Stability

Question 1

Why does a vessel return to the upright position?

Answer 1

When the combination of the center of gravity “G” pulling down coupled with the center of buoyancy “B” pushing up creates a righting action that forces the vessel back to its upright position.

Question 2

What is free surface effect?

Answer 2

Free movement of liquids, which may or may not be confined in tanks.

Question 3

What is a tender vessel?

Answer 3

• Easy to incline initially.
• Small reserve of Stability, GM.
• Small GZ, less able to return to upright once inclined.
• Long slow lazy roll period, comfortable, less movement of weights & less stress on the ship’s structure.

Question 4

What is a stiff vessel?

Answer 4

• Difficult to incline initially.
• Reserve of Stability, large GM.
• Large righting lever, GZ, easy to keep relatively upright.
• Short roll period, violent/jerky uncomfortable nature, racking stresses.

Question 5

What is list?

Answer 5

A vessel is said to be listed when it is inclined by forces within the vessel, e.g. movement of weight within the vessel.

Question 6

What is heel?

Answer 6

Whena vessel is inclined by external forces e.g. wind and waves

Question 7

What is Gross Tonnage?

Answer 7

Gross tonnage is the total of all enclosed space of a vessel. It is a measure of volume and not weight.

Question 8

What is Net Tonnage?

Answer 8

Net tonnage is the total enclosed space available revenue generating cargo including passengers. This will exclude non-revenue generating areas like bridge, engineering spaces and crew quarters.

Question 9

What is FWA?

Answer 9

Fresh Water Allowance

It is the amount that the draught of a vessel will change when a ship passes into fresh water while floating at the loaded draught.

Question 10

What is the meta centre and metacentric height?

Answer 10

Meta centre is the point where the force of buoyancy intersects the centre line of the vessel

Meta centric height is the distance between the centre of gravity and the meta centre otherwise known as GM.

Question 11

What are the three states of equilibrium?

Answer 11

• Stable
• Neutral
• Unstable

Question 12

What is light displacement?

Answer 12

Mass of the vessel when complete and ready for sea but with no passengers, stores, fuel, or cargo on board. An Empty Ship.

Question 13

What is load displacement?

Answer 13

Mass of the vessel fully loaded with cargo, stores, etc. floating at her assigned freeboard.

Question 14

What is TPC immersion?

Answer 14

The mass required to load/discharge in order to change the mean draught by 1cm

Question 15

What are some ways to reduce free surface effect?

Answer 15

Ship design: Longitudinal watertight divisions

During a passage: Press full, empty, use narrow width tanks.

Question 16

How do you find the righting moment?

Answer 16

Displacement x GZ (∆ x GZ)

Question 17

What is reserve buoyancy and what is the formulae to calculate it?

Answer 17

It is the amount of enclosed watertight space above the water line to the freeboard deck. It provides a safety factor to the vessel in the event of damage. Reserve buoyancy = L x B x Fbd(freeboard = depth - draft) x Cb

Question 18

What information do you provide prior to dry docking?

Answer 18

Prior to arrival: Send Docking plan, dockmaster to return arrival conditions, insurance, class, flag , managment company details

Prior to dry docking: Upright, no free surfaces, trim as required by the dock master, minimum ballast, no loose equipment on deck, increase GM. All water tight and bulk head doors closed to ensure continuity of strength through vessels length. Conditions need to be recorded so vessel can be refloated with same characteristics.

Risk Assesment, Pilot, Tug and mooring operations, Weather, Tides.

Meeting with dry dock master: Discus dry docking plan which would include the placemnet of blocks and shores, location of any hull protrusions, sea chests, tank plugs and tranducers.

Post lift: Hook up firefighting services, water, electricity

Question 19

What does a stability booklet contain and which vessels must carry one?

Answer 19

• Vessel over 24m

Gives information of the vessels stability at different loading condition. e.g. Curves or table of GM vs draught and data necessary to maintain stability after damage.

Question 20

What information can be found on the docking plan?

Answer 20

• Locations of: Beams and frames, through hulls, sea chests, transducers, stabilizers, rudder posts, propellors and shafts, thrusters, protrusion.
• Positions of: Blocks shores
• Tank locations
• Docking conditions