GATE NM key terms

The vertical distance between a vessel's centre of gravity and its metacenter. A positive value indicates stable equilibrium in roll; a negative value means the ship heels further once disturbed and may capsize.

The point about which a floating body appears to rotate for small angles of heel, found where the vertical line through the new centre of buoyancy meets the original upright centreline. Its height above the keel is denoted KM.

The horizontal offset between the centre of gravity and the centre of buoyancy at a given angle of heel. Multiplied by displacement and gravity it gives the righting moment restoring the ship upright.

The heel angle at which the righting lever curve falls back to zero, so the ship no longer generates a restoring moment. Beyond it the vessel capsizes unless the heeling cause is removed.

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

The centroid of the displaced water volume, through which the total buoyant force acts vertically upward. Its position moves as the underwater hull shape changes with heel, trim, or loading.

The watertight volume above the waterline that can be flooded before the vessel sinks. It is expressed as a percentage of total volume and is protected by watertight subdivision and closing appliances.

The vertical distance from the summer load waterline to the freeboard deck at side. It provides reserve buoyancy and is fixed by load line assignment rules based on hull geometry and service restrictions.

The disc and horizontal line painted amidships showing the maximum permitted draught in different seasons and water densities. It is assigned under international convention and enforced at each port.

The total mass a ship can carry at a stated draught: cargo, fuel, fresh water, ballast, stores, crew, and passengers. It equals loaded displacement minus lightship displacement.

The mass of the complete vessel with machinery, outfit, and fixed equipment but no cargo, consumables, ballast, crew, or passengers. It is the baseline for deriving deadweight and stability conditions.

The ratio of the immersed volume to the product of length, breadth, and draught of a rectangular block enclosing the hull. Full forms such as tankers have high values near 0.85; fine forms are lower.

The ratio of the immersed volume to the product of midship section area and waterline length. It expresses how the sectional area is distributed along the length and strongly influences wave-making resistance.

A dimensionless ratio of speed to the square root of gravitational acceleration times length. It governs wave-making behaviour, so geometrically similar hulls at equal Froude number have comparable wave resistance.

The component of ship resistance caused by energy radiated as the hull generates a wave system. It rises sharply near a Froude number of about 0.4 and dominates at higher speeds.

Resistance arising from skin friction on the wetted surface plus pressure drag from flow separation, especially at the stern. It is estimated with friction lines and form factors in model extrapolation.

The fractional reduction in inflow speed at the propeller compared with ship speed, caused by the hull slowing the water around it. It links thrust power to effective power in propulsion calculations.

The fractional increase in hull resistance caused by propeller action lowering pressure over the afterbody. It reduces the thrust effectively available to drive the ship forward.

The ratio of effective power needed to tow the ship to the shaft or brake power delivered by the engine. It combines hull efficiency, propeller open-water efficiency, and shafting losses.

Formation and collapse of vapour bubbles in low-pressure regions of a propeller blade or hull. Collapse causes noise, erosion, vibration, and thrust loss, and limits blade loading and ship speed.

The difference between the theoretical advance of a propeller in one revolution if it turned in a solid medium and its actual advance through water, usually expressed as a percentage.

The second moment of area of a hull cross-section divided by the distance to the extreme fibre. It converts bending moment into bending stress, so adequate values limit deck and keel stresses.

The idealised beam formed by the ship's continuous longitudinal structure, assumed to resist overall bending between wave crests and troughs. Its strength is assessed by section modulus and permissible stress.

The bending moment on the hull girder caused by unequal weight and buoyancy distribution along the length. Hogging puts the deck in tension; sagging puts the bottom in tension.

Vertical shear force is the integral of load per unit length along the hull; shear lag describes non-uniform stress across wide decks because plane sections do not remain plane.

A material's ability to withstand repeated cyclic stress below its static yield point without cracking. Welded details in way of stress concentrations are the usual initiation sites.

Sudden cleavage failure with little plastic deformation, often starting at a weld defect in a region of low temperature and high restraint. Toughness testing and steel grade selection guard against it.

Local amplification of stress where geometry changes abruptly, such as at openings, brackets, or weld toes. It raises peak stress above nominal values and promotes fatigue cracking.

The capacity of a steel to be joined by welding into a sound joint without cracking or unacceptable loss of properties. It depends on composition, notably carbon equivalent, and on welding procedure.

An index combining carbon and other alloying elements to indicate how readily a steel hardens and cracks when welded. Higher values demand preheat, controlled heat input, and low-hydrogen consumables.

The parent metal adjacent to a weld that is altered by welding heat but not melted. It can suffer grain growth, hardening, or toughness loss, so it often controls joint performance.

A dimensionless ratio of inertial to viscous forces, equal to velocity times length divided by kinematic viscosity. It determines laminar or turbulent flow and is essential in resistance scaling.

An energy relation along a streamline for steady, incompressible, frictionless flow, stating that pressure head plus velocity head plus elevation head stays constant. It underpins lift, draft, and piping analysis.

The extra inertia a body appears to gain when accelerating through fluid, because surrounding water must also be accelerated. It matters in roll damping, slamming, and mooring load estimates.

The natural time for one complete transverse oscillation of a vessel. It grows with radius of gyration and shrinks with greater metacentric height, so it is a practical stability check.

An on-board test after construction measuring speed, power, manoeuvrability, and anchor handling under defined conditions. Results verify contractual performance and reveal machinery or hull deficiencies.

The power delivered along the propeller shaft after gearbox and bearing losses. It is measured by torsion meter or telemetry and compared with brake power to obtain transmission efficiency.

A rule expressing the probability of a hypothesis given evidence in terms of prior probability and likelihood. Used in General Aptitude for revising estimates when new information arrives.

The probability that event A occurs given that event B has occurred, written P(A|B) and equal to the joint probability divided by P(B). It underlies reliability and diagnostic reasoning problems.