Page 108 - Practical Ship Design

76 Chapter 3

judgments -but it does seem likely to be quite close to an optimum which is not a
highly tuned one.
It was disappointing to be unable to detect any significant effect of either LIB or
BIT, although it must be noted that different types of vessels, each of which
generally has its own particular range of LIB and BIT tend to be concentrated at
different parts of the F,, range. This remark also applies to twin screw propulsion,
which is generally confined to high speed container ships, passenger ships, ferries
and warships.
The types of ships used in the plots are indicated showing the areas in which
each predominates. Ship type may have some significance in relation to selection
of C, because of the different practices in relation to service margin which seem to
apply to different types of ships.
A slightly modified version of the 1975 plot is presented in an enlarged format
in Fig. 3.12.
The use of the Watson-Gilfillan line in a computer programme was made much
more convenient by Dr Townsin who devised the following formula which agrees
almost exactly with the mean line:
(23- lOOF,,)
C, = 0.70 + 1 I 8 tan-' radians (3.14)
4

Adding recent (1991) data to this graph seems to confirm its continuing validity and
prompts the question whether all naval architects are now using the 1975 paper!
Although the block coefficient given by the mean line in Fig. 3.12 should result
in fairly near optimum powering, there can be other factors which may make a
different block coefficient desirable, and it is as well to remember that powering is
more closely related to the prismatic coefficient than it is to the block coefficient
- and implicit in any selection of block coefficient must be an intention to
associate it with an appropriate midship section coefficient.
Both the design of lines and powering are dealt with in later chapters, but before
leaving this discussion of C, it may be worth mentioning that most warships have
much finer values of C, than might appear necessary for their F,, values purely on a
powering criterion. The main reason for this is the requirement that these comp-
aratively small ships have to maintain as much of their service speed as possible in
whatever weather they may encounter which together with the modest draft of
most warships means that much of the fore body can come out of the water as the
ship pitches with a consequent danger of severe slamming.
As the best way to reduce the incidence of slamming is to give the ship a high
rise of floor, these ships tend to have a small midship section coefficient which for
a given prismatic coefficient gives a reduced block coefficient. The increase in
draft obtained in this way can have other advantages such as improving course
stability.