Page 124 - Practical Ship Design

Weight-Based Designs 91

In this expression the indices of C, in the various terms have been left as
alphabetical symbols. It would appear from inspection that X might have a value
close to unity as it has components both from the integration factor and from
Lloyd's modulus formula; both Y and Z are clearly fractional indices.
An extrapolation on log-log paper of information on integration factors similar
to that presented in Fig. 4.8 indicated that overall the steel-weight is proportional to
the square root of the block coefficient.
If it is accepted that for any one type of ship the dimensions L, B, and D are
related, the formula can be simplified to:

W, = Cb1l2. L . B . [K, . L(L/D) + K, . D] (4.7)

This has one modulus-related term and one volume-related term and is very similar
to Sato's expression:

W, = C,,'/'[w,(L)'.' . B/D + w2 . (L), . (B + D),] (4.8)

Determining values of K, and K2 is left to readers, but the author believes that
this method applied to a range of good data would lead to more accurate steel-
weight estimation.

4.3 DETAILED STRUCTURAL WEIGHT CALCULATIONS -ALL SHIP
TYPES

The principal difficulty in making detailed calculations arises from the fact that
this more accurate weight is almost invariably required at a time when very few
plans have been drawn. Plans which must be available for a detailed calculation
include a reasonably complete general arrangement plan, a body plan and a mid-
ship section with the scantlings. Improved accuracy could be obtained if structural
sections away from midships, a shell expansion plan, steel deck plans, bulkhead
plans are also available, but this is unlikely to be the case, although scrap drawings
of these can be made specifically to assist in the calculation.

4.3. I Use of the midship section weight per unit of length~for merchant ship
weight calculutions

Without the plans just mentioned, calculation of the weights of shell, ship side
framing and the double bottom is almost impossible and the best alternative would
seem to be the use of an integration factor in association with a calculated weight
per metre of the midship section.