Page 271 - Practical Ship Design

Powering II 229

The parts of the line between Cb = 0.55 and 0.65 and above cb = 0.80 take the
form that might be expected but the “hump” between cb = 0.70 and 0.80 is not so
readily understandable. One possible explanation may be that less attention has
been paid to the development of models in this region.

7.9.2 Length
Length affects powering in several ways, the most important being the effect it
exercises on Froude number, the significance of which is clearly shown in the C,
figures presented in $7.2 with increasing length reducing F, for any required speed
and thereby reducing C,.
For a required displacement the effect does not end there since an increase in
length as well as reducing the Froude number will also result in a reduction in the
other dimensions of B, T and C,, and reductions in the first and last of these will
also reduce the resistance.
A third effect, or possibly another way of looking at the same thing, dealt with in
96.2, is the reduction in the frictional resistance coefficient with length.
At high Froude numbers, a long slender ship shows to advantage, but the
smaller wetted surface of a short, beamy, deep ship can be advantageous at lower
Froude numbers where frictional resistance predominates.
The reduction in EHP obtainable by increasing length is offset to a small extent
by the reduction in propulsive efficiency which is caused by an increase in length.

7.9.3 The effect of UV’”
Another way of looking at the effect of length is provided by assessing the effect
that LIV“’ has on the resistance.
In $6.8 it was noted that plots of resistance against LIV“’ were used by
Guldhammer and Harvald in association with a standard BIT.
A feel for values is not easy to acquire, but if the values of the ratios LIB, BID
and TID suggested in 93.3 are used, it is instructive to see the values of L/V”’
which result.
For a fast (F, = 0.30) long slender fine lined ship with a B freeboard having Cb =
0.55, LIB = 7.5, BID = 1.65, and TID = 0.69, the value of L1V”3 is 6.24.
For a slow (F, = 0.14) short beamy full ship with an A or B-60 freeboard having
C,, = 0.85, LIB = 5.5, BID = 1.9, and TID = 0.77, the value of L/V’I3 is 4.44.
Diagrams in a 1966 RINA paper “The BSRA Methodical Series” by Lackenby
and Parker indicate that there is no benefit in increasing L/V“3 above about 5.2 for
ships with block coefficients of 0.75 or more.
For C, values between 0.75 and 0.60 the value should increase from 5.2 to 5.6.
For Cb values of less than 0.60 the value should increase to about 6.0.