PRC Naval Architecture & Marine Engineering key terms

The total weight of water displaced by a floating hull, equal to the ship's total weight and expressed in tonnes; calculated from submerged volume multiplied by water density.

The mass a ship can carry when loaded to its assigned summer draught, comprising cargo, fuel, fresh water, ballast, stores, crew and passengers; equal to loaded displacement minus lightship weight.

The mass of a completed ship with machinery, outfit and fixed ballast but without cargo, fuel, consumables, crew or passengers; the baseline from which deadweight is measured.

The ratio of a hull's submerged volume to the volume of a rectangular block of its length, breadth and draught; a measure of hull fullness, typically 0.5 to 0.85.

The ratio of submerged volume to the volume of a prism with the same midship section area and length; indicates longitudinal distribution of hull volume and influences resistance.

The ratio of the immersed midship section area to the area of a rectangle of the same breadth and draught; describes fullness of the midship section.

The vertical distance between the centre of gravity and the transverse metacentre; a positive value indicates initial stability, and its magnitude governs roll period and stiffness.

The horizontal distance between the centre of gravity and the line of buoyancy force at a given heel angle; its integral over heel gives the righting energy used in stability criteria.

The apparent rise in centre of gravity caused by liquid shifting in partly filled tanks during heel, reducing metacentric height; mitigated by longitudinal subdivision of tanks.

Markings on the shell amidships indicating maximum permitted draughts for assigned zones and seasons, certified under the International Convention on Load Lines.

The vertical distance from the waterline to the freeboard deck at side amidships; it provides reserve buoyancy and weathertight integrity margin.

The watertight volume above the waterline expressed as buoyancy, available to support the ship if it is further immersed; a core measure of survivability.

The ability of a ship to remain afloat and stable after flooding of one or more compartments, assessed against deterministic or probabilistic subdivision requirements.

The arrangement of watertight bulkheads and decks dividing the hull into compartments to limit flooding extent and preserve stability and buoyancy after damage.

A vertical partition separating compartments; watertight bulkheads provide subdivision and strength, while collision bulkheads protect against forward flooding.

The watertight space between the inner bottom and outer shell plating, providing grounding protection, tank capacity for ballast and fuel, and longitudinal strength.

The hull's ability to resist bending moments and shear forces along its length caused by uneven weight and buoyancy distribution, especially in sagging and hogging conditions.

The ratio of a hull section's second moment of area about its neutral axis to the distance to the extreme fibre; governs bending stress under longitudinal bending moment.

The longitudinal curvature of the deck, higher at bow and stern than amidships, increasing reserve buoyancy and freeboard at the ends and improving seakeeping.

The transverse rise of the deck from side to centreline, promoting drainage and adding slight transverse stiffness to the deck structure.

The upward slope of the bottom shell from the keel toward the bilge, affecting drainage, deadrise and hydrodynamic form near the turn of the bilge.

A longitudinal fin fitted along the bilge turn to damp roll motion; it is not a strength member and is designed to shed damage without holing the shell.

A rounded protrusion at the stem below the waterline that modifies wave-making, often reducing resistance at design speed by altering the bow pressure field.

Formation and collapse of vapour bubbles in low-pressure regions of propeller blades, causing erosion, noise and thrust loss; mitigated by blade area and wake-adapted design.

The fractional reduction of inflow velocity to the propeller relative to ship speed caused by the hull's wake; used to relate thrust power to effective power.

The ratio of effective power needed to tow the ship to the power delivered to the propeller, combining hull, relative rotative, propeller and shafting efficiencies.

The power required to overcome a ship's total resistance at a given speed, equal to resistance multiplied by speed; the starting point for machinery power estimation.

Controlled full-scale testing after construction to verify speed, power, manoeuvrability, endurance and machinery performance against contractual specifications.

A test using known shifting weights to measure a completed ship's lightship weight, centre of gravity and metacentric height for the stability booklet.

An independent technical organisation that develops and verifies rules for hull and machinery, issuing class certificates and statutory surveys on behalf of flag states.

The state whose nationality a ship bears, responsible for implementing international conventions and exercising jurisdiction over the vessel through surveys and certification.

Inspection by a port authority of foreign-flagged ships to verify compliance with international conventions, with powers to detain vessels for serious deficiencies.

The principal international treaty governing ship construction, subdivision, stability, fire safety, lifesaving and equipment standards for seagoing vessels.

The international treaty regulating pollution prevention from ships, covering oil, noxious liquids, harmful substances, sewage, garbage and air emissions.