Glossary

BLOCK COEFFICIENT (Cb): The ratio of the underwater volume of the vessel to the volume of a rectangular block of the same overall dimensions.

BREADTH (BEAM): Maximum width of the ship.

Moulded Breadth: Width of the ship amidships, measured inside the shell plating.

CENTRE OF FLOTATION (CF): The centroid of the waterplane area. It is the point about which the ship trims, heels and lists.

Longitudinal Centre of Flotation (LCF): The distance of the centre of flotation from midships or the aft perpendicular.

DEADWEIGHT: The difference between the light displacement and the loaded displacement It is the carrying capacity of the ship and includes the weight of cargo, ballast, fuel, water, stores, crew and effects.

DENSITY: The mass per unit volume. Apparent Density: The weight in air of a unit volume.

True Density: The weight in a vacuum of a unit volume.

Relative Density (RD): The ratio of the weight of a substance to the weight of an equal volume of fresh water. RD of fresh water: 1.000 at 4°C RD of salt water: 1.025 at 4°C The term specific gravity, previously used in place of RD, is no longer used with SI units.

DEPTH: Distance from the top of the deck plating at the side of the ship to the bottom of the keel.

Moulded Depth: Distance from the keel to the deck at the ship's side, measured inside of the shell plating.

DISPLACEMENT: The mass of water displaced by the ship. It represents the total weight of the ship and is calculated by: volume of displacement x density of water.

Light Displacement: The displacement of the ship and superstructure with all fixed equipment plus engine room spares and with water in the boilers to working level. Also referred to as Lightship or Lightweight.

Summer Load The displacement of the ship when loaded to her Displacement: summer load line.

DRAUGHT: The distance from the bottom of the keel to the waterline. Sometimes spelled draft.

Aft Draught: The distance from the bottom of the keel to the waterline on the aft perpendicular when the ship is upright.

Midships Draught: The distance from the bottom of the keel to the waterline at a position midway between the perpendiculars.

Forward Draught: The distance from the bottom of the keel to the waterline on the forward perpendicular when the ship is upright.

Arithmetical Mean The arithmetical mean of the forward draught and Draught (AMD): the aft draught.

True Mean Draught The draught at the centre of flotation. (TMD):

EVEN KEEL: When the forward and aft draughts are the same, the ship is said to be on an even keel. The ship's hydrostatic data is calculated for this condition.

FREEBOARD: The distance from the waterline to the upper surface of the freeboard deck at the ship's side.

Assigned Summer Freeboard: The distance from the upper edge of the summer load line to the upper edge of the deck line.

FRESH WATER ALLOWANCE (FWA): The amount by which the summer load line may be submerged in fresh water.

HEEL: The ship is heeled when inclined by an external force.

HOG: The condition of a ship when it is deformed so that the forward and aft draughts are deeper than the midships draught. It is referred to as a hull deflection.

LENGTH BETWEEN PERPENDICULARS Distance from the forward perpendicular to the aft perpendicular. (LBP):

Aft Perpendicular (AP): A perpendicular drawn to the waterline at the point where the aft side of the rudder post meets the summer load line. Where no rudder post is fitted it is taken as the centre line of the rudder stock.

Forward Perpendicular A perpendicular drawn to the waterline at the point (FP): where the foreside of the stem meets the summer load line (or load water line LWL).

LENGTH OVERALL (L or LOA): Maximum length of the ship.

LIST: The ship is listed when inclined by weights within the ship.

MASS: The mass of an object is the quantity of matter it contains and is calculated by: volume x density

MIDSHIPS: The point midway between the forward and aft perpendiculars. The centre of the load line circle should indicate this position.

MOMENT TO CHANGE TRIM PER CENTIMETRE (MCTC or MCT 1 cm): The moment of the weight (i.e. weight x distance) required to change the trim of the ship by 1 cm.

SAG: The condition of a ship when it is deformed so that the midships draught is deeper than the forward and aft draughts. It is referred to as a hull deflection.

SOUNDING: The distance from the bottom of a tank to the top of the liquid within it.

TONNES PER CENTIMETRE IMMERSION (TPC): The weight which must be loaded or discharged to change the ship's mean draught by 1 cm

TRIM: The difference between the forward and aft draughts.

Apparent Trim: The difference between the draughts observed at the forward and aft draught marks.

True Trim: The difference between the draughts at the forward and aft perpendiculars.

ULLAGE: The distance above a liquid, within a tank, to the measuring point at the top of the tank.

VOLUME OF DISPLACEMENT: The volume of water displaced by the ship and is calculated by: length x breadth x draught x Cb.

WATERPLANE COEFFICIENT (Cw): The ratio of the area of the ship's waterplane to a rectangle of the same extreme dimensions. Also known as the coefficient of fineness.

WEIGHT: The weight of an object is the force it exerts on anything which freely supports it.

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