Page 230 - Practical Ship Design

192 Chapter 7

Table 7 1
A comparison of S values using different formulae based on 4tandard 5hip dimensions
L=I22,8=1676,T=732,L B T=14967
Froude SI = 3 4 + 0 5 WV"'
Mumford S = (1 7 L T+ C, L B)/V'"
Modified Taylor for standard dimensions s = 5 74/(Ch) ''

ch V s Froude s Mumtord S Taylor
~~ ~
0.55 8232 6.42 1 6.48 I 6.342
0.60 8980 6.335 6.352 6.250
0.65 9729 6.258 6.246 6.167
0.70 10477 6.188 6. 158 6.092
0.75 11225 6.124 6.086 6.022
0.80 1 I974 6.066 6.025 5.958
0.85 12722 6.013 5.973 5.898
~
Maximum difference about 2%.
v = 1.188 x for S.W.
= 1.139 x for F.W.
If model length is not known, a length of 5 m can be assumed without this being
likely to introduce any significant error.
If the input 0 is Froude then this must be taken back to model size (or to the
length of a new ship design) by the use of the Froude friction line. The correction
from a length L1 to a length L2 (of the model or new ship) is:

where
12.767 f 0.0 16 16
(-0) = and f= 0.00871 +
~0.0875 2.68 + L,,,
m
these two formulae being metricated versions (Lm in metres) of the Froude friction
line formulae.
If the C value is ITTC it can be converted to model length (or the length of a
new ship design) by the formula:

-C,
=
IC',,
[cL2 + (Cf,, ) x 39.8 /S) (7.6)
Once at model length the conversion from 4, to C, is made using the formula:
C, = 1@lf39.8 $ (7.7)

Although rarely required, a direct conversion from C,F to I~ITTC vice versa
or
can be made by reversing direction at the model scale or by the use of Fig 7.3,