Page 209 - Practical Ship Design

Powering I I75

This can be transformed into
C,= C‘,X[(B,/B,)~~~X(T,/T~) 35)
(I
47Fn-0
(6.38)
Moor gives limits for the use of these indices of a 10% change in the Beam ratio
and a 15% change in the Draft ratio, whilst the F, range is stated as from 0.15 to
0.27. The author, in his design work, has gaily extrapolated well beyond all three
limits without his calculations being found in error to any degree by later tank
testing. But caution is of course desirable.
It can be helpful when using Mumford indices to remember that G) increases
with an increase in the beam ratio, but generally reduces with an increase in the
draft ratio except at high speeds.
Moor’s C: values are based on model results with turbulence stimulation but the
Froude friction line was used and the appropriate ship/model correlation factors
must therefore also be used.
The standard ship method was also adopted by B.S.R.A. (British Ship Research
Association) for their very considerable programme of tank test research which
covered block coefficients from 0.525 to 0.875.
Moor revised his “average” attainment data in a 1969 B.S.R.A. report, which
also gives “optimum” values. This time the draft used was 24 ft (7.32 m) and the
range of block coefficient was increased to cover from 0.54 to 0.88 and the VI&
values to cover from 0.40 to 1.10. Although the effect of the draft change is not
very great, this change needs to be carefully noted if great accuracy is required.
Possibly because of the increasing number of ships with bulbous bows by this
time, Moor felt that the standards were better established without reference to the
LCB position, although a plot of the LCB positions of the best forms used to derive
the optimum curves is given.

6.7.2 Twin-screw ships
In another R.I.N.A. paper “Some aspects of passenger liner design” written in
1962 in conjunction with R.V. Turner and M. Harper, Moor presented what is
probably the best of the rather limited data there is available on the resistance and
propulsion of twin-screw ships. The data in the paper referred to “average modern
attainment”, but was revised and extended to give “optimum” data as a B.S.R.A.
report in 1968.
For twin-screw ships the standard ship has the same length and beam, but the
draft is reduced to 18 ft (5.5 m)
Corrections for beam and draft variations from the standard figures are again
made using Mumford indices. These are given for a wider range of beam and draft
ratios than for single-screw ships and cover a greater range of V/& and in this case
the index x varies with F,.