharged.

Question 4

What is a List?

Answer 4

Inclination due to internal imbalance (uneven loading). Correct by shifting ballast/cargo.

Question 5

What is Heel?

Answer 5

Inclination due to external forces (wind, waves).

Question 6

What is Loll?

Answer 6

Unstable equilibrium → vessel leans to one side due to high G. Correct by lowering G (ballast low side first).

Question 7

Chief Officer’s duties before drydocking?

Answer 7

Secure hatches/derricks, reduce free surface, calculate stability, sound tanks, prepare fenders/shores, inform dock, scrub hull if possible.

Question 8

What is a Docking Plan?

Answer 8

Plan showing keel blocks, bilge blocks, and shores to align vessel safely in dock.

Question 9

Critical stage in docking?

Answer 9

Just before vessel takes blocks fore & aft → loss of buoyancy → virtual rise in G.

Question 10

Preparations before flooding dock?

Answer 10

All bottom plugs in, hull watertight, repairs complete, valves shut, anodes fitted, surveys done, stability confirmed.

Question 11

LY3 Stability Criteria?

Answer 11

GZ ≥ 0.20m at ≥30° heel
Max GZ at >30° (not <25°)
GM ≥ 0.15m after FSE correction

Question 12

Damage Stability (short-range yachts exempt)?

Answer 12

Vessel must float 75mm below deck with one compartment flooded; equilibrium heel ≤7°.

Question 13

Bad weather precautions?

Answer 13

Re-route, verify position, update weather, plot storm often, manual steering, reduce speed, close WT doors, rig lifelines, reduce free surface, secure stowage.

Question 14

Gross Tonnage (GT)?

Answer 14

Volume of enclosed spaces (1 GT = 2.83 m³).

Question 15

Net Tonnage (NT)?

Answer 15

GT minus non-revenue spaces (crew, machinery, ballast). Basis for port dues.

Question 16

Reserve Buoyancy?

Answer 16

Volume of watertight hull above waterline (measured by freeboard).

Question 17

Plimsoll Line?

Answer 17

Load line marking minimum freeboard to ensure reserve buoyancy.

Question 18

Freeboard?

Answer 18

Distance from summer load line to freeboard deck.

Question 19

Fresh Water Allowance (FWA)?

Answer 19

Difference in draught between salt and fresh water due to density difference (RD 1.025).

Question 21

Q: Why does a vessel return to upright?

Question 22

A: Because buoyancy acts through the metacentre (M) above G

Answer 22

creating a positive righting moment (GZ).

Question 24

Q: What is Free Surface Effect (FSE)?

Question 25

A: Shift of liquid in tanks → apparent rise in G → reduces GM and stability.

Question 27

Q: What is List?

Question 28

A: Inclination due to internal imbalance (uneven loading).

Question 30

Q: What is Heel?

Question 31

A: Inclination due to external forces (wind/waves).

Question 33

Q: What is Loll?

Question 34

A: Vessel leans to one side due to high G and unstable equilibrium. Correct by lowering G (ballast low side first).

Question 36

Q: Chief Officer’s duties before drydocking?

Question 37

A: Secure hatches/derricks

Answer 37

reduce FSE

Question 39

Q: What is a Docking Plan?

Question 40

A: Plan showing keel blocks

Answer 40

bilge blocks

Question 42

Q: Critical stage in docking?

Question 43

A: Just before vessel takes blocks fore & aft → loss of buoyancy → virtual rise in G.

Question 45

Q: LY3 Stability Criteria?

Question 46

A: GZ ≥ 0.20m @ ≥30° heel

Answer 46

max GZ at >30° (not <25°)

Question 48

Q: Damage Stability?

Question 49

A: Float 75mm below deck with one compartment flooded; equilibrium heel ≤7°.

Question 51

…and so on for FWA

Answer 51

Freeboard

Question 52

Why does a vessel return to upright?

Answer 52

Because buoyancy acts through the metacentre (M) above G, creating a positive righting moment (GZ).

Question 53

What is Free Surface Effect (FSE)?

Answer 53

Shift of liquid in tanks → apparent rise in G → reduces GM and stability.

Question 54

What is List?

Answer 54

Inclination due to internal imbalance (uneven loading).

Question 55

What is Heel?

Answer 55

Inclination due to external forces (wind/waves).

Question 56

What is Loll?

Answer 56

Vessel leans to one side due to high G and unstable equilibrium. Correct by lowering G (ballast low side first).

Question 57

Chief Officer’s duties before drydocking?

Answer 57

Secure hatches/derricks, reduce FSE, check stability, sound tanks, prepare fenders, inform dock, scrub hull.

Question 58

What is a Docking Plan?

Answer 58

Plan showing keel blocks, bilge blocks, shores to align vessel safely in dock.

Question 59

Critical stage in docking?

Answer 59

Just before vessel takes blocks fore & aft → loss of buoyancy → virtual rise in G.

Question 60

LY3 Stability Criteria?

Answer 60

GZ ≥ 0.20m @ ≥30° heel, max GZ at >30° (not <25°), GM ≥ 0.15m after FSE correction.

Question 61

Damage Stability?

Answer 61

Float 75mm below deck with one compartment flooded; equilibrium heel ≤7°.