Page 328 - Introduction to Naval Architecture

SHIP DESIGN 313

little cost. The same techniques can be used for merchant ships, taking
the natural hazards and human failings into account rather than
deliberate enemy action.
In such studies the statistics of past accidents are very valuable. For
instance, from the data on the damaged length in collisions and
groundings the probability of a certain fraction of the ship's length being
damaged in this way in some future incident, can be assessed. This is the
basis of the latest IMO approach to merchant ship vulnerability. The
probability of two events occurring together is obtained from the
product of their individual probabilities. Thus the designer can combine
the probabilities of a collision occurring (it is more likely in the English
Channel than in the South Pacific), that the ship will be in a particular
loading condition at the time, that the impact will occur at a particular
position along the length and that a given length will be damaged. The
crew competence in dealing with an incident is again a factor. The IMC)
have proposed standard shapes for the probability density functions for
the position of damage, length of damage, permeability at the time and
for the occurrence of an accident. There is a steady change to these
probabilistic methods of vulnerability assessment and passenger and
cargo ships are now studied in this way.
It must be accepted, however, that no ship can be made absolutely
safe under all possible conditions. Unusual combinations of circum-
stances can occur and freak conditions of wind and waves will arise
from time to time. In 1973 the Benchruachen, with a gross tonnage of
12 000, suffered as a result of a freak wave. The whole bow section 120 ft
forward of the break in forecastle was bent downwards at 7°. When an
accident does occur the question to be asked is whether the design was
a reasonable one in the light of all the circumstances applying. No
matter how tragic the incident the design itself may have been sound.
At the same time the naval architect must be prepared to learn as a
result of experience.

THE DESIGN PROCESS

Some of the various factors a designer must take into account have
been discussed. How then does the designer go about developing the
design? What is the design process? Different authorities use different
terms for the various design stages. For the present purposes the terms
feasibility studies, contract design and full design will be used.

Feasibility studies
The starting point is usually a type ship. That is one which is carrying out
all or most of the functions asked of the new ship and which is judged