PE Naval Architecture flashcards

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

Metacentric height GM

Vertical distance from the centre of gravity to the transverse metacentre, computed as GM = KB + BM − KG. Positive GM gives initial stability; negative GM produces an angle of loll.

Block coefficient

Ratio of the displaced volume to the volume of the enclosing rectangular block L×B×T. It ranges from about 0.5 for fine vessels to 0.85 for full tankers.

Free surface correction

Reduction in metacentric height caused by liquid with a free surface in a tank. It equals the liquid density times the free-surface second moment divided by displacement, FSC = ρ·i/Δ.

Tons per centimetre immersion

Mass in tonnes that must be loaded to change mean draft by one centimetre, given by TPC = A_w·ρ/100 where A_w is the waterplane area at that draft.

Angle of loll

The heel angle at which a ship with negative metacentric height rests in neutral equilibrium, satisfying tan²θ = −2GM/BM. The vessel is unstable upright but stable at the loll angle.

Moment to change trim

Moment required to change trim by one centimetre, MCTC = Δ·GM_L/(100L). It converts a longitudinal weight shift into the resulting change of trim in metres.

ITTC 1957 friction line

Standard skin-friction coefficient correlation C_F = 0.075/(log₁₀Re − 2)², used with Reynolds number Re = VL/ν to scale model frictional resistance to full scale.

Advance ratio

Non-dimensional propeller operating parameter J = V_A/(nD), the ratio of advance velocity to rotational speed times diameter. It fixes the point on the open-water diagram.

Thrust deduction fraction

Fractional loss of thrust, t = (T − R)/T, caused by the propeller lowering pressure over the stern and thereby increasing hull resistance. Typical values are 0.1 to 0.3.

Wake fraction

Ratio w = (V − V_A)/V relating the undisturbed ship speed to the reduced velocity at the propeller disc. Single-screw full hulls commonly show values of 0.2 to 0.4.

Open water efficiency

Ratio of useful propeller work to shaft work in uniform inflow, η_O = J·K_T/(2πK_Q). Well-designed propellers typically reach 0.5 to 0.7.

Admiralty coefficient

Empirical relation P = Δ^(2/3)·V³/C used to estimate required power for a new design from a similar existing ship of known displacement, speed and power.

Section modulus

Ratio SM = I/y of the hull girder second moment of area to the distance from the neutral axis to the extreme fibre. Bending stress follows as σ = M/SM.

Wave bending moment

Wave-induced hull girder moment defined by IACS rules: hogging 0.19·C·L²·B·C_B and sagging 0.11·C·L²·B·(C_B + 0.7) in kN·m, with C depending on length.

Miner's rule

Linear cumulative fatigue damage rule D = Σ n_i/N_i, where n_i is the number of cycles at a stress range and N_i the allowable cycles from the S-N curve. D of one or more predicts failure.

Longitudinal framing

Framing arrangement using closely spaced longitudinal stiffeners supported by widely spaced transverse webs. It gives efficient resistance to hull girder bending and is standard in deck and bottom.

Shear force and bending moment

Along the hull girder the load curve integrates to shear force and shear force integrates to bending moment, so dM/dx = Q. This underlies still-water bending moment calculations.

Encounter frequency

Wave frequency experienced in the ship frame, ω_e = ω − (ω²V/g)·cos μ, where μ is the heading angle. Head seas increase it; following seas decrease it.

Significant wave height

Statistical sea-state measure defined as the mean of the highest third of wave heights. It equals four times the square root of the spectral area, H_1/3 = 4√m₀.

Roll natural period

Period of free roll in calm water, T_R = 2π·k_x/√(g·GM), with roll gyradius k_x about 0.35B. A large metacentric height shortens the period and stiffens rolling.

Response amplitude operator

Transfer function giving ship motion amplitude per unit wave amplitude as a function of frequency. Squaring it and multiplying by the wave spectrum yields the response spectrum.

Parametric roll

Large resonant roll excited by periodic variation of metacentric height as waves pass the bow and stern alternately, occurring typically in head or following seas at about twice roll frequency.

Propeller law

For a fixed-pitch propeller, absorbed power varies as the cube of shaft speed and thrust as the square. It defines the propeller demand curve used for engine matching.

Specific fuel consumption

Mass of fuel consumed per unit of brake work per hour, SFOC = fuel mass flow divided by brake power. Large two-stroke marine diesels achieve about 170 to 190 g/kWh.

Sulphur emission control area

Sea area designated under MARPOL Annex VI where fuel sulphur content is limited to 0.10% m/m, compared with the 0.50% global cap applied since 2020.

Oily water separator

Equipment separating oil from machinery bilge water before overboard discharge. MARPOL Annex I limits the effluent to 15 ppm oil content and requires monitoring and recording.

Energy Efficiency Design Index

Mandatory index for new ships expressing grams of CO₂ per tonne-mile of transport work. Required values tighten in phases to drive efficient hull, propeller and machinery design.

Design spiral

Iterative design process in which dimensions, weights, power and cost are estimated and refined in successive passes until the design satisfies all requirements consistently.

Prismatic coefficient

Ratio C_P = ∇/(A_M·L) of displacement volume to the volume of a prism with the midship section area and the same length. It expresses the fineness of the ends.

Deadweight

Total mass a ship can carry at the assigned load line, comprising cargo, fuel, lubricating oil, fresh water, ballast, stores, crew and passengers. Added to lightweight it gives displacement.

Required freight rate

Annual capital and operating cost of a ship divided by the annual cargo quantity carried. It is the standard economic measure for comparing competing design alternatives.

Block construction

Shipbuilding method in which the hull is assembled from prefabricated blocks in a building dock or berth, allowing parallel outfitting and reduced construction time.