Stability

Question 1

What is displacement?

Answer 1

The amount of water displaced by a vessel.
Volume in cubes
Weight in tonnes

Question 2

What is buoyant volume?

Answer 2

Volume of the entire watertight part of the hull. From keel to uppermost watertight deck.

Question 3

What is reserve of buoyancy?

Answer 3

Volume of the waterline to the uppermost watertight deck.

Question 4

What is freeboard?

Answer 4

Height from the waterline at the ships side to the highest continuous deck.

Question 5

What is centre of buoyancy?

Answer 5

Point of action of the upward thrust of the water on the ships hull.
Centroid of the underwater volume.

Question 6

What is ponding effect?

Answer 6

The free surface within a tank is reduced if the tanks are nearly full or nearly empty.
Creates an almost solid weight ballasting, aiding stability.

Question 7

Why do we have minimum liquid loading restrictions?

Answer 7

Preserve transverse stability by ensuring centre of gravity doesn’t rise too far. GM must be greater than 0.3m.

Question 8

What is heeling motion?

Answer 8

When vessel is inclined by an external force such as wind or waves. The shape of the underwater volume changes, moving the centre of buoyancy. Centre of buoyancy and centre of gravity are now separated horizontally by distance GZ (righting lever).
Weight and buoyancy remain equal and form a couple, this produces the righting movement which brings the ship back to the upright position.

Question 9

How do we reduce free surface effect?

Answer 9

Pump overboard
Longitudinal sub division of compartment
Drain down to lower, narrower compartment
Fill compartment up

Question 10

What is a naval ship safety certificate?

Answer 10

Produced after inclining experiment
Given by the naval authority group
Lasts 10 years

Question 11

What are the characteristics of stiff?

Answer 11

Larger GM/GZ
More stable
Shorter roll period
Quick to return upright

Question 12

What are the characteristics of tender?

Answer 12

Smaller GM/GZ
Less stable
Longer roll period
Slow to return upright

Question 13

What is equilibrium?

Answer 13

Stable equilibrium means ship will return to upright after being inclined.

Question 14

What is unstable equilibrium?

Answer 14

Centre of gravity is above meta centre. Vessel in this state will loll.

Question 15

Explain list?

Answer 15

GM is positive when ship is upright.
Stable.
Steady roll.
Caused by an off centre weight.
Heel towards the weight or damage.

Question 16

How do we correct list?

Answer 16

Remove the off centre weight
Add an equivalent weight to the other side
Transfer weight to centre

Question 17

Explain loll?

Answer 17

GM is negative when ship is upright
Unstable
Unsteady roll
Cause by free surface, unsteady top weight or both
Will cause vessel to heel until stability is restored
Tendency to heel away from damage

Question 18

How do we correct loll?

Answer 18

Remove weight from high up
Break up the free surface
Add weight low down
Pump out
Drain to lower compartment
Fill compartment

Question 19

Where do we find stability information?

Answer 19

CBRNDC class book
Stability and survive-ability handbook
BR 2170
Pumping and flooding board
Stability computer
Personal notes

Question 20

When are levelling lines open or closed?

Answer 20

Tanks full and in use- shut and opened routinely
Tanks empty- left open
Tanks full and not in use- shut

Question 21

Explain positive GM?

Answer 21

Stable
G is below M
GZ= righting lever

Question 22

Explain neutral GM?

Answer 22

G is next to M
GZ= no righting lever

Question 23

Explain negative GM?

Answer 23

G is above M
ZG= upsetting lever