The main parts of a typical ship together with the terms applied to the principal parts are illustrated in Fig. 4. I. Because, at first, they are of little interest or influence, superstructures and deckhouses are ignored and the hull of the ship is considered as a hollow body curved in all directions, surmounted by a watertight deck. Most ships have only one plane of symmetry, called the middle line plane which becomes the principal plane of reference. The shape of the ship cut by this plane is known as the sheer plan or profile. The design waterplane is a plane perpendicular to the middle line plane, chosen as a plane of reference at or near the horizontal; it may or may not be parallel to the keel. Planes perpendicular to both the middle line plane and the design waterplane are called transverse planes and a transverse section of the ship does, normally, exhibit symmetry about the middle line. Planes at right angles to the middle line plane, and parallel to the design waterplane are called waterplanes.
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