Page 335 - Practical Ship Design

292 Chapter 10

the hull girder, whilst transverse framing is used for the ship side (the girder web)
and also for the supporting structure of any decks near the neutral axis.

10.3 SPECIAL STRENGTH CONSIDERATIONS FOR PARTICULAR SHIP
TYPES

Apart from the structural considerations already mentioned which affect all ships
to a greater or lesser extent, there are some special considerations which are
applicable to particular types, such as fast cargo ships, passenger ships and warships.

10.3.1 Fast cargo ships

The fine lines of a fast cargo ship tend to result in there being a lack of material in
the upper deck in way of the forward hatch and particularly at the forward comers
of this. There have been a number of incidents in which the structure in this vicinity
has been damaged with in some cases the whole bow of the ship being lost.
As a consequence, Classification Societies now have special strength require-
ments for the strength of this area, depending on the ship’s speed and the shape of
the cross section -but wise design will try to avoid the problem.
An over-heavy flare forward should be avoided as this may result in bow flare
slamming, which has led to considerable damage on some ships.

10.3.2 Large passenger ships

Whereas on most ship types there is a clearly defined deck which forms the upper
flange of the hull girder with the superstructures above this level being relatively
short and therefore not contributing to the overall longitudinal strength and conse-
quently fairly lightly constructed, passenger ships tend to have a mass of super-
structure decks which in most of today’s designs extend for almost the complete
length of the ship.
In passenger liners built before World War II, attempts were made to relieve the
superstructures of stress by fitting these with expansion joints. In a number of
liners built after World War 11, aluminium superstructures were used. Whilst the
main reason for the use of this material was its light weight and the greater extent of
superstructures which could therefore be built within the limit imposed on the
VCG by stability considerations, it was hoped that the fact that aluminium has a
much lower Young’s modulus than steel would enable the junction of the two
materials to provide much the same effect as the expansion joints of earlier ships.
In both cases, however, cracking seems to have been a frequent problem
indicating that the superstructures were taking stresses for which they had not been
designed.