Definitions & Drawings

Question 1

Statical Stability (Ships inherent stability)

Answer 1

Statical stability is the ability of a vessel to return to her upright position after she has been inclined by an external force.

Question 2

Forward perpendicular. FP

Answer 2

The line perpendicular to the moulded base line and will pass through the intersection of the designed load waterline and the forward side of the stem.

Question 3

Aft perpendicular. AP

Answer 3

The line perpendicular to the moulded base line and will pass through the intersection of the designed summer load waterline and the middle of the rudder stock.

Question 4

Length between perpendicular. LBP

Answer 4

Horizontal distance measured on summer load waterline. Naval architects calculate all hydrostatic data using LBP.

Question 5

Moulded depth. D

Answer 5

Is the depth measured from the top of the keel to the heel of the deck beam at side at the middle of the length between perpendiculars.

Question 6

KB (Vertical centre of buoyancy) or VCB

Answer 6

The distance of Centre of buoyancy (B) above the keel (K)

Question 7

Block Coefficient Cb

Answer 7

Ratio of the underwater volume occupied by a ship in relation to the block defined by the waterline length beam & draft.
Cb = Volume (V) divided by the LWL x BWL x d (draft)

Question 8

Freeboard

Answer 8

Vertical distance from waterline to upper edge of the deckline
Freeboard = Depth - Draft

Question 9

Dynamic Stabilty

Answer 9

External work to be done on a vessel to incline her to a certain angle of heel

Question 10

Draft

Answer 10

Depth of the lowest point of the keel below waterline

Question 11

Centre of Gravity (G)

Answer 11

The point through which the total mass of the ship is assumed to act vertically downwards.

Question 12

Draw Stable Equilibrium

Answer 12

Question 13

Draw neutral equilibrium

Answer 13

Question 14

Draw Unstable equilibrium

Answer 14

Question 15

Force

Answer 15

Product of mass and acceleration
Unit of mass = kg
Unit of acceleration = meters per second squared m/s2
Unit of force = kg.m/s2 or newton (N) when acceleration is 9.81m/s2

Question 16

Moment

Answer 16

Product of force and distance

Question 17

Draw curve for tender/stiff ship

Answer 17

Question 18

Draw curve of statical stability

Answer 18

Question 19

Angle of Vanishing Stability

Answer 19

Angle of Vanishing Stability is the angle of heel where GZ changes from positive to negative.

Question 20

Range of Stability

Answer 20

Range of Stability is the range of heel angles where GZ is Positive.

Question 21

GM (Metacentric Height)

Answer 21

Is the distance between the ships centre of gravity and the transverse metacentre

Question 22

Transverse Metacentre (M)

Answer 22

Transverse Metacentre is the intersection of the force of buoyancy with the ships centerline

Question 23

Critical moment & Critical period (dry dock)

Answer 23

Critical Moment- Is the moment just before the vessel takes blocks overall

Critical Period - The period since the keel first touches the block until the vessel takes blocks overall

Question 24

KG (vertical centre of gravity)

Answer 24

The distance of G above the Keel (K)

Position of KG for any other condition than light ship depends on the distribution of weights and must be calculated

Question 25

Centre of buoyancy (B)

Answer 25

The centre of gravity of the underwater volume (volume of displacement)

Question 26

Free surface effect

Answer 26

Free Surface Effect - When a vessel that has a slack tank (partially filled) is heeled through e by an external force, the liquid in the tank will flow to the low side. The center of gravity of the liquid will move from its original position (g) to the new centroid of the liquid in the tank (g1) - thus causing a parallel shift of the Centre of Gravity (G to G1) of the ship - thereby causing a virtual loss of Metacentric Height (GM).

Question 27

Define centre of floatation (F)

Answer 27

The centroid of the water plane area