A fundamental drawing in the design of a ship-particularly relative to resistance is the sectional area curve, shown in Figure below for a ship with some parallel middle body.

The sectional area curve represents the longitudinal distribution of cross sectional area below the DWL. The ordinates of a sectional area curve are plotted in distance squared units. Inasmuch as the horizontal scale, or abscissa, of Figure above represents longitudinal distances along the ship, it is clear that the area under the curve represents the volume of water displaced by the vessel up to the DWL, or volume of displacement.

Alternatively, the ordinate and abscissa of the curve may be made non-dimensional by dividing by the midship area and length of ship, respectively. In either case, the shape of the sectional area curve determines the relative "fullness" of the ship.

The presence of parallel middle body is manifested by that portion of the sectional area curve parallel to the baseline of the curve. The shoulder is defined as the region of generally greater curvature (smaller radius of curvature) where the middle body portion of the curve joins the inward sloping portions at bow or stern.

The centroid of the vessel's sectional area curve is at the same longitudinal location as the center of buoyancy, LCB, and the ratio of the area under the sectional area curve to the area of a circumscribing rectangle is equal to the prismatic coefficient, .

Figure above also shows the customary division of the underwater body into forebody and afterbody, forward of and abaft amidships, respectively. Entrance and run, which represent the ends of the vessel forward of and abaft the parallel middle body, are also shown.

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