Page 195 - Practical Ship Design

Powrt-ing I 161

After discussion, tank superintendents agreed that skin friction was being
over-estimated by the Froude method, and decided to change to the 1957 ITTC
formulation. More recently, appreciation of the effect of the form has changed the
emphasis with the frictional element now being seen once again as much the
largest part of the resistance of most ships.

6.2.6 Hull finish, AC and (I + x)
The ITTC 57 model-ship correlation line was intended to eliminate ship-model
correlation making (1 + x) = 1 .OO for all-welded ships having a shell roughness
amplitude of 165 microns which was found to be typical of new construction in the
decade 1960- 1 970.
Prior to the general use of AC, various formulae had been suggested for the
calculation of (1 + x) for different shell roughness values, with the most authori-
tative probably being that given in a 1980 R.1.N.A paper “Speed, power and
roughness - the economics of outer bottom maintenance” by Townsin et al.,
although the physics of this have been challenged.
In this paper the value of x is shown to change with a change in roughness (h) in
microns mean apparent amplitude (MAA) as follows:

A(x) = 0.058 [(h,)”’ - (h2)i’3] (6.1 1)

Applied to a base of (1 + x) = 1 .OO at h, = 165, this gives the following values.
MAA (1 +x)
80 0.932
125 0.968
165 1 .ooo
230 1.037
400 1.109
125 microns is quoted in a 1972 NMI Report No. 172 as the best figure achieved on
the ships measured in that survey; 230 is quoted as typical of a rather poor
performance, and 400 was the worst.
As has already been said, modern practice favours the use of AC rather than
( I + x). A formula for AC suggested by Townsin and accepted by ITTC, although
here once again the physics of this empirical formula have been challenged, is
IO3 AC =44[(h/LWL)“3 -10(R,)-”3]+0.125 (6.12)

where h is an average measure of the height of the elements of roughness of the
entire hull, LWL is the waterline length, both in identical units.
It has been pointed out that AC can vary quite considerably even on hulls with
the same h, depending on the number and spacing, individual height and shape of