Page 16 - Introduction to Naval Architecture

Definition and

2 regulation

DEFINITION

A ship's hull form helps determine most of its main attributes; its
stability characteristics; its resistance and therefore the power needed
for a given speed; its seaworthiness; its manoeuvrability and its load
carrying capacity. It is important, therefore, that the hull shape should
be defined with some precision and unambiguously. To achieve this the
basic descriptors used must be defined. Not all authorities use the same
definitions and it is important that the reader of a document checks
upon the exact definitions applying. Those used in this chapter cover
those used by Lloyd's Register and the United Kingdom Ministry of
Defence. Most are internationally accepted. Standard units and
notation are discussed in the Appendix.

The geometry
A ship's hull is three dimensional and, except in a very few cases, is
symmetrical about a fore and aft plane. Throughout this book a
symmetrical hull form is assumed. The hull shape is defined by its
intersection with three sets of mutually orthogonal planes. The
horizontal planes are known as waterplanes and the lines of intersection
are known as waterlines. The planes parallel to the middle line plane cut
the hull in buttock (or bow and buttock) lines, the middle line plane itself
defining the profile. The intersections of the athwartships planes define
the transverse sections.
Three different lengths are used to define the ship (Figure 2.1). The
length between perpendiculars (Ibp), the Rule length of Lloyd's Register, is
the distance measured along the summer load waterplane (the design
waterplane in the case of warships) from the after to the fore
perpendicular. The after perpendicular is taken as the after side of the
rudder post, where fitted, or the line passing through the centreline of
the rudder pintles. The fore perpendicular is the vertical line through the
intersection of the forward side of the stem with the summer load
waterline.

5