Page 131 - Practical Ship Design

98 Chapter 4

4.3.2 Items not in the weight per unit length

Items within the main hull not included in the weight per metre such as platform
decks, bulkheads and casings together with all items such as superstructure, masts,
etc. outside the main hull must be added to give the total net structural weight.
Rapid and accurate calculation of the weight of all the other structural weight
items demands the maintenance of careful records of weights from ships for which
the designer has complete plans. These will usually be best recorded as weights per
square metre, but sometimes weights per lineal metre are appropriate and occasion-
ally it is best to record the total weight of an item. If at all possible, these weights
should be plotted against a suitable base, with the length B.P. with which many
scantlings scale often being the best parameter, so that values appropriate to the
dimensions of a new design can be selected.
If ingenuity is used in selecting suitable numerals for these plots, they can be
remarkably accurate and very useful.
For example, the numeral N = B x 0* x C,, (at 0.80) was found to give a
remarkably good plot of the average weight of a watertight bulkhead for a
particular ship type, with different lines being identified for ships with different
numbers of decks supporting the bulkhead and influencing stiffener spans and
therefore scantlings and weights. The use of this numeral avoids the necessity of
drawing each bulkhead so that its area can be calculated.
In this calculation, as in all detailed calculations, probably the most important
thing is to make sure that no items are completely omitted - an error in estimating
can be bad enough but a complete omission can be disastrous and to avoid this it is
important that a standard checklist, such as that given below, is followed.
1. Corrections for local structure
1.1 Shell
1.2 Decks
1.3 Double bottom
2. Side to side erections: forecastle, bridge, poop
2.1 Front, sides, end
2.2 Decks
3. Houses
3.1 Front,sides and end
3.2 Decks
3.3 Machinery casings
3.4 Other internal steel casings
3.5 Doors, access hatches, etc.
4. Fittings on or above the upper deck
4.1 Bulwarks
4.2 Hatch coamings