It is obvious that displacement will vary with the
depth of a ship’s keel below the water line that is known
as draft. As the draft increases, the displacement
increases. This is indicated in figure 12-13 by a series
of displacements shown for successive draft lines on
the midship section of a ship. The volume of an
underwater body for a given draft line can be measured
in the drafting room by using graphic or mathematical
means. This is done for a series of drafts throughout the
probable range of displacements in which a ship is
likely to operate. The values obtained are plotted on a
grid on which feet of draft are measured vertically and
tons of displacement horizontally. A smooth line is
faired through the points plotted, providing a curve of
displacement versus draft, or a DISPLACEMENT
CURVE as it is generally called. An example of this for
a typical warship is shown in figure 12-14.