Stability Theory

Question 1

True Mean Draught (TMD)

Answer 1

The draught of the vessel at the Longitudinal Centre of Flotation, and the draught used in the Hydrostatic Particulars.

Question 2

Fresh Water Allowance (FWA)

Answer 2

The amount the ships draught changes when moving from salt water to fresh water, due to the difference in water density.

Question 3

Righting Moment

Answer 3

GZ x Displacement (In Tonnes)
Indicates the ships ability to resist heeling forces.

Question 4

Relative Density

Answer 4

The ratio of the density of a liquid (Such as ballast, cargo or surrounding water) to the density og fresh water (Taken as 1.000).

Question 5

Neutral Equilibrium

Answer 5

A ship is considered to be in neutral condition of equilibrium if, when acted upon by an external force in still water, it remains at that angle of inclination. The ship has no righting lever (GZ), resulting in no MSS. However, should the vessel be inclined further, the Centre of Buoyancy will move further out, resulting in a GZ being created.

Question 6

List

Answer 6

A list is a permenant or semi-permanent inclination of a ship to port or starboard, due to an uneven distribution of weight.

Question 7

Heel

Answer 7

A heel is a temporary inclination to one sid caused by external forces such as wind, turning or waves.

Question 8

Loll

Answer 8

A loll is an unsafe, unstable inclination of a ship to port ot starboard due to negative initial stability (I.e. Negaive GM)

Question 9

Free Surface Effect

Answer 9

Occurs when a liquid in a partially filled tank moves, reducing the ships stability by raising the Centre of Gravity

Question 10

Transverse Statical Stability

Answer 10

The ability of a ship to return to the upright when subjected to an external force when floating in still water.

Question 11

Longitudinal Centre of Flotation (LCF)

Answer 11

The LCF is the fore-and-aft position about which a ship trims when weight is added or removed. It is the centre of the waterplane arrea at a given draught.

Question 12

Forward Perpendicular (FP)

Answer 12

The FP is a vertical line drawn at the point where the ships summer load waterline intersects the foremost part of the ships stern.

Question 13

Centre of Gravity (G)

Answer 13

The point through which the total weight of the ship acts vertically downward.
Moves when weights are added, removed or shifted.

Question 14

Initial Transverse Metacentre (M)

Answer 14

M is the point about which a ship starts to roll given a very small angle of heel. It is the intersection of the buoyant force line at a small heel and the ships original vertical centreline when upright.

Question 15

Centre of Buoyancy

Answer 15

The Centre of Buoyancy is the point through which the buoyant force (Upthrust) acts on a submerged or floating body. It is the centroid of the displaced volume of fluid.

Question 16

Block Coefficient (CB)

Answer 16

Is the ratio of the underwater volume of the ships hull (Displacement volume) to the volume of a rectangular block having the same overall length, breadth and draught as the ship.

Question 17

Arithmetic Mean Draught

Answer 17

Considered to be the mid-point between the two draughts.

Question 18

Stable Equilibrium

Answer 18

A ship is considered to be in a stable condition of equilibrium if, when acted upon by an external force (Wind, waves) in still water, it returns to the upright when that force is removed. When at sea, the MSS will return the vessel to the upright.

Question 19

MCTC

Answer 19

The moment required to change the trim of the vessel by one centimentre

Question 20

Deck Edge Immersion

Answer 20

The GZ curve can be used to determine the angle of heel at which the deck is immersed. The deck immersed angle occurs at the point where the gradient of the curve starts to reduce.
This is also known as the “Point of Inflection”.

Question 21

Metacentric Height (GM)

Answer 21

The vertical distance between the Centre of Gravity (G) and the metacentre (M).

Question 22

Aft Perpendicular (AP)

Answer 22

The AP is a vertical line drawn at the after end of the designed waterline, typically at the centreline of the rudder stock or at the intersection of the stern with the waterline.

Question 23

Unstable Equilibrium

Answer 23

The ship is considered to be in an unstable state of equilibrium if, when acted upon by an external force (Wind and Waves) in still water, it will continue to heel further over when that force has been removed. The ship has a GZ which is now a capsizing lever. As the Centre of Buoyance moves further out, resulting in B and G in line, the ship will hopefully settle at an angle of loll.

Question 24

Dock Water Allowance (DWA)

Answer 24

The amount by which a ships draught must be reduced to account for the difference in water density between dock water and sea water.

Question 25

Archimides' Principle

Answer 25

Any object, wholly or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. This buoyant force acts vertically upward through the Centre of Buoyancy, which is the centroid of the displaced fluid volume.

Question 26

Peak GZ requirements

Answer 26

GZ should be atleast 0.20m at an angle of heel of 30 degrees.

Question 27

Righting Arm (GZ)

Answer 27

The horizontal distance between the lines of action of buoyancy and gravity. The bigger the GZ, the greater the ships ability to return to upright.

Question 28

Why load lines assigned to cargo ships

Answer 28

-To ensure adequate freeboard -To prevent overloading -To account for seasonal and geographical conditions -To comply with international regulations -To ensure structural integrity and seaworthiness

Question 29

TPC (Tonnes per Centimeter)

Answer 29

The TPC is the mass (in tonnes) that must be added to or removed from a ship in order to change its mean draught by one centimetre in a water of a given density.