Page 208 - Practical Ship Design

174 Chapter 6

These values are then corrected for block coefficient and position of the
longitudinal centre of buoyancy, for both of which optimum values are laid down,
so that all corrections are additions.
The corrections for block coefficient are fairly modest if the ship is finer than
standard, but severe if it is fuller.
The corrections for the LCB position tend to be more severe if the LCB is
forward of the standard position, those applicable if the LCB is abaft the standard
position being smaller.
Interestingly, the standard block coefficient for a twin-screw ship is set 0.01
fuller than for a single-screw ship. The data used in preparing the curves of C2 is
now unfortunately outdated. In its day, the method not only provided an accurate
estimate of EHP but also gave some very useful guidance towards the optimisation
of the ship’s lines.

6.7 MOOR’S METHOD

6.7.1 Single-screw ships
In a paper entitled “The effective horsepower of single-screw ships -average
modern attainment” presented to R.I.N.A. in 1959, Moor and Small give 0 values
for standard ship dimensions of 400 x 55 x 26 ft (122 x 16.76 x 7.93 m) for a range
Of
(i) block coefficients - from 0.625 to 0.80 by 0.025 intervals;
(ii) LCB positions - from 2.00% aft to 1.75% forward;
(iii) speeds - from 10 knots to 18 knots, corresponding to F, = 0.15 to
F, = 0.27.
Corrections to 0 for other lengths of ship are read from a slightly complicated
graph given in the paper, which the author has been able to simplify with only a
trivial reduction in accuracy to the formula given below:

d@ = 4(& - L,) lo4 6.37
where do is the change in 0 for a change in length from a basis length of L, to a
new ship length of L2, both in metres.
The corrections for differences in beam and draft from the standard values are
made using indices of a type devised by Mumford, who was at one time Super-
intendent of the Denny Tank.
Mumford postulated that P, varies as lF and F. Moor investigated the values of
x and y which had been found to apply to 1 1 different standard series before settling
on x = 0.90 and a y value varying with Froude number from 0.54 for F, = 0.15 to
0.76 for F, = 0.30.