Page 272 - Practical Ship Design
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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.
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