Page 286 - Practical Ship Design

244 Chapter 8

1 .oo

0.90

Cm

0.80

0.70
0.30 0.40 0.50 0.60 0.70 0.80 0.90
Cb

Fig. 8.9. Approximate relationship between C, and C,.

The area of the sectional area curve is Cp = cb I C,
The area curve is divided into three parts:
(i) the entrance,
(ii) the parallel middle body,
(iii) the run.
The parallel middle body should be made as long as possible without distorting
either the entrance or run and avoiding hard shoulders at the junctions with these.
The length of parallel middle body is determined largely by the block (or
prismatic) coefficient, whilst the length of entrance and run depends on both the
block coefficient and the LCB position. In some respects it is more logical to
reverse this statement and say that the LCB position is the outcome of decisions on
the shape of the sectional area curve taken to minimise resistance.

8.4 DRAWING LINES USING A BASIS SHIP

With both Cb and C, fixed, a sectional area curve, or for that matter a body plan,
can be drawn for the new ship based respectively on the sectional area curve or the
lines plan of a “basis” ship of known good performance.
The advantage of working with the lines of a basis ship is that a body plan is the
direct result. This should not only be completely fair but its sections should have
the general characteristics of the basis ship albeit distorted by differences in the
LIB and BIT ratios of the basis and new ships.
It is, of course, rare for an initial lines prepared in this way to meet all the
designers wishes, but it does provide a good basis on which the modifications