Page 226 - Practical Ship Design

Powering I1 189

7.1.2 Re-analysis by an up-to-date method

The first step is to recreate the model results by reversing the process used for the
ship estimate and then subject these to the ITTC’78 treatment, all generally as
illustrated in Fig. 7.1.
Having drawn Fig. 7.1, the author decided to check how well the two E.H.P.
formulae agreed with one another. As the process used also illustrates a quick
method to approximate ship dimensions it seemed worth including as a digression.
A bulk carrier with a deadweight of 24000 tonnes and a service speed of 15
knots was used as a sample.
From Fig. 3.3 dwtldisp = 0.8 making disp = 30000 tonnes
From Fig. 3.8 LIB = 6.25; BID = 1.88; TID = 0.71
From Fig. 3.12 C, = 0.75 assuming F, = 0.2 approxf= 1.023; (1 = S) = 1.05

Using eq. (3.5)

30000x 6.25’ x 1.88
L=[
1.025 x 1.5 x 0.75 x 0.7 1
l’li
L = 156.65 m; B = 25.06 m; D = 13.33; T = 9.47

From Fig. 6.1 C = 2.55; S = 2.55 (30000 x 156.65)”2
S = 5528 m2
IS = 1.0166 x 55281(30000)2’3
From 56.3~)
= 5.82
Assume C,, = 2.5 x lo-’
Equation (6.16) gives IC = 39.8 x 2.5 x lo-’ x 5.82 = 0.579
. A2/3 ,v
3
P, =
580 = 3254 kW iremarkable agreement
J
P, = 0.0697 C,, .S .Vi = 325 1 kW

Returning to the re-analysis:
A computer spreadsheet provides a convenient way of handling what is quite an
involved calculation and one that a design office is likely to have to do repeatedly.
Figure 7.2 gives the headings of a Lotus 123 spread sheet used by the author. This
is reasonably comprehensive and can accommodate a variety of different input
formats and give alternative outputs.
The formulae involved in the various steps are as follows:

N1 - Circular M