Page 237 - Practical Ship Design

Powering II 199

7.2 POWERING DATA

7.2. I A “C,” Method based on Moor’s and similar data

In giving the title “Practical Ship Design” to this book, the author intended not only
to present the theory of ship design, but to provide sufficient data to enable a reader
to prepare at least an outline design for many types of ship without the need to refer
to other data.
The discussion of powering methods has so far stopped short of providing such
data, partly because full data on each of the methods described is readily available
in the reference documents quoted and partly because an abstract suitable for this
book could cover only a limited area of Froude Number, block coefficient etc. It is
now time to give some data.

7.2.2 C,,,, for single screw ships
For single screw ships the author originally intended to draw on his 198 1 Parsons
memorial paper “Designing ships for fuel economy”. In writing that paper he felt it
important to establish a “base case” of a thoroughly efficient powering perform-
ance for a range of ships against which comparisons could be made, as too many
claims of substantial improvements in performance - of increased speed and/or
reduced fuel consumption - could be shown on analysis to be based on poor
performance by the ship or machinery used as the reference point. For the perform-
ance criteria he turned to Moor’s work which has already been mentioned and to
the B.S.R.A. Methodical series, updating these to transform the lines to Froude
number from the historic V/& basis (V in knots and L in feet), which had become
anachronistic in an SI unit age, and changing the ordinate from 0 Froude to C,,
ITTC’57, retaining the standard ship dimensions of 122 x 16.76 x 7.32 m.
By the time this book came to be written, C, ITTC’57 had itself become out of
date with the introduction of form factor and C, ITTC’78. To transform the Moor
and BSRA data into this new format was quite easy using a spread sheet and Table
7.4 shows the transformation of Moor’s single screw average values.
Because each block coefficient has a different (1 + K) and this value is needed
when correcting to the dimensions of a new design, plotting C, ITTC’78 is
impractical. Before discussing the plot which the author ultimately decided to use,
it is worth looking at the comparison of C,‘57 and Ct‘78 given in Table 7.4. These
differ quite considerably, with C,‘57 the greater by from about 5% to 20%. The
difference is greatest on full ships at slow speeds reducing for finer ships and faster
speeds. The comparison is not strictly correct as AC, C,,, and Capp have to be added
to ITTC’78 when calculating the total resistance coefficient and this brings the two
values a little closer. The fact that there are some negative Cr‘78 figures in Table
7.4 appears to stem from an anomaly in the original data in which resistance