1
Q
- What is the definition of the Center of Gravity (G)?
A
The point through which the total weight of the ship is considered to act vertically downwards.
2
Q
- What is the definition of the Center of Buoyancy (B)?
A
The geometric center of the underwater volume of the ship, through which the force of buoyancy acts vertically upwards.
3
Q
- What is the Metacenter (M)?
A
The point of intersection between the vertical lines drawn through the center of buoyancy (B) at two consecutive angles of small heel.
4
Q
- What does “GM” represent?
A
The Initial Metacentric Height, which is the vertical distance between the Center of Gravity (G) and the Metacenter (M).
5
Q
- What is the formula for calculating GM?
A
GM = KM - KG
6
Q
- What is a “Stable Equilibrium”?
A
A state where the vessel, when inclined by an external force, tends to return to its original upright position (Initial GM is positive).
7
Q
- What is an “Unstable Equilibrium”?
A
A state where the vessel, when inclined, continues to heel further away from the upright position (Initial GM is negative).
8
Q
- What is “Neutral Equilibrium”?
A
A state where the vessel, when inclined, remains in the inclined position without returning or heeling further (Initial GM is zero).
9
Q
- What is the Righting Lever (GZ)?
A
The horizontal distance between the vertical line of action of buoyancy and the vertical line of action of gravity.
10
Q
- What is the formula for GZ at small angles of heel (up to 10°)?
A
GZ = GM x sin (0)
11
Q
- What is “KM”?
A
The vertical distance from the Keel (K) to the Metacenter (M), calculated as $KB + BM$.
12
Q
- What is “KG”?
A
The vertical distance from the Keel (K) to the Center of Gravity (G).
13
Q
- What happens to G when weight is added to the ship?
A
G moves directly toward the center of gravity of the added weight.
14
Q
- What happens to G when weight is removed from the ship?
A
G moves directly away from the center of gravity of the removed weight.
15
Q
- What happens to G when weight is shifted within the ship?
A
G moves in a direction parallel to the shift of the weight’s center of gravity.
16
Q
- What is the formula for the shift of G due to weight movement (GG1)?
A
GG1=wxd/W
17
Q
- What is “Stiff” ship behavior?
A
A vessel with a large GM, resulting in a very short rolling period and jerky movements.
18
Q
- What is “Tender” ship behavior?
A
A vessel with a small GM, resulting in a long, slow rolling period.
19
Q
- What is the Free Surface Effect (FSE)?
A
The reduction in effective GM caused by the movement of liquid in partially filled tanks as the ship heels.
20
Q
- How is the Virtual Loss of GM due to FSE calculated?
A
FSC= i x p/w
(where i is the moment of inertia of the free surface and p is the density of the liquid).
21
Q
- What is “Transverse Statical Stability”?
A
The ability of a ship to return to an upright position after being heeled by an external force.
22
Q
- What is a “Heel”?
A
An inclination of the ship caused by external forces like wind, waves, or centrifugal force during a turn.
23
Q
- What is a “List”?
A
An inclination of the ship caused by internal forces, such as the asymmetrical distribution of weight within the vessel.
24
Q
- What is “BM” (Transverse Metacentric Radius)?
A
The vertical distance between the Center of Buoyancy (B) and the Metacenter (M).
25
25. What is the formula for BM?
bm=I/v
(where I is the Moment of Inertia of the waterplane and V is the underwater volume).
26
26. What is "KB"?
The vertical distance from the Keel to the Center of Buoyancy.
27
27. What is the approximate KB for a box-shaped vessel?
kb= draft/2
28
28. What is "Displacement" (W)?
The actual weight of the water displaced by the ship, which equals the total weight of the ship and its contents.
29
29. What is "Deadweight"?
The weight a ship can carry, including cargo, fuel, water, and crew (Displacement minus Lightship).
30
30. What is "Lightship"?
The weight of the empty ship as built, including only permanent items like machinery and boiler water.
31
31. What is the "Center of Flotation" (F)?
The centroid of the waterplane area about which a ship trims.
32
32. What is "Trim"?
The difference between the forward draft and the aft draft.
33
33. What is "MCTC"?
Moment to Change Trim by 1 Centimeter.
34
34. What is "TPC"?
Tonnes per Centimeter Immersion—the weight required to change the mean draft by 1 cm.
35
35. What is the "Angle of Loll"?
The angle at which an unstable ship (negative GM) will eventually settle and find a state of equilibrium.
36
36. How do you correct an Angle of Loll?
By lowering the Center of Gravity (G), usually by filling bottom tanks, starting with the low side first to avoid a violent flop.
37
37. What are "Hydrostatic Curves"?
Graphs or tables providing the ship's stability data (KM, KB, TPC, etc.) for different drafts.
38
38. What is "Statical Stability Curve" (GZ Curve)?
A plot showing the righting lever (GZ) against various angles of heel.
39
39. What is the "Range of Stability"?
The extent of angles through which the vessel has a positive righting lever.
40
40. What is the "Angle of Maximum GZ"?
The angle of heel where the righting lever is at its greatest value.
41
41. What is "Dynamic Stability"?
The work done in heeling a ship to a particular angle, represented by the area under the GZ curve.
42
42. What is "Initial Stability"?
The stability of a ship at small angles of heel (usually up to 7°–10°).
43
43. What is "Reserve Buoyancy"?
The volume of the watertight part of the ship above the waterline.
44
44. What is the "Load Line"?
A marking on the hull indicating the maximum depth to which the ship may be safely loaded.
45
45. What is "Density" effect on draft?
A ship moving from saltwater to freshwater will increase its draft due to the lower density of freshwater.
46
46. What is "Fresh Water Allowance" (FWA)?
The distance the draft changes when moving between saltwater (1.025 t/m^3) and freshwater (1.000 t/m^3)
47
47. What is the formula for FWA?
FWA= W/40xtpc
48
48. What is "DWA" (Dock Water Allowance)?
The reduction in FWA when the water density is between 1.000 and 1.025
49
49. What is "Wall-Sided" Formula?
Used to calculate GZ at larger angles of heel when the deck edge is not yet immersed.
GZ = GM + 1/2 BM X tan^2 theta)x sin theta
50
50. What is "Cross Curves of Stability" (KN Curves)?
Curves used to find the GZ value for any displacement and angle of heel, assuming KG is zero.
GZ = KN - KG X SIN THETA
51
1. What is ship stability?
The ability of a vessel to return to an upright position after being inclined by an external force such as wind or waves.
52
2. Define the term "center of gravity (G)."
The point through which the total weight of the ship and its contents is considered to act vertically downwards.
53
3. What is the center of buoyancy (B)?
The geometric center of the underwater volume of the ship, through which the force of buoyancy acts vertically upwards.
54
4. What is metacenter (M)?
The point of intersection between two successive vertical lines of action of buoyancy as the ship heels through small angles.
55
5. Define metacentric height (GM).
The vertical distance between the center of gravity (G) and the metacenter (M).
56
6. What is the difference between initial stability and overall stability?
Initial stability refers to stability at small angles of heel (up to 10°); overall stability refers to the ship's ability to resist capsize at all angles of heel.
57
7. What is meant by "positive stability"?
A condition where the metacenter (M) is above the center of gravity (G), resulting in a positive GM and a vessel that returns to upright.
58
8. What condition does a vessel have if GM is negative?
The vessel is in unstable equilibrium and will not remain upright; it will either capsize or settle at an angle of loll.
59
9. What does it mean when a ship is in neutral equilibrium?
A condition where GM is zero (G and M coincide), and the ship will remain at whatever angle of heel it is placed in.
60
10. What is the importance of GM in determining stability?
GM serves as a primary measure of a vessel’s initial stability; a larger GM indicates a greater resistance to heeling at small angles.
61
11. Differentiate between static and dynamic stability.
Static stability is the ability to resist a constant heeling force; dynamic stability is the work required to heel the ship to a certain angle (area under the GZ curve).
62
12. What is angle of loll?
The angle at which an unstable ship (negative GM) will eventually reach a state of equilibrium and stop heeling.
63
13. What causes a vessel to be in a condition of angle of loll?
A negative initial GM caused by the center of gravity (G) rising above the metacenter (M).
64
14. What is the angle of heel?
The degree of inclination of a vessel from the vertical, caused by external forces like wind or waves.
65
15. What is the difference between stiff and tender ships?
A "stiff" ship has a large GM and rolls quickly; a "tender" ship has a small GM and rolls slowly and sluggishly.
66
16. What are the effects of a low GM on a vessel's rolling period?
A low GM increases the rolling period, making the vessel roll slowly, which can be comfortable for the crew but dangerous for stability.
67
17. What happens when weight is added above the center of gravity?
The vessel's center of gravity (G) moves upward, which reduces the GM and decreases stability.
68
18. What happens when weight is removed from below the center of gravity?
The vessel's center of gravity (G) moves upward, reducing the GM and decreasing stability.
69
19. What is the righting arm (GZ)?
The horizontal distance between the vertical lines of action of gravity (G) and buoyancy (B) when the ship is heeled.
70
20. How is the righting moment calculated?
By multiplying the displacement by the righting arm (GZ): Moment = Moments x GZ
71
21. What is the heeling moment?
A moment produced by external or internal forces (wind, weight shift) that tends to tip the vessel away from the vertical.
72
22. How does wind cause a heeling moment?
By exerting pressure on the ship's side above the waterline, creating a force that acts against the lateral resistance of the water.
73
23. What is the purpose of calculating a vessel's righting arm?
To determine the magnitude of the force available to return the ship to an upright position at any given angle of heel.
74
24. What is the relationship between GM and the righting arm?
At small angles, GZ = GM x sin(theta) Therefore, a larger GM produces a larger righting arm.
75
25. What is a stability curve?
A graph showing the value of the righting arm (GZ) plotted against the angle of heel.
76
26. At what angle is the maximum righting arm usually found?
Typically between 30° and 45°, often shortly after the deck edge is immersed.
77
27. What is the range of stability?
The range of angles through which the ship has a positive righting lever (from 0° to the angle of vanishing stability).
78
28. What does the area under the GZ curve represent?
The dynamic stability, or the amount of work required to heel the ship to a specific angle.
79
29. What information can be derived from a GZ curve?
Initial GM, maximum GZ, range of stability, and the angle of vanishing stability.
80
30. What is the free surface effect?
The reduction in stability caused by the shifting of liquid in partially filled tanks as the vessel heels.
81
31. How does free surface effect influence GM?
It causes a virtual rise in the center of gravity (G), which reduces the effective GM.
82
32. How can free surface effect be minimized?
By keeping tanks either completely full ("pressed up") or completely empty, and by using longitudinal swash bulkheads.
83
33. What is the effect of longitudinal weight shifts on trim?
Shifting weight forward or aft moves the center of gravity longitudinally, changing the drafts at the bow and stern.
84
34. What happens to a ship's stability when cargo is loaded high in the ship?
The center of gravity (G) rises, the GM decreases, and the vessel becomes more tender or potentially unstable.
85
35. How does loading heavy cargo at the bottom affect the GM?
The center of gravity (G) lowers, which increases the GM and makes the vessel more stiff.
86
36. What is damage stability?
The ability of a ship to remain afloat and stable after the flooding of one or more compartments.
87
37. What is the purpose of a watertight bulkhead?
To contain flooding within a specific compartment and prevent it from spreading to the rest of the ship.
88
38. How does flooding affect a ship's stability?
It increases displacement, changes trim, and usually decreases GM due to the free surface effect of the floodwater.
89
39. What is the effect of a breach in the hull on the ship's stability?
It causes a loss of buoyancy in the damaged area and often creates a massive free surface effect, leading to a list or capsize.
90
40. How can transverse subdivision improve stability?
By limiting the longitudinal extent of flooding, thereby maintaining enough buoyancy and stability to keep the ship afloat.
91
41. Why is it important to monitor the ship's stability regularly?
To ensure the vessel remains within safe limits as fuel is consumed, water is used, and cargo is moved.
92
42. What is the purpose of an inclining experiment?
To accurately determine the vessel's lightship displacement and the vertical position of its center of gravity (KG).
93
43. Who is responsible for ensuring ship stability during operations?
The Master of the vessel, supported by the Chief Officer.
94
44. What are the consequences of poor stability management?
Vessel capsize, loss of life, cargo damage, and environmental disasters.
95
45. How is GM determined during an inclining experiment?
By moving known weights across the deck and measuring the resulting angle of heel using pendulums or clinometers.
96
46. If a vessel is listing and GM is negative, what should be done?
Lower the center of gravity (G) by filling bottom tanks, starting with the low side first to avoid a violent "flop" to the other side.
97
47. During cargo operations, how can you avoid excessive trim?
By distributing the cargo weight evenly forward and aft of the center of flotation.
98
48. What precautions must be taken when ballasting?
Ensure the free surface effect is managed by filling tanks one at a time and monitoring the change in GM and stresses.
99
49. In heavy weather, how can a ship's stability be affected?
Icing on the superstructure (weight high up) or water on deck can raise the center of gravity and reduce GM dangerously.
100
50. If stability is marginal, should the vessel proceed to sea?
No. The vessel must be ballasted or cargo rearranged to ensure a safe positive GM and adequate range of stability before departure.
INSW
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