Page 100 - Practical Ship Design

The Design Equutions 69

3.3.3 DeptWhreadth relationship D = .f(B)

This relationship is closely related to stability since KG is a function of depth and
KM is largely a function of beam.
In the 1962 paper the relationship was presented as:
B-K
D=-
1.4
with two different values of K stated as representing moderate and good stability,
respectively.
The BID values varied from 1.5 for a large ship with “moderate” stability to 1.8
for a small ship with “good” stability.
By 1975 better standards of stability were demanded by International rules and
the distinction between moderate and good stability had become academic and the
BID values had increased.
Figure 3.9, which was originally presented in the 1975 paper, reverts to lines of
constant BID and shows a plot of depth against beam for a number of ship types. It
was found that there were two distinct groups. The first group consisted of
deadweight carriers comprising coasters, tankers and bulk carriers had an BID ratio
of about 1.9. The second group consisted of volume carriers comprising fishing
vessels and cargo ships whose depth was limited by stability considerations and
which had a BID ratio of about 1.65.
The 199 1 plot included in Fig 3.8 largely confirms these groupings with tankers
and bulk carriers again averaging at a BID of 1.9.
The second group brought container ships and refrigerated cargo ships together
at the slightly increased BID value of 1.7. The higher BID value (1.7 vs 1.65) for
volume carriers in 1991 may be a consequence of the need to limit the depth of
these ships because of the stability implications of making provision for the
carriage of containers on deck.
Factors which in general may require an increased BID value include: higher
standards of stability for whatever reason these may be needed; the carriage of
deck cargo; reductions in machinery weight raising the lightship KG; and the finer
lines needed for high speeds giving reduced KM for a given beam.
Factors which may permit a reduction in BID include the provision of a large
ballast capacity in the double bottom; absence of deck cargo; relatively light
superstructure and cargo handling gear; absence of sheer and/or camber; and lines
designed to provide a particularly high KM value (see Chapter 8).
Although L and B values are freely available for warships, figures for D are
rarely quoted. Because of the need to allow for the considerable variety in the
amount of superstructure and other topweight on these ships, there tends to be quite
a wide scatter in the BID ratios used, but a new line for frigates has nevertheless
been added to Fig. 3.9 showing a mean BID ratio of 1.55.