Page 229 - Practical Ship Design

Powering 11 191

S -Circular S
Froude’s formula was one of two most commonly used by British tanks to arrive at
SI when moving from the measured model resistance to 0; other made use of
the
Mumford’s formula for S given in eq. (6.17). When reversing direction from
~
~
~
~
I C , back ~to C,, it is important to use the same formula
~
Froude’s 1s = 3.4 + 0.5 (7.2)
Another formula for (S> can be devised as follows:

= 1.0124. c @‘I2 (7.3)
where c is the constant in Taylor’s wetted surface formula metricated (see Fig.
6.1).
This can be transformed to:

For most ships with C, between 0.95 and 0.99, c = 2.55.
For the BSRA standard ship with dimensions 122 x 16.76 x 7.32 m, LIB = 7.28
and TIL = 0.06, this transforms to:

A comparison of the (3 values obtained using these three different formulae is
given in Table 7.1.
0 and (K - Circular L and K

L’ = F,, x 3.545; @ = L) x @$”’
It is most important that (9 data is designated as either Froude or ITTC and is
related to a particular ship length or to a model.