Page 272 - Practical Ship Design
P. 272
230 Chapter 7
Further reductions in resistance can be obtained by increasing towards 7.0,
although the gain by doing so must be assessed against the added constructional
weight and cost that this will entail.
7.9.4 B/T ratio
Diagrams in the Lackenby paper can also be consulted to evaluate the effect of
changes in BIT.
At a value of 2.4, which appears to be about the average value for most ships, the
resistance values seem, in general, to be only 2% or so above the optimum values
which generally correspond to a BITvalue of a little less than 2.2 - a figure which
incidentally it would be almost impossible to achieve in most ships because of the
constraints which stability and freeboard rules apply to ship proportions.
In the case of a slow speed (F, < 0.14) and full bodied ship (Cb > OB), these
diagrams show that a change of BIT from 2.0 to 3.0 causes an increase in resistance
of 3%.
In the case of a fast (F, > 0.30) and fine lined ship (C, < 0.55) a similar change in
BIT increases the resistance by 5%.
It may be noted that the adoption of a low BIT value or, since the breadth is more
likely to be fixed in relation to the depth than to the draft, the adoption of as large a
draft as possible, is doubly advantageous as, for a fixed displacement it will reduce
the capital cost.
Much the same lessons can be drawn from the Mumford Indices which have
already been mentioned.
7.9.5 B/T ratio of the ballast condition
In the ballast condition there is a major change in the BIT ratio from that applying
in the load condition. Typically for a large tanker or bulk carrier BIT laden is about
2.2 but in the ballast condition this changes to 5-6. This results in a major change in
the hydrodynamics, which is rarely given any consideration although these vessels
will spend 40-50% of their time in ballast.
Ships with controllable pitch propellers can adapt their rpm to suit either regime
but there may be a case for more consideration being given to the ballast condition
in the design of conventional propellers.
Other aspects of the design of the lines to minimise the power required are dealt
with in Chapter 8.