PE Naval Architecture key terms

The weight of water displaced by a ship's hull, which equals the total weight of the ship and everything aboard when floating in equilibrium. Expressed in tonnes, it is the basis for most hydrostatic calculations.

The total weight a ship can carry when loaded to its assigned load line, comprising cargo, fuel, fresh water, ballast, stores, crew and passengers. It equals loaded displacement minus lightship weight.

The point where the vertical line through the centre of buoyancy of a slightly heeled vessel crosses the ship's centreline. For small angles the vessel rotates about it, and its height above the keel governs initial stability.

The vertical distance between the centre of gravity and the transverse metacenter. A positive value indicates initial stability; its magnitude shows whether a ship is stiff, with a short roll, or tender.

The horizontal distance GZ between the vertical through the centre of gravity and the vertical through the centre of buoyancy at a given heel angle. Multiplied by displacement it gives the righting moment.

The apparent rise in a ship's centre of gravity caused by liquid shifting in a partly filled tank as the vessel heels. It reduces metacentric height and is minimised by longitudinal subdivision of tanks.

The angle of heel at which a ship with negative metacentric height rests in equilibrium with zero righting arm, the GZ curve rising on either side. The vessel may flop violently to the opposite side.

The ratio of a hull's displaced volume to the volume of a rectangular block with the ship's waterline length, breadth and draught. High values indicate full, box-shaped forms such as tankers.

The vertical distance from the waterline to the freeboard deck line at side amidships for the assigned load line. It provides reserve buoyancy and is governed by international load line rules.

A test in which known weights are shifted transversely to heel a completed lightship, the resulting heel is measured, and the lightship centre of gravity and metacentric height are calculated.

A non-dimensional speed parameter equal to velocity divided by the square root of gravitational acceleration times a length. It governs wave-making resistance and the wave pattern a hull generates.

The resistance component caused by energy radiated away in the wave system the hull creates. It rises steeply with Froude number and dominates total resistance at higher speeds relative to length.

Resistance arising from shear stress as water flows over the wetted surface of the hull. It is estimated from friction lines such as ITTC-57 and depends on length, surface roughness and speed.

The power needed to tow the ship at a given speed, equal to total resistance multiplied by speed. It is the useful output that the propulsion system must ultimately deliver.

The fraction by which propeller thrust exceeds hull resistance, because the propeller alters pressure and flow around the stern and so increases the resistance the hull experiences.

The ratio of the speed of water entering the propeller to the ship's speed through the water, representing the slowing of flow around the hull. It links thrust power to effective power.

The ratio of effective power to thrust power, equal to (1 − t) divided by (1 − w). It expresses how hull and propeller interaction modifies overall propulsive efficiency.

The ratio of effective power to the power delivered to the propeller, combining hull efficiency, open-water propeller efficiency and relative rotative efficiency. It is used to predict required shaft power.

The formation and collapse of vapour bubbles on a propeller blade where local pressure falls below vapour pressure. It causes erosion, noise, vibration and thrust loss, and limits blade loading.

The second moment of area of the hull's transverse section about its neutral axis divided by the distance to the extreme fibre. Checked at deck and keel, it measures resistance to longitudinal bending.

The bending moment on the hull girder calculated from the distribution of weight and buoyancy in calm water for a particular loading condition, with no wave effects included.

The two extremes of longitudinal bending. Hogging occurs when excess buoyancy amidships makes the ends droop; sagging occurs when the ends are more buoyant and the midship section bends downward.

The dimensions of structural members such as plate thickness and stiffener size, determined by classification society rules so the structure withstands design loads with adequate margin.

Failure of a slender member or plate by lateral deflection under compressive load before the material yields. It depends on slenderness, end conditions and initial imperfections in the structure.

A watertight transverse bulkhead fitted a specified minimum distance aft of the forward perpendicular, intended to limit flooding if the bow is damaged in a collision.

The six independent motions of a ship: three translations called surge, sway and heave, and three rotations called roll, pitch and yaw. Seakeeping analysis predicts each response to waves.

The period of free roll oscillation of a ship in calm water, approximately proportional to beam divided by the square root of metacentric height. It sets the condition for resonance with waves.

A transfer function giving the amplitude of a ship motion or load per unit wave amplitude at each encounter frequency, used with a wave spectrum to predict responses in irregular seas.

Large roll oscillations excited by periodic variation of metacentric height as a ship meets waves alternately at bow and stern, typically in following or head seas with wavelength near ship length.

Severe impact loading when the bow or bottom leaves the water and re-enters at high relative velocity, producing high local pressures and whipping vibration of the hull girder.

The highest power an engine or propulsion system is designed to deliver continuously under stated conditions. It is the reference for loading limits, fuel consumption and ship speed trials.

The mass of fuel burned per unit of power produced per unit time, commonly in grams per kilowatt-hour. It is the key indicator of engine efficiency and voyage fuel cost.

A propeller whose blades rotate about their own axes to change pitch while running, so thrust magnitude and direction can be adjusted without stopping or reversing the shaft.

Shipboard systems that remove or kill organisms in ballast water before discharge, meeting the Ballast Water Management Convention to prevent transfer of invasive aquatic species between ports.

An iterative design process in which principal dimensions, weights, power, capacities and cost are estimated and successively refined until a consistent design satisfying the requirements is reached.

A drawing defining hull form through body plan, half-breadth plan and sheer plan, showing waterlines, buttocks and sections from which the hull surface is faired and built.

A non-dimensional measure of a ship's total enclosed volume under the international tonnage convention, used for regulatory, manning and port-charge purposes rather than as a weight.

The practice of installing pipes, ducts, cabling and equipment into hull blocks in the workshop before erection, improving access, safety and productivity during ship construction.