Page 52 - Practical Ship Design

Introduction, Methods and Data 23

can be a considerable temptation in work of this sort to omit data points that do not
seem to plot well with the majority of the data.
Whether such a divergence is due to error in the input data or whether it means
that the theory is not as exact as one would like, must trouble anyone working in
this way. Caution clearly needs to be exercised when deducing design information
from this or similar sources, but provided an adequate number of samples are used
and extreme values are discarded or at least are not allowed to influence conclu-
sions to any significant extent, useful lessons can be learnt.

1.3.3.2. Datu from Significant Ships

In the past, designers obtained a lot of information from articles in the technical
press. These articles, based on data provided by the shipbuilders concerned, gave
much useful design information including plans. In recent years, fewer such articles
have appeared and it was for this reason that the Register was used. Since writing
the first draft of this section however it has been a pleasure to see this information
gap largely filled by R.I.N.A’s Significant Ships series.
The author of Significant Ships plainly wanted to quote displacements and
lightship weights as well as deadweights. In some cases the shipbuilders appear to
have been coy, but it is most satisfactory to see that this data is given for almost
50% of the designs shown. Knowing the displacement provides a most important
key to any design as it enables the block coefficient to be calculated and if the
deadweight is known it gives the lightweight. It is perhaps understandable that this
information used to be kept confidential but the freer interchange of it that
Significant Ships seems to be achieving will certainly be a great help to designers.
Table 1.2 uses data from Significant Ships of 1990 and 1991. It will be noted that
because Significant Ships gives displacements there are no columns for F,, as there
was in Table 1.1, but the columns for C, are retained for use as a check on the
displacements quoted.
The deadweight/displacement ratios calculated in Tables 1.1 and 1.2 are plotted
in Fig. 3.3, whilst the dimensional ratios are given in Fig. 3.8.
Cargo capacity is also given in Significant Ships and a form for tabulating and
using this data in the way discussed in $3.2 is given as Table 1.3, which has been
completed for the same ships used in Table 1.1 enabling the information on the
block coefficient at the load draft given in that figure to be used to estimate the
block coefficient at the hull depth and thus the total hull volume. The data from this
investigation is plotted in Fig. 3.5.
To use Significant Ships effectively in design, it is desirable to have a form of
index which identifies the location in the volumes, of which there are now six, of
ships of the type and size which may be suitable as guidance for a particular design
and this is given in Chapter 16, 516.9.