The Geometry of Ships

THE GEOMETRY OF SHIPS 55
Fig. 41
A sonar dome formed as an appendage to the original
fair surface.
dome, and does not have to worry about side effects on
the remainder of the surface.
A cavity such as a thruster tunnel (Fig. 26) is sometimes
treated as a negative appendage. The net freeflooding
volume of the tunnel is subtracted from the sum
of displacement volumes of other (positive) appendages.
12.5 Sailing Yacht Keels. A sailing yacht needs to
generate hydrodynamic lift forces to resist the component
of sail force that tends to push it sideways. It also
needs roll stability to resist the heeling moments arising
from sail forces. In monohull yachts, a keel appendage is
the most common answer to both these needs. A keel is
the repository for a substantial fraction of the yacht’s
total “all-up” weight — more than 80 percent in extreme
cases — and is shaped so as to carry this weight as low
as possible, while providing an effective lifting shape of
sufficient lateral area, adequate streamlining, and low
wetted surface. It must be stressed that the “lift” required
for a sailboat to sail is a horizontal force component,
not vertical as in an airplane. A sailing yacht can be
viewed (to a degree) as an airplane flying on its side,
with its two wings — keel on one side and sails on the
other — having quite different shapes and proportions
primarily because of the large difference in density (a
factor of about 830) between the two fluids they operate
in. (However, the analogy can only be taken so far; no
airplane derives significant propulsive force from the difference
in velocity at its right and left wings!)
Section 13
Arrangements and Capacity
13.1 Cargo Capacity and Tonnage. A basic characteristic
of any cargo ship is the quantity of cargo she is
able to carry — her cargo capacity. Two fundamental aspects
of capacity are
• Volume: how much space is available for cargo
stowage?
• Mass or weight: how much load can she carry?
These characteristics are, of course, crucial to the
ship’s commercial success.
The gross deadweight of a ship is the difference
between the full-load displacement (mass) and the
light-ship mass, i.e., mass of hull steel, machinery, and
outfit. The cargo deadweight is the result of deducting
from gross deadweight the maximum values of variable
masses of fuel, stores, fresh water, crew, and their
effects.
Registered tonnage is a volume measurement expressed
in “register tons” of 2.885 cubic meters (100
cubic ft.). The gross tonnage is the volume of all
enclosed spaces of the hull and superstructure. Net
tonnage is the gross measure, less deductions for nonrevenue-producing
spaces such as machinery space
and crew quarters. Net tonnage measurements are the
basis for some important operating costs such as harbor
dues, dockage fees, and canal tolls. Gross tonnage
is used as the basis for drydocking charges, and applicability
of various safety rules and regulations. Details
of tonnage and its determination are discussed in
Chapter 8 of Lamb (2003).
13.2 Compartmentation and Subdivision. The interior
space of a ship is subdivided into functional
subspaces or compartments suited to the vessel’s mission
and purpose. This is accomplished by partitions
analogous to the floors, ceilings, and interior walls that
subdivide a building into rooms. The partitions have
structural requirements related to the loads they must
support, and also are integrated into the general structure
of the ship, providing critical stiffening and
reinforcement for the hull shell, weather deck, and
superstructure.
Common classes of compartments are: cargo holds for
dry cargoes, cargo tanks for liquid cargoes, water ballast
tanks, machinery spaces, tanks for consumables, spaces
for stacking containers, accommodation spaces for crew
and passengers, and void spaces. Efficient layout of all