Blueprint Reading

AFTER PERPENDICULAR - The aft perpendicular can be the perpendicular drawn through the aft side of the rudder post or through the center-line of the rudder pintles. The aft perpendicular is the aft reference line for all hydrostatic calculations.

ALIGNED SECTION - A section view in which some internal features are revolved into or out of the plane of the view.

APPLICATION BLOCK - A part of a drawing of a subassembly showing the reference number for the drawing of the assembly or adjacent subassembly.

ARCHITECT’S SCALE - Scale used when dimensions or measurements need to be expressed in feet and inches.

AXONOMETRIC PROJECTION - A set of three or more views in which the object appears to be rotated at an angle, so that more than one side is seen.


BASE LINE - A straight horizontal line at or near the bottom of the molded surface from which vertical heights are measured.

BLOCK DIAGRAM - A diagram in which the major components of a piece of equipment or a system are represented by squares, rectangles, or other geometric figures, and the normal order of progression of a signal or current flow is represented by lines.

BLUEPRINTS - Copies of mechanical or other types of technical drawings. Although blueprints used to be blue, modern reproduction techniques now permit printing of black-on-white as well as colors.

BREAK LINES - Lines to reduce the graphic size of an object, generally to conserve paper space. There are two types: the long, thin, ruled line with freehand zigzag and the short, thick, wavy freehand line.


CENTER LINES - Lines that indicate the center of a circle, arc, or any symmetrical object; consist of alternate long and short dashes evenly spaced.

COFFERDAM - An insulating space between two watertight bulkheads or decks within a ship. A cofferdam can be a void (empty) space or a ballast space. Cofferdams are usually employed to ensure that oil or other chemicals do not leak into machinery spaces. If two different cargoes that react dangerously with each other are carried on the same vessel, one or more cofferdams are usually required between the cargo spaces.

CONSTRUCTION LINES - Lightly drawn lines used in the preliminary layout of a drawing.

CUTTING PLANE LINE - A line indicating a plane or planes from which a sectional view is taken.


DIMENSION LINE - A thin broken line (except in the case of structural drafting) with each end terminating with an arrowhead; used to define the dimensions of an object.


ELEVATION - A drawing showing the front, rear, and sides of a structure, as they would appear projected on vertical planes.


FORWARD PERPENDICULAR - A vertical line through the intersection of the stem with the load water line.

FULL SECTION - A sectional view that passes entirely through the object.


HATCHING - Lines that are drawn on the internal surface of sectional views. Hatching is used to define the kind or type of material the section surface consists of.

HIDDEN LINES - Medium, short, dashed lines that indicate the hidden features of the object being drawn.


ISOMETRIC PROJECTION - A form of graphical projection, or more specifically, a form of axonometric projection; a method of visually representing three-dimensional objects in two dimensions, in which the three coordinate axes appear equally foreshortened and the angles between any two of them are 120 degrees.


LONGITUDINAL FRAMING - Longitudinal framing is a method of ship construction in which large, widely spaced transverse frames are used in conjunction with light, closely spaced longitudinal members. This method lends a ship much greater longitudinal strength than in ships built in the traditional method, where a series of transverse frames were fit together closely from the keel upwards.


OBLIQUE PROJECTION - A view produced when the projectors are at an angle to the plane of the object illustrated. Vertical lines in the view may not have the same scale as horizontal lines.

OFFSET SECTION = A section view of two or more planes in an object to show features that do not lie in the same plane.

ORTHOGRAPHIC PROJECTION - A means of representing a three-dimensional object in two dimensions; with multiview orthographic projections, up to six pictures of an object are produced, with each projection plane parallel to one of the coordinate axes of the object.


PARTIAL SECTION - A sectional view consisting of less than a half section, which is used to show the internal structure of a small portion of an object, also known as a broken section.

PHANTOM LINES - Lines showing the alternate position of a movable object.

PLAN VIEW - A view of an object or area as it would appear from directly above.


SECTION LINES - Thick solid lines with an arrowhead used to indicate the direction in which a section or plane is viewed or taken.

STATION NUMBERS - Designations of reference lines used to indicate linear positions along a component such as an airframe or a ship’s hull.