Page 177 - Practical Ship Design

Volume, Area and Dimension-Based Designs 143

used to be a feature, suggests a change in design methods, with the use of the
following equation:

or the corresponding equation for L is

where
V,, is the volume taken up by lifeboats if these are to be stowed under
overhanging superstructure decks, which is now becoming common practice on
the larger ships of this type (such stowage usually takes two tweendeck
heights, a breadth for both sides of about 10 metres and a length to suit the
number of lifeboats to be carried); and
D,d is the depth to the topmost continuous or nearly continuous deck ignoring
comparatively short houses; and cb,d is the corresponding block coefficient,
which can be estimated, admittedly fairly approximately because of the
considerable extrapolation involved, by the use of eq. (3.2.4).
It will be noted that this eliminates the need to divide the total volume into hull and
a superstructure percentages. If the value of Dtd is well chosen in relation to the
value of B this will go a long way towards ensuring that the chosen dimensions will
result in satisfactory stability in the same way that the BID ratio does for cargo
ships.
For large modern cruise liners and for large passenger ferries the ratio

Dtd/B = 1.2 (k 0.05) is remarkably constant.
This ratio is based on data for seven cruise liners: Crown Princess, Crystal Harmony,
Fantasy, Horizon, Monarch of the Seas, Asuka and Statendaam, and two passenger
ferries: Silja Serenade and Olau Britannia. The D, figures used have been estimated
by scaling from small plans or by adding standard tween deck heights to the
moulded depth in the absence of more accurate information, but the error is
unlikely to be significant.
Having reached an initial set of dimensions, the depth to the top deck will
usually require some adjustment to make it a convenient summation of the
required tween deck heights, etc. The beam should then be modified to keep the
Dtd/B ratio within the suggested band and the length of the ship adjusted to give the
required volume. This may seem a very approximate method but sensibly used it will
generally ensure that the dimensions adopted for a preliminary design need little
modification when the results of later more detailed calculations become available.