Page 17 - Introduction to Naval Architecture

6 DEFINITION AND REGULATION

The length overall (loa) is the distance between the extreme points
forward and aft measured parallel to the summer (or design) waterline.
Forward the point may be on the raked stem or on a bulbous bow.
The length on the waterline (Iwl) is the length on the waterline, at which
the ship happens to be floating, between the intersections of the bow
and after end with the waterline. If not otherwise stated the summer
load (or design) waterline is to be understood.
The mid-point between the perpendiculars is called amidships or
midships. The section of the ship at this point by a plane normal to both
the summer waterplane and the centreline plane of the ship is called
the midship section. It may not be the largest section of the ship. Unless
otherwise defined the beam is usually quoted at amidships. The beam
(Figure 2.2) most commonly quoted is the moulded beam, which is the
greatest distance between the inside of plating on the two sides of the
ship at the greatest width at the section chosen. The breadth extreme is
measured to the outside of plating but will also take account of any
overhangs or flare.
The ship depth (Figure 2.2) varies along the length but is usually
quoted for amidships. As with breadth it is common to quote a moulded
depth, which is from the underside of the deck plating at the ship's side
to the top of the inner keel plate. Unless otherwise specified, the depth
is to the uppermost continuous deck. Where a rounded gunwhale is
fitted the convention used is indicated in Figure 2.2.
Sheer (Figure 2.1) is a measure of how much a deck rises towards the
stem and stern. It is defined as the height of the deck at side above the
deck at side amidships.