Page 329 - Introduction to Naval Architecture

314 SHIP DESIGN

to be close to the size needed. From this base the designer can get a first
approximation to the principal dimensions of the new ship. Allowance
would be made for different capacities, perhaps higher speed, smaller
crew and so on. A feel for the size of the ship will be obtained from the
weight or volume of cargo. The type ship will then give a guide to the
ratio of the dimensions but these can be modified to give the form
coefficients (block, prismatic, etc.) desired. The values of ratios such as
length to beam or draught need to be checked as being within the
usually accepted limits. Absolute dimensions must be compared with
limiting values for ports and waterways the ship is to use.
From the principal dimensions first assessments of draughts, stability,
power, etc., can be made. Each of these will lead to a better picture of
the design. It is an iterative process which has been likened by some to
a spiral. This analogy is helpful in that each ship feature must be
considered more than once and at the end of a cycle the designer
should have approached the final design more closely. However it gives
the impression that design is a steady progression which it is not. It
ignores the step functions that occur such as when a larger machinery
set has to be fitted or an extra bulkhead added. A better analogy might
be a network which showed the many interdependencies present in the
design. This network would really be a combination of a large number
of interactive loops.
Not all design features will be considered during every cycle of the
design process. Initial stability would be considered early on, large
angle stability would follow later but damaged stability would not be
dealt with until the design was better defined. The first estimate of
power, and hence machinery required, would be likely to be changed
and this might introduce a step change in machinery weight and
volume. There would be corresponding changes in structural weight
and so the design develops. Some of the initial assessments, possibly
that of the longitudinal bending moment, would be by using
approximate formulae. When the design is reasonably defined more
advanced computer programs can be employed.
When a reasonably conventional ship is being built to the rules of a
classification society the scantlings of the structure can be obtained
from the rules. For merchant ships of novel design, and for warships, it
is necessary to carry out a full strength investigation. Even when
classification society rules have been used, a still water bending
moment calculation will often be demanded. When the structure has
been determined and approved a final check can be made of its
weight.
The aim at the feasibility stage is to confirm that a design to meet the
requirements is possible with the existing technology and to a size and
cost likely to be acceptable to the owner. Several design options will