Page 19 - Introduction to Naval Architecture

8 DEFINITION AND REGULATION

The draught of the ship at any point along its length is the distance
from the keel to the waterline. If a moulded draught is quoted it is
measured from the inside of the keel plating. For navigation purposes
it is important to know the maximum draught. This will be taken to the
bottom of any projection below keel such as a bulbous bow or sonar
dome. If a waterline is not quoted the design waterline is usually
intended. To aid the captain draught marks are placed near the bow
and stern and remote reading devices for draught are often provided.
The difference between the draughts forward and aft is referred to as
the trim. Trim is said to be by the bow or by the stern depending upon
whether the draught is greater forward or aft Often draughts are
quoted for the two perpendiculars. Being a flexible structure a ship will
usually be slighdy curved fore and aft. This curvature will vary with the
loading. The ship is said to hog or sag when the curvature is concave
down or up respectively. The amount of hog or sag is the difference
between the actual draught amidships and the mean of the draughts at
the fore and after perpendiculars.
Freeboard is the difference between the depth at side and the draught,
that is it is the height of the deck above the waterline. The freeboard is
usually greater at the bow and stern than at amidships. This helps
create a drier ship in waves. Freeboard is important in determining
stability at large angles (See Chapter 4).

Representing the hull form

The hull form is portrayed graphically by the lines plan or sheer plan
(Figure 2.4). This shows the various curves of intersection between the
hull and the three sets of orthogonal planes. Because the ship is
symmetrical, by convention only one half is shown. The curves showing
the intersections of the vertical fore and aft planes are grouped in the
sheer profile, the waterlines are grouped in the half breadth plan; and the
sections by transverse planes in the body plan. In merchant ships the
transverse sections are numbered from aft to forward. In warships they
are numbered from forward to aft although the forward half of the ship
is still, by tradition, shown on the right hand side of the body plan. The
distances of the various intersection points from the middle line plane
are called offsets.
Clearly the three sets of curves making up the lines plan are inter-
related as they represent the same three dimensional body. This inter-
dependency is used in manual fairing of the hull form, each set being
faired in turn and the changes in the other two noted. At the end of
the iteration the three sets will be mutually compatible. Fairing is
usually now carried out by computer. Indeed the form itself is often