Page 134 - Practical Ship Design

Weight-Based Designs 101

Provided a good “basis” ship is available and the corrections for known
differences are made with care the method is the best available short of detailed
calculations (see later), which are time consuming and difficult to make with
worthwhile accuracy at the early design stage.
A warning about the accuracy of square number proportioning may be taken
from Fig. 4.12, which is a modified version of the figure presented in the 1976
paper. This shows that even for a particular type of ship the ratio outfit weight/
square number is not always constant, although near constant values do seem to
apply to general cargo ships and container ships.
On the other hand, values slowly diminishing with length seem to apply to
tankers and bulk carriers, possibly because some items such as the accommodation
on these ships vary only slightly with ship size.
In the 1976 paper the comment was made that the value of the ratio for passenger
ships increased quite rapidly with length, probably reflecting the increase in the
number of decks which tended to go with increasing length and breadth emphas-
king the fact that for these ships volume is a better parameter than area.
When data for modern cruise liners was added to this graph it was found to be
much higher than the 1976 line suggesting that the latter was now out of date. The
reason for this difference appears to be the extra decks which ships of a given
length now have, which naturally increases the outfit weight per square metre of
L x B. To maintain satisfactory stability, the breadth of these modern ships has
been increased relative to their length reducing the LIB ratio to near 6 as opposed to
about 8+ for the ships which formed the basis of the 1976 plot. It may be worth
noting that the lower speeds of the cruise liners permitted this change without there
being an unacceptable penalty on powering.
This reasoning suggested that the new data might be reconciled with the 1976
spots if both were plotted on a base of beam rather than length and the result is Fig.
4.13.
The passenger line of the original 1976 plot has now been replaced in Fig. 4.12
by a series of lines of different LIB values with the 1976 spots falling happily into
place. There is a lesson to be learnt here. In 1976 the available data seemed to
justify the line given at that time but the base used can now be seen to be inherently
wrong. It is vitally important that all approximate formula have a rational scientific
basis if they are to continue to be relevant with significant design changes.
Reverting to Fig. 4.12, it may be noted that a line for frigates and corvettes has
been added.
The use of this type of graph for warships is complicated by the fact that, as
discussed in $04.1.1 and 4.4.6 and again in relation to specification writing in
Chapter 17 and to cost estimating in Chapter 18, outfit is not a concept used by
warship designers. But as a number of warship designers are used to merchant ship
practice and because outfit can be a helpful concept at the initial design phase a line