Page 203 - Practical Ship Design

Powering I 169

As R,, values are not exactly memorable, readers may find a few figures helpful:
R, (SW) for 122-m ship R, (FW) for 5-m model
For F, = 0.12 4.26 x lo8 3.69 x IO6
For F,, = 0.48 17.05 x los 14.76 x lo6

(x) Water transport efficiency q(wt) (see Fig. 2. I)

work done - S.04SAVk
Nwt) = - (6.33)
energy used P,, (kW)

(xi) Froude ’s circular notation

One of the coefficients CI which forms part of this notation has already been
mentioned, but as a number of the other notations are in use in the presentation of
powering data it may be helpful to summarise these and explain their basis.
The “circular” notations which were devised by R.E. Froude in I888 are a series
of non-dimensional coefficients based in general on the dimensions of a cube
which has the same volume as the displacement volume of the ship.
The side of this cube U = VI” = 0.991 8 A”’
The face of the cube has an area U2 = 0.9837 A213

Where the non-dimensional expressions are converted below to dimensional units,
the units used are:
L,, in metres
S, in square metres
A in tonnes
V, in knots.
ilcl‘ the 1engtWdisplacement ratio relates the ship’s length to the length of a side
of the cube having the same volume of displacement as the ship.
M = LJU = 1.00826 L, IA”’

S 1 the wetted surface coefficient relates the wetted surface S to the area of one
side of the cube.

IS = SJU’ = 1.0166 S, IA‘”
Froude’ s approximate formula for ,S>, which may be compared with Mumford’s
formula for S (see eq. (6.17)) is: