Glossary for the Statics and Dynamics of Marine Vessels Adapted from Introduction to Naval Architecture (1983), The ITTC Dictionary of Ship Hydrodynamics (1975), The ITTC Symbols and Terminology List (2005) Ship Design and Construction (1980), U. S. Navy Damage Control Manual (DCM, 1945), various textbooks, current IMO documents and Wikipedia Added mass—The total hydrodynamic force, per unit acceleration, exerted on a ship or other body in phase with and proportional to the acceleration. Advance—The distance by which the center of gravity (CG) of a ship advances in the first quadrant of a turn. It is measured parallel to the approach path, from the CG position at rudder execute to the CG position where the ship has changed heading by 90°. Maximum advance is the distance, measured parallel to the approach path from the CG position at rudder execute to the tangent to the path of the CG normal to the approach path. The first of these terms is the most commonly used. Advance coefficient (J)—A parameter relating the speed of advance of the propeller VA to the rate of rotation n, given by J = VA/nD, where D is the propeller diameter. The advance coefficient may also be defined in terms of ship speed V, in which case it is given by .JV, = V/nD. Afterbody—That portion of a ship's hull abaft amidships. After peak—The compartment in the stern, abaft the aftermost watertight bulkhead. After perpendicular—See length between perpendiculars. Amidships—In the vicinity of the midlength as distinguished from the ends. Technically it is exactly halfway between the forward and the after perpendiculars. Amplitude—The magnitude of the extreme of a sinusoidal quantity with respect to the mean value. The double amplitude is the magnitude of the difference of the extreme. Angle of attack (α)—The angle, measured in the plane containing the lift vector and the inflow velocity vector, between the velocity vector representing the relative motion between a body and a fluid and a characteristic line or plane of the body, such as the chord line of an airfoil or hydrofoil. Synonymous with angle of incidence. Appendages—The portions of a vessel extending beyond the main hull outline, including such items as rudder, shafting, struts, bossings, and bilge keels. Archimedes' Principle—The principle that states that buoyant force acting on a body partially or completely immersed in a fluid is equal to the weight of fluid displaced. Aspect ratio (AR)—The ratio between the span of a hydrofoil or airfoil, measured at right angles to the liquid flow, to the chord c of the hydrofoil, in the direction of flow. When the chord varies in length across the span, the aspect ratio is the span b divided by the mean chord c obtained generally by dividing the hydrofoil projected area A, into the square of the span b, 2 i.e. b /AP. Athwartship—Across the ship, at right angles to the fore-and-aft centerline. Back (of blade)—The side of a propeller which faces generally in the direction of ahead motion. This side of the blade is also known as the suction side of the blade because the average pressure there is lower than the pressure on the face of the blade during normal ahead operation. This side of the blade corresponds to the upper surface of an airfoil or wing. Ballast—Any solid or liquid weight placed in a ship to increase the draft, to change the trim, or to regulate the stability. Baseline—A fore-and-aft reference line at the upper surface of the flat plate keel at the centerline for flush shell-plated vessels, or the thickness of the gar board strake above that level for ships having lapseam shell plating. Vertical dimensions are measured from a horizontal plane through the baseline, often called the molded baseline. Beam (B)—A dimension expressing breadth or width of a body or ship in a transverse horizontal direction. When not otherwise defined the beam is the molded breadth of a ship, measured amidships at the designed waterline. According to the position where the breadth is measured, it is named beam, extreme: maximum beam wherever it occurs on the hull above or below water. beam, immersed, maximum: maximum beam of the underwater body. beam, maximum section (BX): beam measured on the designed waterline at the maximum section area. beam, midlength (BM): beam at the midsection of the designed waterline. Beam, molded—The maximum breadth of the hull measured between the inboard surfaces of the side shell plating of flush-plated ships, or between the inboard surfaces of the inside strakes of lap seam- plated vessels. Beam, deck—An athwartship horizontal structural member, usually a rolled shape, supporting a deck or flat. Bilge—Intersection of bottom and side. May be rounded or angular as in a chine form hull. The lower parts of holds, tanks, and machinery spaces where bilge water may accumulate. Bilge keel—A long longitudinal fin fitted at the turn of the bilge to reduce rolling. Commonly it consists of plating attached to the shell plating. Bilge strake—Course of shell plates at the bilge. Blockage—The effect of the boundaries of a channel or tunnel on the flow around a body. Blockage correction—A correction made to the results of hydrodynamic experiments made in a channel or tunnel of one cross-section in order to estimate the equivalent results for another cross-section. Specifically a correction made to the results of a resistance experiment in a towing tank in order to estimate the equivalent results in unrestricted water. Block coefficient (CB)—The ratio of the underwater volume of a ship to the volume of a rectangular block, the dimensions of which are the effective length, draft, and beam. The relationship is expressed as a decimal. Body plan—A drawing consisting of two half transverse elevations or end views of a ship, both having a common vertical centerline, so that the right-hand side represents the ship as seen from ahead, and the left- hand side as seen from astern. On the body plan appear the forms of the various cross sections, the curvature of the deck lines at the side, and the projections, as straight lines of the waterlines, the buttock lines, and the diagonal lines. Bollard pull—The pull force exerted by a ship at zero ship speed. It is the sum of the propeller thrust and the interaction force on the hull. Bonjean curves—A set of curves, each of which represents a plot of the cumulative area of a station on the lines plan, from the base line to any point above it. Bossing or boss—The curved swelling outboard portion of the ship's shell plating that surrounds and supports the propeller shaft. Boundary layer—The region of fluid close to a solid body where, due to viscosity, transverse gradients of velocity are large as compared with longitudinal variations, and shear stress is significant. The boundary layer may be laminar, turbulent, or transitional. Boundary layer thickness (δ)—The distance normal to the surface of a body at which the speed attains that in an equivalent inviscid flow. For practical purposes this is sometimes taken as 99 percent of the inviscid flow speed or 98 percent of the total head. Bow line—The intersection of the molded hull surface forward of amidships with any vertical longitudinal plane not on the centerline. See buttock. Bracket—A plate used to connect rigidly two or more structural parts, such as deck beam to frame, or bulkhead stiffener to the deck or tank top (usually triangular in shape). Breadth, molded— See beam, molded. Breakwater—Inclined bulwark-like structure on a weather deck to deflect overboard water coming over the bow or over the gunwale and moving aft. Breasthook—A triangular plate bracket joining port and starboard side stringers at the stem. Broaching—An involuntary and dangerous change in heading produced by a severe following or quartering sea. Bulkhead—A term applied to the vertical partition walls that subdivide the interior of a ship into compartments or rooms. The various types of bulkheads are distinguished by their location, use, kind of material, or method of fabrication, such as forepeak, longitudinal, transverse, watertight, wire mesh, and pilaster. Bulkheads that contribute to the strength of a vessel are called strength bulkheads, and those that are essential to the watertight subdivision are watertight or oil tight bulkheads. Gastight bulkheads serve to prevent the passage of gas or fumes. Bulkhead, after peak—A term applied to the first main transverse bulkhead forward of the sternpost. This bulkhead forms the forward boundary of the after peak tank. Bulkhead, collision or forepeak—The foremost main transverse watertight bulkhead. It extends from the bottom of the hold to the freeboard deck and is designed to keep water out of the forward hold in case of bow collision damage. Bulkhead deck—The bulkhead deck is the uppermost deck up to which the transverse watertight bulkheads and shell are carried. Bulkhead, screen—A term applied to a light nonwatertight transverse bulkhead fitted in some Great Lakes ore carriers. Its greater flexibility allows it to survive the effects of the unloading machinery. Buoyant Volume— Buttock—The intersection of the molded surface abaft amidships with any vertical longitudinal plane not on the centerline. See bow line. Camber—The rise or crown of a deck, athwartship; also called round of beam. Camber (of a hydrofoil)—The maximum separation of the mean line and the nose-tail line. Capacity plan—A plan outlining the spaces available for cargo, fuel, fresh water, water ballast, etc., and containing cubic or weight-capacity lists for such spaces and a scale showing deadweight capacities at varying drafts and displacements.