Definitions & Drawings Flashcards
Statical Stability (Ships inherent stability)
Statical stability is the ability of a vessel to return to her upright position after she has been inclined by an external force.
Forward perpendicular. FP
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.
Aft perpendicular. AP
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.
Length between perpendicular. LBP
Horizontal distance measured on summer load waterline. Naval architects calculate all hydrostatic data using LBP.
Moulded depth. D
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.
KB (Vertical centre of buoyancy) or VCB
The distance of Centre of buoyancy (B) above the keel (K)
Block Coefficient Cb
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)
Freeboard
Vertical distance from waterline to upper edge of the deckline
Freeboard = Depth - Draft
Dynamic Stabilty
External work to be done on a vessel to incline her to a certain angle of heel
Draft
Depth of the lowest point of the keel below waterline
Centre of Gravity (G)
The point through which the total mass of the ship is assumed to act vertically downwards.
Draw Stable Equilibrium
Draw neutral equilibrium
Draw Unstable equilibrium
Force
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
Moment
Product of force and distance
Draw curve for tender/stiff ship
Draw curve of statical stability
Angle of Vanishing Stability
Angle of Vanishing Stability is the angle of heel where GZ changes from positive to negative.
Range of Stability
Range of Stability is the range of heel angles where GZ is Positive.
GM (Metacentric Height)
Is the distance between the ships centre of gravity and the transverse metacentre
Transverse Metacentre (M)
Transverse Metacentre is the intersection of the force of buoyancy with the ships centerline
Critical moment & Critical period (dry dock)
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
KG (vertical centre of gravity)
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
Centre of buoyancy (B)
The centre of gravity of the underwater volume (volume of displacement)
Free surface effect
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).
Define centre of floatation (F)
The centroid of the water plane area
