Page 193 - Practical Ship Design

Powering I 159

when the viscous or frictional components respectively of the model resistance are
subtracted from the model total resistance to establish the respective residuary
resistance coefficients, with the further change caused by the multiplication of the
ship friction coefficient by the form factor being of lesser significance.
An example with some figures may help to make the difference clear. A ship of

L = 330 m, V = 300,000 m3, 15 knot speed, F, = 0.136 (V = volume of
displacement)

was tested using a model with the following particulars:
I = 7 m, F, = 0.136, speed = 1.127 ds
K from test = 0.33
model R, = 6.856 x lo6 ship R, = 2.14 x lo9
Ctm = 4.309 x
Cfm = 3.203 x
AC = 0.10 10-~
Car = 0.05 x

By the ITTC’57 method:
Cr = (4.309 - 3.203) x = 1.106 x
ct-\ = 1.39 10-~

Ct, = (1.39 + 0.10 + 1.106 + 0.05) x = 2.646 x
Proportion of frictional resistance = (Cf\ + AC)/C,,
= (1.39 + 0.10y2.646
= 56%
By the ITTC’78 method:
Cr = (4.309 - 1.33 x 3.203) x = 0.05 x lo-’
cv\ = (1.33 x 1.39 + 0.10) x = 1.949 x
Ct, = (1.949 + 0.05 + 0.05) x = 2.049 x
Proportion of viscous resistance = 1.949/2.049
= 95%
Apart from the change in the proportion of frictional/viscous resistance, the very
large reduction in C,,, from 2.646 x to 2.049 x or 29% should be noted.
This change in value is of course tied to the K value of 0.33 used in this example, a
figure which appears to agree with Holtrop and Mennen’s formula given in 86.9.
Although most tank authorities appear to have adopted the new method, others
are sticking to the use of ITTC’57. Designers can only hope that there will shortly
be an end to the succession of changes and variety of methods used by tanks which
have caused them so much difficulty in the last two decades.
This hope may, however, be a little premature as a 1993 R.I.N.A. paper by
C.W.B. Grigson “An accurate smooth friction line for use in performance