Page 285 - Practical Ship Design

Design of Lines 243
I
B.T .(1 -C,) -F(B / 2-K / 2)
R=[ (8.3)
2{( 1 - .n / 4) - F / (B - K)}

and

B.T .(l-C,) 112
(8.4)
2(1 -IT/ 4)
Whether C, or R should be fixed first is a matter for debate and there may need to
be an interactive process.
There seem to be three motives for keeping the bilge radius small:
(i) the greater resistance to rolling provided by a “square” bilge;
(ii) the easier cargo stowage of a squarer hold; and
(iii) for a given C,, the finer Cp associated with a larger C, will generally, but
not always, reduce the resistance.
On the other hand, the radius should be sufficiently large to be production-kindly,
which probably means about 2.5 m for ships with a beam greater than about 16 m.
On fine lined ships it may be desirable to increase it above this figure to assist in
marrying it in to the fore and aft lines. Generally however if a fine C, is desired for
any reason - say to increase the draft of a “volume” type ship - this is usually
better achieved by the use of a high rise of floor.
For ships with a beam of less than 20 m and no rise of floor an approximate
empirical formula for the bilge radius, in metric units, is:
R = (1.7 - C,,) x (B/3.3)”* (8.5)

On ships with rise of floor the bilge radius may be somewhat reduced.
A bilge radius to the above formula and with no rise of floor will result in a C,
Of
(1.7-C,)2
c,, =I-
7.7T

Although C, is usually best determined as the product of practical decisions on
the dimensions of the bilge radius and the rise of floor, it is sometimes convenient
for powering calculations (see $6.9) to have a quick method of estimating a
reasonable value in terms of the block coefficient and an approximate C,,-C,,
relationship is given as Fig. 8.9.
The big difference between the lines which appear to apply to most merchant
ships and that which applies to most warships confirms the view that C, is best
determined by deciding on the bilge radius and the rise of floor rather than vice
versa.