DEFINING THE SHIP GEOMETRY
As with any engineering product, precision is necessary in defining the geometry of a ship. Again, in common with most disciplines, an internationally recognised terminology has grown up over the years to aid this definition.
A ship’s hull is three dimensional and, as is usually the case, it is assumed here to be symmetrical about its middle line plane. The shape is defined by its intersections with three mutually orthogonal planes. The intersections with horizontal planes, known as waterplanes whether below or above water, are known as water lines. Those with the athwartships planes define the transverse sections of the hull and planes parallel to the middle line plane lead to what are termed bow and buttock lines. The external hull shape can be defined by the distances of the hull from the centreline plane at each transverse section and waterplane. In tabular form these are known as a table of offsets.