Page 22 - Introduction to Naval Architecture

10 DEFINITION AND REGULATION

generated directly from the early design processes in the computer,
Manual fairing is done first in the design office on a reduced scale
drawing. To aid production the lines used to be laid off, and re-
faired, full scale on the floor of a building known as the mould loft.
Many shipyards now use a reduced scale, say one-tenth, for use in the
building process. For computer designed ships the computer may
produce the set of offsets for setting out in the shipyard or, more
likely, it will provide computer tapes to be used in computer aided
manufacturing processes.
In some ships, particularly carriers of bulk cargo, the transverse
cross section is constant for some fore and aft distance near
amidships. This portion is known as the parallel middle body.
Where there are excrescences from the main hull, such as shaft
bossings or a sonar dome, these are treated as appendages and faired
separately.

Hull characteristics

Having defined the hull form it is possible to derive a number of
characteristics which have significance in determining the general
performance of the ship. As a floating body, a ship in equilibrium will
displace its own weight of water. This is explained in more detail in
Chapter 4. Thus the volume of the hull below the design load
waterline must represent a weight of water equal to the weight of the
ship at its designed load. This displacement, as it is called, can be
defined as:

D=pg

where p - the density of the water in which the ship is floating
g - the acceleration due to gravity
V = the underwater volume

It should be noted that displacement is a force and will be measured in
newtons.
For flotation, stability, and hydrodynamic performance generally, it is
this displacement, expressed either as a volume or a force, that is of
interest. For rule purposes Lloyd's Register also use a moulded
displacement which is the displacement within the moulded lines of the
ship between perpendiculars.
It is useful to have a feel for the fineness of the hull form. This is
provided by a number of form coefficients or coefficients of fineness. These
are defined as follows, where V is the volume of displacement: