Page 120 - Practical Ship Design

Weight-Based Designs 87

Lack of data prevents the calculation of a corresponding figure for the changes
which have undoubtedly occurred since 1976. It seems likely there was a further
reduction possibly of about 5% for mild steel construction in the early years of this
period - with another reduction if higher tensile steel was used to a major extent
as was increasingly the case for large ships. In the last five years, concern over
recent ship casualties has reduced the trend towards an increased use of higher
tensile steel and seen the recognition of the need for some structural redundancy
with a consequent increase in the structural weight of ships now under construction
towards, but keeping a little below, the 1976 figures.

4.2.5 Scrap and invoiced steel-weight
Whilst the net steel-weight is required for calculating the light weight, it is still
necessary to consider the scrap allowance to arrive at the invoiced weight needed
for cost estimation.
In 1962, a figure of 12% of the invoiced steel was suggested as an average scrap
percentage. By 1975 scrap percentages had been considerably reduced particularly
for larger ships and it was suggested that the amount of shape in the ship was the
major relevant factor, with full ships having a lower scrap percentage than fine
lined ships.
In general the following factors were seen as likely to influence the scrap
percentage:
- shipyard steel ordering methods: the use of standard plates; the necessity of
ordering sections for stock to ensure supply when needed;
- shipyard constructional methods: the allowance of overlaps on prefabricated
units to be cut at the ship to ensure a good fit; the use of numerical or optical
lofting methods involving nesting procedures; extra lengths on sections to
suit the operation of cold frame benders;
- the effect of increases in the cost of steel enforcing economy in its use; the
skill of draughtsmen in utilising material, particularly by the use of nesting
when ordering plates; the accuracies of the calculations and the weighing
methods employed to assess net and invoiced weights, respectively.
Figure 4.2 shows a plot of scrap percentage against block coefficient at 4/5 depth
with some suggestions for fine tuning in relation to size and some other factors.
When making up the lightship weight from a calculated net weight an addition
of 1 % should be made to allow for deposited weld metal and rolling margin on the
steel.

4.2.6 Other approximate formulae for steel-weight
In a 1974 R.I.N.A. paper, K.W. Fisher summarised a number of alternative steel-
weight estimating methods. Most of the formulae quoted appear to have been