Page 48 - Practical Ship Design

Introduction, Methods and Data 19

(1) Lines

At least one firm offers lines, hydrostatics etc. for asymmetric forms. The
parameters used for the design of lines seem to be based on block coefficient and
LCB only and do not appear to include consideration of the features required to
ensure good sea-keeping and manoeuvrability or high KM which are discussed in
Chapter 8. The alternative approach via the digitising of an approximate lines plan
enables these features to be built in by the designer.
The system of co-ordinates used by one (and possibly some others) of the
systems examined involves the use of a load waterline or similar datum which
facilitates the definition of both the ship’s bottom and the superstructures with
negative and positive Z values. Longitudinal or X values are positive forward,
whilst Y values are positive to starboard.

(2) and (3) Outline structural plans

The use of these outlines along with computerised area measurement holds out
hope of speeding and improving the accuracy of steel-weight calculations.

(4) Capacities and centres

The ability to update these for minor changes on a regular basis enables the
design to be updated regularly and goes a long way to ensuring the avoidance of the
nasty surprise which not infrequently occurred in the days when such updates were
made manually at too lengthy intervals.

(13) and (14) Ship motions

In the past these calculations were only rarely made and then only for ships
designed for a special-purpose role. Ship motions were investigated for ships
involved in work at sea, either of a defence or scientific nature or in support of oil
exploration or production.
Manoeuvring was only investigated for ships requiring a special capability.
With the programs now available both of these important ship characteristics can
be assessed early in the design process almost as a routine matter.

(15) and (I 6) Resistance and effective horsepower

Each of the firms offering this software provides options as to the data and
calculation methods used for normal displacement ships and in addition offer the
use of special methods and data for particular ship types. For normal displacement
ships, the methods on offer include: