Page 123 - Practical Ship Design

90 Chapter 4

A, + Ab = K(Z/D) - A,/3.8

substituting this in eq. (4.4) gives
steel-weight per metre = R[K(Z/D) + 0.74 A, + A,]
or

hull steel-weight = R . If x L . [K(Z/D) + 0.74 A, + A,]

where If = integration factor which is a function of cb.
A formula for the modulus Z from Lloyds rules which applies to ships with a
still water bending moment not exceeding 70% of the wave bending moment is:

Z = C, L2 B(Cb + 0.7) cm3

In this formula C, is not a constant but varies from 7.84 to 10.75 as L changes from
90 m to 300 m, at which point it becomes substantially constant. For present
purposes it is treated as constant, although its variation with length may explain the
slightly higher index of L which Sato suggests.
A, = 0.80 t, D and t, =f(L) from which A, = n2(L x D) and A, = n,(B x D)

Hence
steel-weight of hull = R . If . L{ n1 . Cb(L)2 . B/D + n2 . (L x 0) + ng . (B x D)}

To obtain the total steel-weight it is necessary to add three more items for each of
which a rational expression is suggested:
(i) weight of bulkheads = n4 x cb x L x B x D
(ii) weight of platform decks = n5 x Cb x L2 x B
(iii) weight of superstructure, masts and deck fittings = n6 x (V) or n7 x B2 x L

where V = volume of superstructure and n, to n7 are constants.
An expression for hull structure weight may be deduced as:
(i) modulus related
+ (ii) side shell and longitudinal bulkheads
+ (iii) transverse frames, beams and bulkheads
+ (iv) platform decks and flats
+ (v) superstructure and deck fittings
W, = 11, . (t13 . B/D . (Cb)' + n2 . (L)* . D . (CJ' + n3 . L . B . D . (cblY

+ n4 . (L)* . B . (Cb)" + ns . (v) or ns B~ L (4.6)
It should be noted that constants n, to n5 in eq. (4.6) are not identical with those in
the earlier expressions as some of the items have been combined.