Extended Operational and Trade Vocabulary

Naval Architecture, Stability, Resistance, and Structural Design glossary

Terminology of naval architecture and hydrodynamics: finite-element solvers such as ABAQUS for ultimate-strength and collision analysis, added mass in seakeeping, the Admiralty coefficient and advance ratio, perpendiculars and air draft, and the resistance, stability and structural-design concepts governing hull form, powering and scantling calculation.

215 defined terms.

A

Abaqus: Nonlinear finite element analysis suite developed by Dassault Systemes under the SIMULIA brand, widely used for ship structural and impact studies.
Added mass: Apparent increase in vessel mass due to fluid acceleration around the hull during motion.
Admiralty coefficient: Empirical powering relation P = (D^(2/3) V^3)/C used for first-cut shaft-power estimates.
Advance ratio J: Non-dimensional propeller operating parameter defined as J equals VA divided by (n times D), where VA is the speed of advance, n is revolutions per second, and D is propeller diameter.
Aft Perpendicular (AP): Reference point at the after side of the rudder post; basis for draft and trim calculations.
Air draft: Vertical distance from waterline to highest fixed point of the ship.
Allowable Still Water Bending Moment: Maximum hogging or sagging bending moment permitted by class rules in calm water for a given loading condition.
Amidships: Longitudinal midpoint between the forward and aft perpendiculars.
Angle of downflooding: Heel angle at which any unprotected opening through which progressive flooding could occur becomes submerged, a key stability cutoff in the IS Code.
Angle of loll: Static heel of a vessel with negative initial GM that has rolled to a position of positive righting arm.
Angle of vanishing stability: Heel angle at which the righting arm GZ returns to zero on the static stability curve, beyond which the ship will capsize.
Ansys Mechanical: Implicit finite element analysis solver from Ansys used for ship structural strength, vibration, and fatigue assessment.
Appendage resistance: Drag contribution of rudders, bilge keels, shafts, brackets, bossings, and stabilizer fins.
Archimedes' principle: Hydrostatic law stating that the buoyant force on a floating body equals the weight of fluid it displaces.
Attained Subdivision Index (A): Calculated probabilistic damage stability value under SOLAS Ch II-1 Part B-1; must equal or exceed R.
AUTOSHIP: An EU Horizon 2020 project that designed and demonstrated next generation autonomous ships for short sea and inland waterway operation.
AVEVA Marine: Integrated shipbuilding CAD/CAM suite from AVEVA combining hull, outfit, and diagrams capabilities derived from Tribon M3.
Axial inflow velocity field: Time-averaged or instantaneous axial wake distribution at the propeller plane that drives wake-adapted blade design.

B

Ballast condition: Loading condition with no cargo, only ballast water and consumables.
Bare hull resistance: Calm-water resistance excluding appendages.
Beam (B): Maximum breadth of the hull.
Becker Mewis Duct: Pre-swirl wake-equalizing duct with internal fins, developed by Becker Marine Systems, fitted ahead of the propeller.
Bilge keel: Longitudinal fin on the turn of bilge to damp roll motion.
Bjorn Andersson stator: Pre-swirl stator concept developed at SSPA Sweden for energy-saving retrofit applications.
Block coefficient (CB): Ratio of displaced volume to L*B*T.
Body plan: Drawing showing transverse section curves of the hull, used to fair the lines and define plate geometry.
Boundary layer: Thin viscous region adjacent to the hull.
BSRA series: British Ship Research Association methodical resistance series for single-screw merchant hulls of varying block coefficient, published in the 1960s.
Bulbous bow: Forward protrusion at the stem to reduce wave-making resistance.
Buoyancy: Upward hydrostatic force on a submerged or partially submerged body equal to the weight of displaced fluid.
Burrill cavitation chart: Empirical chart by L C Burrill (1943) relating propeller thrust loading to back cavitation percentage for merchant ship propellers.
BV (Bureau Veritas): Classification society issuing Statements of Compliance with HKC to recycling facilities including several Alang plots.

C

CADMATIC: Finnish CAD/CAM/CAE software developer whose marine suite covers hull, outfitting, electrical, and information management.
CCM: Computer-controlled manufacturing tooling and nesting suite used in steel hull fabrication shops.
Center of buoyancy (B): Centroid of the displaced volume.
Center of flotation (F): Centroid of the waterplane area.
Center of gravity (G): Mass centroid of the ship.
CFD: Computational fluid dynamics used for offshore structure hydrodynamic loading.
China Ship Scientific Research Center CSSRC: National hydrodynamic institute at Wuxi (founded 1951) with deep-water towing tank, cavitation tunnel, and seakeeping basin.
CLT propeller: Contracted and Loaded Tip propeller patented by Sistemar of Spain with end-plates that suppress tip vortex.
Coffin diagram: Plot of allowable hogging and sagging bending moments versus mean draft showing the loading-condition envelope.
Common Structural Rules CSR-H: IACS Harmonised CSR for bulk carriers and oil tankers, effective 1 July 2015, replacing the 2006 separate CSR-BC and CSR-OT.
Container ship design: Specialized design discipline producing cellular hulls with high deadweight to displacement ratio, hatchcover-free open-top variants, and torsional stiffness for open hatches.
CSSRC towing tank: 474 m by 14 m by 7 m deep-water tank at Wuxi, among the longest in the world.
CTO Polish Ship Hydromechanics Centre: Centrum Techniki Okretowej in Gdansk, the principal Polish marine model basin.

D

Damage Stability: Required for passenger and cargo ships under SOLAS II-1, Parts B-1 and B-2.
Deadweight (DWT): Difference between loaded and lightship displacement.
Deadweight scale: Tabulation of drafts versus deadweight at given densities.
Depth D: Vertical distance from baseline to the molded freeboard deck at side amidships.
Design wave method DWM: Direct calculation approach in which a single regular wave reproduces a target long-term response amplitude operator output for structural analysis.
Displacement (Δ): Mass of water displaced equal to vessel mass at floating equilibrium.
Displacement-length ratio: Δ/(0.01 L)^3 in long-ton/foot units.
DNV: Det Norske Veritas (DNV); IACS member classification society.
DNV Sesam: Integrated DNV software suite for hydrodynamic and structural analysis of ships and offshore structures.
Double hull: Submarine construction with separate pressure and outer hulls.
Double-bottom: Structural arrangement providing tanks below cargo holds.
Draft (T): Vertical distance from keel to waterline.
Dynamic stability: Energy required to heel the ship from upright to a given angle, equal to the area under the GZ curve between those angles times displacement.

E

Effective horsepower (EHP): Power required to tow the hull at given speed equal to R_T*V.
Effective wake fraction wT: Wake fraction derived from thrust identity in the propulsion factor analysis of self-propulsion tests.
Endo Watanabe trial analysis: Self-propulsion factor analysis procedure used in Japanese yards for converting trial data to power predictions.
Energopac: Wartsila integrated rudder and propeller bulb energy-saving device.
Entrance: Forward portion of the underwater hull from the bow to the start of parallel middle body, characterized by its half-angle of entrance.
Equivalent design wave EDW: Regular wave whose response amplitude operator output for a dominant load equals a target long-term value, used in CSR-H direct analyses.
Euler angles: Three rotation angles (roll phi, pitch theta, yaw psi) defining ship orientation relative to an earth-fixed frame.

F

FEM finite element method: Numerical technique discretizing structure into elements to solve stress, vibration, and buckling problems.
Foran: Shipbuilding CAD/CAM/CAE system from SENER (Spain) covering hull, outfitting, electrical, and accommodation.
FORCE Technology: Danish research and consulting institute operating the SimFlex Navigator series of full mission bridge simulators in Lyngby.
Form factor (1 plus k): Multiplier on flat-plate friction in the Prohaska/ITTC 1978 method that accounts for three-dimensional viscous effects.
Forward perpendicular (FP): Vertical line at the intersection of waterline and stem.
Free Surface Correction (FSC): Subtraction from GM to account for tank free surfaces.
Free surface effect FSE: Loss of stability due to liquids with free surfaces in slack tanks, modeled by the FSC.
Freeboard: Distance from waterline to freeboard deck, governed by the Load Lines Convention but referenced in SOLAS.
Froude number (Fn): V/sqrt(gL), governing wave-making similitude.
Froude's law: Similitude rule stating that wave-making resistance scales with the same Froude number between model and full scale.

G

Gawn series: Propeller series tested by R. W. L. Gawn at Haslar in the 1950s, used for naval and high-speed craft preliminary design.
GM (metacentric height): Distance from G to transverse metacenter M.
GM longitudinal GML: Distance from G to the longitudinal metacentric height, governing trim response and MCT1cm.
GTT Mark III: GTT membrane LNG containment system using stainless steel primary barrier with corrugations on insulation panels.
GTT Mark V: Latest GTT membrane system combining stainless steel primary barrier and Invar secondary barrier.
GTT NO96: GTT membrane LNG containment system using Invar primary and secondary barriers with plywood boxes filled with perlite.
Guldhammer-Harvald method: Resistance prediction procedure by H E Guldhammer and S A Harvald (Copenhagen 1974) based on systematic merchant-ship data.
GZ righting arm: Horizontal distance between the lines of action of weight and buoyancy at heel angle phi, equal to KN minus (KG times sin phi).

H

Half-breadth plan: Top view of waterlines, half-breadths shown.
Heeling moment: External moment about the longitudinal axis from wind, turning, or shifted weight that the righting moment must balance.
Hollenbach Method: Resistance prediction method for single and twin screw vessels published by Hollenbach in 1998.
Holtrop-Mennen method: Statistical resistance and powering prediction.
HSVA: Hamburgische Schiffbau-Versuchsanstalt (Hamburg Ship Model Basin), founded 1913, operating a deep-water tank, ice tank, and cavitation tunnel.
Hull efficiency etaH: Ratio of effective to thrust power equal to (1 minus t) divided by (1 minus w).
Hull form coefficients: Set of non-dimensional ratios (CB, CM, CP, CWP, CVP) characterizing fullness of the underwater hull.
Hull Girder Bending Moment: Vertical and horizontal bending moments governed by IACS UR S11 and S11A.
Hull girder section modulus (Z): Bending section modulus at deck and keel.
Hyundai Heavy Industries HHI HMRI: Hyundai Maritime Research Institute towing tank and cavitation tunnel in Ulsan, South Korea.

I

IACS UR S Series: Unified Requirements on Strength of Ships from the International Association of Classification Societies (S1 longitudinal strength, S11 longitudinal strength standard, S17 watertight integrity, etc).
Ice class strengthening: Hull, propulsion, and machinery enhancements per IACS Polar Class or Finnish-Swedish Ice Class Rules to operate in ice-covered waters.
IMO Severe Wind and Rolling criterion: Weather criterion of the 2008 IS Code requiring that the ship withstand a beam wind plus rolling without exceeding downflooding angle.
INA Institute of Naval Architects: Predecessor (founded London 1860) of the Royal Institution of Naval Architects, granted Royal status in 1960.
Initial stability: Stability range described by GM at small heel.
IS Code 2008: International Code on Intact Stability adopted by IMO Resolution MSC.267(85), mandatory under SOLAS and Load Line Convention.
ITTC: International Towing Tank Conference, the worldwide hydrodynamic research body.
ITTC 1957 friction line: Standard skin-friction line used in ITTC powering predictions, the framework within which propeller open-water data are scaled.
ITTC 1978 Performance Prediction Method: Procedure combining resistance, propulsion, and open-water test results with form factor to predict full-scale power.
ITTC procedure 7.5-02-02-01: Recommended procedure for resistance test execution and analysis.
ITTC procedure 7.5-02-03-01.1: Recommended procedure for propulsion test execution and analysis (1978 method).
ITTC procedure 7.5-02-07-04: Recommended procedure for seakeeping experiments in regular and irregular waves.

J

J advance ratio: See Advance ratio J.
JASNAOE: Japan Society of Naval Architects and Ocean Engineers, formed 2007 by merger of the Society of Naval Architects of Japan (1897) and other regional societies.

K

Kappel propeller: Tip-loaded propeller with smoothly curved tip developed by J. J. Kappel and MAN Diesel & Turbo (MAN ES).
KB: Vertical distance from keel (baseline) to the center of buoyancy.
KCD propellers: Kelvin and Clyde Dock methodical propeller series for transom-stern hulls, tested at Denny tank.
KCM: Korean Containership Model used as a benchmark hull in many CFD validation studies (with KVLCC2 from KRISO).
KG: Kommanditgesellschaft, German limited partnership, vehicle for retail ship funds.
KM: Vertical distance from keel to the transverse metacenter, equal to KB plus BM.
KN curves: Cross curves of stability giving the righting arm of form KN versus displacement at fixed heel angles, independent of KG.
KQ: Non-dimensional propeller torque coefficient KQ equals Q divided by (rho times n squared times D to the fifth).
KRISO: Korea Research Institute of Ships and Ocean Engineering, source of public domain KCS and KVLCC hulls.
KT: Non-dimensional propeller thrust coefficient KT equals T divided by (rho times n squared times D to the fourth).
KVLCC2: KRISO Very Large Crude Carrier benchmark hull.

L

LBP length between perpendiculars: Horizontal distance from forward perpendicular to aft perpendicular, the standard length for hydrostatic and rule calculations.
LCB longitudinal center of buoyancy: Longitudinal coordinate of the center of buoyancy, expressed as percentage of LBP forward or aft of amidships.
LCF longitudinal center of flotation: Longitudinal coordinate of the center of flotation about which the ship trims.
LCG longitudinal center of gravity: Longitudinal coordinate of the ship's center of gravity.
Length displacement ratio: See Slenderness ratio.
Lewis form: Two-parameter family of cross sections used in strip theory.
Lightship weight LWT: Weight of the completed ship excluding cargo, fuel, water, stores, crew, and effects, estimated by Watson-Gilfillan or built up from steel, outfit, and machinery weights.
Lines plan: Drawing of body, sheer, and half-breadth views.
LNG carrier MOSS: Spherical independent Type B tank design originated by Moss Maritime (Norway), recognizable by external aluminum spheres.
LOA length overall: Maximum length of the hull including bulwarks, stem, and stern extensions.
Longitudinal Strength: hull-girder bending and shear capacity, governed by IACS UR-S11 and CSR-H Part 1 Chapter 5.
LWL length waterline: Length of the hull at the design waterline.

M

MAESTRO: MAESTRO Marine global ship structural analysis software combining FEM modeling, rule checks, and optimization.
MARIN: Maritime Research Institute Netherlands hydrodynamic facility.
MARPOL Annex I probabilistic outflow: Regulation 23 methodology for calculating mean outflow performance of oil tankers in collision and grounding scenarios.
Maximum righting arm GZmax: Peak value of the GZ curve, criterion in the IS Code requiring GZmax at angles not less than 25 degrees.
MCT1cm: Moment to change trim 1 cm.
Metacenter (M): Intersection of buoyancy verticals at small heel.
Metacentric radius (BM): I_T/V, transverse moment of waterplane area over volume.
Mewis Duct: Pre-swirl wake-equalizing duct developed by Friedrich Mewis and Becker Marine Systems, widely retrofitted on tankers and bulkers.
Midship section coefficient CM: Ratio of midship section area below waterline to (B times T).

N

NAPA: Naval Architecture Package, Finnish naval architecture software suite originally developed in 1971.
NASTRAN: NASA Structural Analysis program, ancestor of MSC Nastran and NX Nastran.
Neutral axis: Horizontal axis through the centroid of the hull cross section about which longitudinal bending stresses change sign.
Newton-Rader propeller series: Three-bladed high-speed propeller series for planing craft tested by R N Newton and W A Rader in 1961.
NMRI: National Maritime Research Institute, Tokyo, Japan, formed 2001 from the former Ship Research Institute SRI.
Nyborg seakeeping basin: SSPA's seakeeping and maneuvering basin in Gothenburg, Sweden.

O

Offsets table: Tabulated half-breadths defining hull lines.
Open-water diagram: KT, KQ, eta_O versus J for a propeller in open water.
Open-water efficiency etaO: Efficiency of the isolated propeller eta0 equals (J times KT) divided by (2 pi times KQ).
Outflow performance: Probabilistic mean oil outflow parameter under MARPOL Annex I Regulation 23 for tankers above 5000 DWT.

P

Parallel middle body: Section of the hull, typically amidships, where cross-sectional shape is constant along the length.
Pinkster's frequency domain method: Mean and low-frequency wave drift force formulation widely used in seakeeping software.
Pre-swirl stator: Energy-saving device with vanes ahead of propeller.
Prismatic coefficient (CP): Volume divided by midship-section area times length.
Probabilistic Damage Stability: SOLAS Ch II-1 Part B-1 framework using R and A indices.
Promas: Integrated rudder-propeller propulsion concept from Kongsberg (formerly Rolls-Royce Marine).
Propeller pitch P: Theoretical axial advance per revolution of the helical blade reference surface.
Propulsive coefficient PC: Ratio of effective horsepower to shaft horsepower, equal to eta_D times shaft transmission efficiency.

Q

Quasi-propulsive coefficient QPC: Ratio of effective power to delivered power, equal to etaO times etaH times etaR.

R

Relative rotative efficiency etaR: Ratio of propeller efficiency behind the hull to its open-water efficiency at the same thrust or torque identity.
Required Subdivision Index (R): Minimum probabilistic damage stability index under SOLAS Ch II-1/6.
Residuary resistance: Total resistance minus frictional resistance, used in classical Froude decomposition.
Reynolds number (Re): VL/nu, governing viscous similitude.
Rhino Marine: Marine plug-in suite (Orca3D, RhinoMarine) extending McNeel Rhinoceros NURBS modeler for hull form fairing and hydrostatics.
Righting moment: Δ*GZ, restoring moment at given heel.
RINA: Royal Institution of Naval Architects, London, founded 1860.
Roll damping: Sum of wave-radiation, viscous skin friction, eddy-making, lift, and bilge-keel damping that limits resonant roll amplitude.
Roll Period: Time for one complete oscillation; approximated by T = 2 pi k / sqrt(g GM).

S

Schneekluth Duct: Wake-equalizing duct invented by H Schneekluth in 1984, an upstream nozzle ahead of the propeller.
Sea Trials: Pre-commissioning tests of naval vessels.
Second generation intact stability criteria: New IMO performance-based criteria (2nd Gen IS) addressing pure loss, parametric roll, dead ship, surf-riding/broaching, and excessive acceleration failure modes, finalized 2020 as MSC.1/Circ.1627.
Section modulus deck Zdeck: Section modulus to the upper deck, used to check tensile stresses in hogging.
Section modulus keel Zkeel: Section modulus to the bottom shell, used to check compressive stresses in hogging.
Sheer plan: Side view of waterlines.
Ship Constructor: ShipConstructor Software (now part of Hexagon AB) AutoCAD-based shipbuilding production CAD suite.
Simpson's first rule: 1-4-1 numerical integration applied to evenly spaced ordinates of hull section areas to compute displacement and moments.
Slenderness ratio: Length-displacement ratio L divided by the cube root of volume of displacement, large values denoting slender hulls.
Smith-McTaggart sea trial correction: Wind, wave, and current corrections applied to measured trial power per ITTC and ISO 15016 procedures.
SNAME: Society of Naval Architects and Marine Engineers, US-based.
SOLAS Chapter II-1 Part B-1: Probabilistic subdivision and damage stability regulations for cargo ships above 80 m and passenger ships, in force since 1 January 2009.
Speed-length ratio: Traditional speed parameter V (knots) divided by the square root of L (feet), related to Froude number.
SSPA: Statens Skeppsprovningsanstalt, Swedish national ship model basin founded 1940 in Gothenburg, now SSPA Sweden AB.
Stability booklet: Approved booklet of loading conditions and curves.
Standard wave bending moment: Vertical wave bending moment formula in IACS UR S11, MwH and MwS for hogging and sagging.
Static stability curve: Plot of righting arm GZ versus heel angle for a given displacement and KG, fundamental output of stability assessment.
Still water bending moment SWBM: Hull girder bending moment in calm water, computed from the longitudinal distribution of weight minus buoyancy.

T

Tanker design: Specialized design for liquid bulk carriage featuring double hull, segregated ballast tanks, inert gas systems, and compliance with MARPOL Annex I and IGC/IBC codes.
TCB transverse center of buoyancy: Transverse coordinate of the center of buoyancy, zero in upright symmetric hulls.
Thrust deduction t: Fraction by which propeller-induced pressure reduction increases hull resistance; T(1 - t) = R_T at self-propulsion.
Tonnes per centimeter immersion TPC: Mass that must be added to immerse the hull by one centimeter, equal to (Aw times rho) divided by 100.
Total resistance RT: Sum of frictional, viscous-pressure, wave-making, air, and appendage components acting opposite to motion.
Transverse stability: Stability of the ship about the longitudinal axis (roll), the primary intact stability concern.
Trim: Difference between forward and aft drafts.
Two-parameter Pierson-Moskowitz spectrum: Wave energy spectrum for fully developed seas used in seakeeping calculations.

U

Ultimate hull girder strength: Maximum longitudinal bending moment the hull girder can sustain before collapse, computed by progressive collapse methods (Smith method, ALPS/HULL, MAESTRO).
Underdeck volume: Total enclosed volume below the freeboard deck, basis for tonnage measurement under the 1969 Tonnage Convention.

V

VCB vertical center of buoyancy: Synonym for KB measured from the baseline.
VCG vertical center of gravity: Synonym for KG.
Vertical wave bending moment VWBM: Sagging or hogging moment induced by waves, given by IACS S11 formulas or direct calculation.
Viscous resistance: Sum of frictional and viscous-pressure resistance components, dominant at high Reynolds and low Froude numbers.
Volumetric coefficient CV: Non-dimensional ratio of displaced volume to length cubed, used in resistance regressions.

W

Wageningen B-series: Open-water propeller series from MARIN.
Wake fraction (w): (V - VA)/V at propeller plane.
Wasim: DNV Sesam time-domain seakeeping and slamming solver.
Waterplane area Aw: Area of the hull section cut by the waterline at a given draft.
Waterplane coefficient CWP: Ratio of waterplane area to circumscribing rectangle L by B.
Watson and Gilfillan: D G M Watson and A W Gilfillan, authors of preliminary ship design methods widely cited for lightship weight estimation (1976 RINA Transactions).
Wave-Making Resistance: Resistance component due to ship-generated waves resolved in free surface CFD.
Weather Criterion: See Severe Wind and Rolling Criterion.
Wetted surface area (S): Submerged hull area used in CF*0.5*rho*V^2*S.

X

X-Bow: Inverted-bow architecture pioneered by Ulstein, used on REV Ocean and other explorers.
X-type rudder: Cruciform rudder arrangement, common in submarine practice.

Y

Yaw: Rotational motion about the vertical axis.
Young's modulus: Elastic constant E for hull plating; for normal-strength steel grade A, E equals approximately 206 GPa.

Z

Z section modulus: See Hull girder section modulus Z.
Zero-speed seakeeping: Hydrodynamic analysis of moored or station-keeping vessels with Froude number near zero, dominated by wave-frequency and drift forces.