Page 242 - Practical Ship Design

204 Chapter 7

and C, and are appreciably better (5% or so) at low F, (<O. 18) and high cb (>0.75)
values. The great virtue of the BSRA data to a designer is that it not only provides a
power estimate but also gives a method of developing a body plan which will
match the power estimate.
Moor’s results on the other hand are derived from a large number of unrelated
models and there is no guidance as to the characteristics of the lines which will
match the power estimates.
In Fig. 7.6 the Watson/Gilfillan block coefficient line is shown as an indication
of the area of this figure which is of most practical importance.

7.2.3 C,, for twin screw ships

The same procedure was applied to Moor’s twin screw data and is presented in Fig.
7.9. A superimposition of the twin screw data on any of the single screw figures
indicates the generally better naked resistance of the former, but this advantage is
of course reversed when appendage resistance is taken into account.
The C,, values for twin screws ships are also presented for a standard model
length of 5 m. The LIB ratio remains 7.28 but TIL is reduced to 0.045, a figure more
appropriate to twin screw ships.

7.2.4 C,, for frigates, corvettes and high speed ferries
Most warships of the corvette and frigate type are required to have maximum
speeds which result in Froude numbers ranging from 0.40 to 0.60, with most of
them between 0.45 and 0.55.
Little warship powering data escapes the security net, so it can be helpful to fill
out whatever data is available with merchant ship data and the twin screw data
given in the previous paragraph can be used in this way, provided care is taken.
In any attempt to do this it is essential to note is that the block coefficient of most
warships is much finer than would be dictated by powering considerations.
This fine C, results from the requirement that these ships should have a high rise
of floor to avoid, or at least minimise, slamming which is a very important
consideration on these relatively small and lightly constructed ships which are
required to maintain high speeds in rough seas. The fine C,, also helps to increase
the draft which again improves seakeeping.
A corollary of the fine C, that these ships have is that C, ceases to be as good a
parameter for C, as it is for merchant ships and warship designers tend to use Cp.
The author would have liked to follow this practice but unfortunately lacked the
necessary information on Cp or C, values to do so.
Anyone wishing to use Cp as a base and having only a C, value available may
find it helpful to use the rough guide to C, values given in the following formulae
(see also Fig. 8.9).