GATE NM flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Block coefficient C_B

Ratio of the displaced volume to the volume of a rectangular block of length L, breadth B and draft T: C_B = ∇/(LBT). It expresses hull fullness; tankers and bulk carriers have high values near 0.8–0.85.

Metacentric height GM

Distance from the centre of gravity G to the transverse metacentre M, equal to KM minus KG. Positive GM gives initial stability; GZ = GM sin θ for small heel angles.

Free surface correction

Reduction in effective GM caused by liquid free surfaces in slack tanks, equal to Σ(ρᵢIᵢ)/(ρ∇). It lowers stability without changing the ship's actual KG.

Froude number

Dimensionless speed ratio Fn = V/√(gL), where L is waterline length. It governs wave-making resistance and is the similarity parameter used to scale model wave patterns.

Reynolds number

Ratio of inertial to viscous forces, Re = VL/ν. It governs boundary layer regime and frictional resistance; model tests must correct for scale effects using the ITTC-57 line.

Advance ratio J

Propeller parameter J = V_A/(nD), where V_A is advance velocity, n revolutions per second and D diameter. Thrust and torque coefficients K_T and K_Q are plotted against J.

Section modulus Z

Ratio of second moment of area to distance from neutral axis, Z = I/y. Bending stress is σ = M/Z, so larger Z reduces deck and bottom stresses under the same bending moment.

Hogging and sagging

Hogging is a longitudinal bending condition with deck in tension and bottom in compression; sagging is the reverse. Wave bending moments add to still-water moments in design.

Shear flow q

Shear force per unit length in a thin-walled section, q = VQ/I, where Q is the first moment of area outboard of the cut. Shear stress follows as τ = q/t.

Deadweight

Total mass a ship can carry when loaded to its assigned load line: cargo, fuel, fresh water, stores, crew and effects. Displacement equals lightship weight plus deadweight.

Load line freeboard

Minimum distance from the load line to the freeboard deck, set by ICLL rules. It depends on tabular freeboard, block coefficient, depth and superstructure credits.

Brake thermal efficiency

Ratio of brake power output to fuel energy input, η_b = 3600/(SFC × LCV) when SFC is in g/kWh and LCV in kJ/g. It expresses how much fuel energy becomes shaft work.

Specific fuel consumption

Fuel mass burned per unit of brake work, usually g/kWh: SFC = fuel flow (g/h) divided by brake power (kW). Lower values indicate a more efficient engine.

Pump affinity laws

For a geometrically similar pump at speed n: flow Q ∝ n, head H ∝ n² and power P ∝ n³. These laws scale performance curves and locate new operating points.

SOLAS steering gear rule

At rated speed the main steering gear must turn the rudder from 35° on one side to 30° on the other in not more than 28 seconds, with the ship at full load draft.

Bernoulli equation

Along a steady, inviscid, incompressible streamline, p + ½ρV² + ρgz is constant. It links pressure, velocity and elevation and gives the total head.

Added mass

Effective inertia added to a body accelerating in a fluid, caused by accelerating surrounding water. Ship heave and sway responses use mass plus added mass in the equations of motion.

Natural roll period

Approximate roll period T ≈ 2πk/√(g·GM), where k is the roll radius of gyration. Large GM gives short, stiff rolling; small GM gives long, easy rolling.

Encounter frequency

Wave frequency experienced by a moving ship, ω_e = ω − kU cos μ, where U is ship speed and μ the heading angle. It shifts the frequency of excitation seen by the hull.

Carbon equivalent CE

Formula CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 used to assess weldability. Higher CE raises hardenability and cold-cracking risk, requiring preheat.

Hydrogen cold cracking

Delayed cracking in weld metal or heat-affected zone caused by hydrogen, restraint and hard microstructure. Controlled by low-hydrogen electrodes, preheat and post-weld heat treatment.

Lamellar tearing

Step-like cracking under welds in thick plate, caused by through-thickness strain and non-metallic inclusions. It occurs in restrained T-joints and is reduced by low-sulphur steel.

ICCP system

Impressed current cathodic protection uses an external DC source and inert anodes to polarise the hull below its corrosion potential, supplementing or replacing sacrificial anodes.

Simpson's 1/3 rule

Numerical integration rule for an even number of equal intervals: integral ≈ (h/3)[f₀ + 4f₁ + 2f₂ + … + 4f_{n−1} + f_n]. Its error is order h⁴.

Beam deflection formula

For a simply supported beam with a central point load P, maximum deflection is PL³/(48EI). For uniform load w it is 5wl⁴/(384EI), with E modulus and I second moment.

Work–rate formula

If A alone takes x days and B alone y days, together they take xy/(x+y) days. Convert each worker's contribution to work per day before combining.

Successive percentage change

Two successive changes of +a% and −b% produce a net change of a − b − ab/100 percent. The cross term means the result is not simply a − b.

Prismatic coefficient C_P

Ratio of displaced volume to the volume of a prism of midship area and length: C_P = ∇/(A_M L) = C_B/C_M. It indicates how full the ends of the hull are.

MCT1cm

Moment to change trim by one centimetre, MCT1cm = Δ·GM_L/(100L), where GM_L is the longitudinal metacentric height. It converts trimming moments into trim change.

Quasi-propulsive coefficient

Overall propulsive efficiency η_D = η_O·η_H·η_R, combining open-water efficiency, hull efficiency and relative rotative efficiency. It links delivered power to effective power.

Plate buckling stress

Critical buckling stress of a plate is σ_cr = kπ²E/(12(1−ν²))·(t/b)². It falls rapidly as the stiffener spacing b increases relative to plate thickness t.

Continuity equation

For steady incompressible flow, A₁V₁ = A₂V₂: volume flow is conserved through a stream tube. Combined with the momentum equation it gives forces on control volumes.