Page 287 - Practical Ship Design
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Design of Lines 245
required can be sketched and eventually faired. If the form derived in this way is not
suitable for the new ship it is however, probably better to work with the sectional
area curve and avoid being influenced by the form of the sections of the basis ship.
8.4. I Retaining the same LCB position
The method used is essentially one of adding parallel middle body if an increase in
C, is required or deducting it if a finer C, is wanted. If
L, is the length of the basis ship, and
C,, is the block coefficient
C, the midship section coefficient
C,, is the required block coefficient of the new ship
X is the parallel middle body to be addedsubtracted
Then
(8.7)
and
Starting at the F.P. and A.P. respectively new stations are then drawn on the
lines plan at a spacing of (L, + X)/lO.
The waterline offsets read at these stations are multiplied by the ratio of the new
beam to the basis beam and are plotted on waterlines whose spacing is adjusted
from the basis spacing by the ratio of the new load draft to the basis load draft to
give a body plan on displacement sections for the new ship.
A comprehensive treatment of ways of varying ships forms may be found in
“On the Systematic Geometrical Variation of Ship Forms” H. Lackenby R.1.N.A
92 (1950) p. 289. Computer programs using a number of these methods are
available from the firms mentioned in 9 1.2.
8.4.2 Changing the LCB position
If the new ship’s LCB position as a percentage of the length is to be changed from
that of the basis ship, then the station spacing should be calculated separately for
the fore and aft bodies using the respective half body block coefficients.
These half body block coefficients can conveniently be obtained from Fig. 8.10
which shows the addition to one body and deduction from the other which
produces a particular LCB position. It is interesting that a midship LCB corresponds
to a forebody C,, = C, + 0.007 and a corresponding aft body C,, = C, - 0.007, and
not, as might have been expected, of equal fore and aft body cbs.
