The Geometry of Ships

48 THE PRINCIPLES OF NAVAL ARCHITECTURE SERIES
eficial in this respect (Fig. 27). A longitudinal (body plan)
view gives good indications of the quantity and quality of
data, whether or not appendages were included, etc., but
of course lacks the crucial information of where each station
was located. An oblique perspective or orthographic
view is a good supplement to the body plan.
11.1 Curves of Form. For a vessel that operates at a
significant range of loadings (displacements), the hydrostatic
properties must be presented for a range of
flotation conditions. It is customary to use vessel draft
as the independent variable, and to tabulate properties
at a reasonable number of draft values. This information
is presented graphically in the curves of form drawing
(Fig. 36). In the curves of form, the draft is the vertical
axis (presumably, because draft is a vertical measurement),
and dependent quantities are plotted horizontally.
The plot is complicated by the fact that the various
hydrostatic quantities have different units and widely
varying magnitudes. Therefore, a generic “scale of
units” from 0 to 10 is used, and quantities are scaled by
powers of 10 (and sometimes other factors) to fit on the
plot. The scaling factors and units for each curve must
be supplied in the legends or keys. The range of draft
should go from somewhat below the minimum working
displacement to somewhat above the deepest loading
expected.
Curves of form present information primarily relevant
to zero-trim conditions. Hydrostatic quantities that are
expected in the curves of form include:
• Displacement (fresh and salt water)
• Longitudinal center of buoyancy
• Vertical center of buoyancy
• Waterplane area (displacement per unit immersion)
• Longitudinal center of flotation
• Transverse metacentric height (above keel)
• Longitudinal metacentric height (above keel).
Other quantities sometimes presented in curves of
form are:
• Wetted surface
• Form coefficients, e.g., block and prismatic
coefficients.
11.1.1 Displacement. Volume displacement is
the volume of the vessel below the plane of flotation.
Displacement is the weight of displaced fluid, i.e., g.
In SI units, displacement is given in metric tons, or kilograms
for small craft. Curves of form usually include
three displacement curves:
• Molded displacement in salt water
• Total (gross) displacement in fresh water
• Total (gross) displacement in salt water.
Fig. 36 Curves of form for the ship of Fig. 32.