Stability

Question 1

Why does a vessel return upright?

Answer 1

GZ - righting lever : produces a righting moment.
GM - metacentric height :
+ GM = stable
- GM = unstable
0 GM = if inclined to a small angle of heel it will tend to remain at that angle. (neutral)

Question 2

law of flotation states..

Answer 2

“a body wholly of partially immersed in a fluid will experience an upward force equal to the weight of the fluid displaced.”

Question 3

Whats free surface effect?

Answer 3

an apparent rise in G, leading to a reduction in GM.
due to the movement of the CoG of a liquid

Question 4

explain a tender vessel

Answer 4

small positive GM therefore smaller righting lever (GZ)
ship roll angles and motions are long and gentle.
no risk of structural damage but
risk of shifting stores and capsizing in rough conditions.

Question 5

explain a stiff vessel

Answer 5

The larger the value of GM, the more ‘stiff’ (stable) the vessel. A larger righting lever (GZ)
roll angles and motions are short + aggressive.
risk of structural damage AND risk of shifting stores.

Question 6

‘List’ is..

Answer 6

ship inclined by forces within the vessel

Question 7

‘Loll’ is…

Answer 7

vessel unstable when upright due to a negative metacentric height ( - GM )
and will take on an angle of heel without any external force

Question 8

‘Heel’ is…

Answer 8

ship inclined by external force

Question 9

Duties for dry docking
(list 6)

Answer 9

1. engine and steering checks.
2. crew briefed and agreement made with yard.
3. dry dock plan sent to dock authorities
4. all hatches and cranes are secured
5. free surface in tanks reduced or removed.
6. stability calculation made to ensure adequate GM to take into account rise of G when the vessel has taken the blocks.
7. adequate fenders rigged

Question 10

docking plan

Answer 10

includes:
-profile, plan and sections of vessel aligned with dock drawings
-any projections from the hull of the vessel

Question 11

how do we compensate for bad weather?
(list 6)

Answer 11

1. consider re routing
2. verify vessels position
3. update weather report
4. plot storm position often
5. engage manual steering
6. reduce speed

Question 12

What’s GT?

Answer 12

measurement of vessels internal volume

Question 13

What’s NT?

Answer 13

the earning capacity of the vessel. obtained by deducting non-cargo spaces from the GT.

Question 14

What’s FWA?

Answer 14

fresh water allowance

calculated to allow for the change of draught when the vessel moves from SW to FW. (less dense, therefore sits lower in water)