Stability

Question 1

what are the 6 degrees of freedom

Answer 1

surge sway heave roll pitch yaw

Question 2

What is vessel displacement

Answer 2

the amount of water that the vessel displaces due to its weight

Question 3

what are some of the preparations you would make for heavy weather

Answer 3

Ensure pre departure checklist was completed thoroughly, heavy weather checklist, stowed cargo, freeing ports clear, water holds (Pools etc) and bilges empty, sounding tank levels and checking stability calcs.

Question 4

what is free surface effect and where can it occur

Answer 4

When a compartment is not completely filled with liquid so it is able to move and thus move the G of the vessel. It can occur in bilges, tanks, pools, on deck.

Question 5

what determines acceptable stability

Answer 5

MNZ rule 47. IMO requires 6 criteria:
- GM of at least 0.15m (0.35m for FV)
- Max GZ must be at least 0.2m
- Angle of max GZ must exceed 30*
- Area under GZ curve to 30* must be at least 0.055mRads
- Area under GZ curve to 40* must be at least 0.090mRads
- Area under GZ curve from 30-40 must be at least 0.03mRads

Question 6

Draw an angle of loll diagram

Answer 6

As per notes

Question 7

What is gross tonnage and net tonnage.

Answer 7

GT- overall volume of the interior of the vessel. NT- overall volume of cargo space.

Question 8

what is the minimum legal GM before departure.

Answer 8

When GM is more than 0.15m (0.35m for FV)

Question 9

what defines a stable and unstable vessel?

Answer 9

Stable is when G is below M. Unstable is when G is above M

Question 10

If G was at M what would be occurring and what would be your action.

Answer 10

Angle of loll. Move weight to lowest point to lower G.

Question 11

Draw a FSE diagram.

Answer 11

See notes

Question 12

Draw an unstable vessel diagram

Answer 12

See notes

Question 13

Draw a stable vessel diagram

Answer 13

See notes

Question 14

Draw a diagram for a vessel at a angle of list

Answer 14

See notes

Question 15

Explain the main points of a load line and reference a diagram

Answer 15

See notes

Question 16

What is the difference between heel and list

Answer 16

Heel is due to external circumstances, a righting lever is trying to right the vessel
List is due to internal circumstances such as loading. The G of the vessel has moved and thus there is no righting lever.

Question 17

State what the major risks in dry docking are with regards to stability. And discuss the precautions that would be taken.

Answer 17

As the vessels stern touches down (point loading), you theoretically loose weight (🔺) from the keel, this causes the position of G to move away from the area of weight loss (upthrust of P). As the rest of the vessel touches down, the area under the water starts to decrease, this brings ‘B’ down which in turn drags ‘M’ down, thus reducing GM.(potential for -GM)

Stability book and dry docking plan needs to be referenced.

The required GM for dry docking needs to be larger than when at sea.

When DEPARTING from the dry dock, you must ensure the vessel is the same condition as when it entered.

Question 18

Define Stability.

Answer 18

A vessels ability to keep itself upright.

Question 19

What is the metacenter of a vessel?

Answer 19

It is the point intersection of the B and the centreline of the vessel

Question 20

What is the angle of inflection on a GZ curve.

Answer 20

The change of direction on the first part of the curve. It is the point where the deck edge of the vessel goes underwater.

Question 21

Briefly describe the relationship between roll period and GM.

Answer 21

As the roll period increases, GM decreases.

Question 22

Who determines the rules and regulations with regards to stability.

Answer 22

ICLL and SOLAS determine the rules and regulations around stability. These get written into flag state rules, such as the large yacht code, or MNZ rule 47.

Question 23

Define freeboard

Answer 23

The distance from the watertight deck edge to the water level. It is also known as reserve buoyancy.

Question 24

What is the benefits of fore/aft trim

Answer 24

trimmed to the aft= more manoeuvrability
Trimmed forward = more efficiency

Question 25

How many degrees in a radian and what is a radian

Answer 25

57.3
It is the radias of the circle arced around the circumference. Half a circle = Pi

Question 26

Draw and label a GZ curve (list, Approx GM, IMO criteria)

Answer 26

See notes

Question 27

Name 7 types of ship stresses

Answer 27

Sagging, Hogging, Torsional, Transverse, Sloshing, Pounding, Racking,

Question 28

What is the distance between draught marks

Answer 28

100mm for each number and the gap between them.

Question 29

What NZ vessels must have draught marks

Answer 29

Vessels over 16m or more than 50 passengers

Question 30

What is MNZ rule 47

Answer 30

Load lines