Page 339 - Practical Ship Design

296 Chapter 10

The relative absence of large deck openings on warships as compared with
merchant ships means that torsion rarely presents any difficulties. The number of
decks and bulkheads prevents lateral strength being a problem.
The high speed of these ships and in particular the frequent need to maintain this
in adverse weather, can result in severe slamming in spite of measures which are
usually taken in the design of the lines to minimise this. The structure in almost the
whole fore body and in the stern must be designed with this in mind.
Underwater explosions can result in intense shock effects, which may induce
whipping of the hull structure. The damage done by shock to machinery and
equipment is more important than that done to the structure of a well designed ship,
but measures are nevertheless taken to improve the structural resistance to shock.
These measures include the use of symmetrical rather than asymmetrical sections
for stiffeners, the design of connections in a manner that helps to ensure continuity
of strength and stiffness and sometimes involve the fitting of additional tripping
brackets. There is usually a requirement for resistance to air blast due to nuclear or
other explosions. This affects the above water structure including the super-
structure. Specialist publications should be consulted for details of the methods
used which make use of the ultimate strength of the structural material allowing
permanent deformation but avoiding complete collapse.

10.4 OTHER STRENGTH CALCULATIONS

10.4.1 Torsional strength
Although torsion is not usually an important factor in ship design for most ships, it
does result in significant additional stresses on ships, such as container ships,
which have large hatch openings. These warping stresses can be calculated by a
beam analysis which takes into account the twisting and warping deflections.
There can also be an interaction between horizontal bending and torsion of the hull
girder. Wave actions tending to bend the hull in a horizontal plane also induce
torsion because of the “open” cross section of the hull which results in the shear
centre being below the bottom of the hull. Combined stresses due to vertical
bending, horizontal bending and torsion must be calculated and must meet classi-
fication society rules. A “closed” structure such as side tanks can add torsional
stiffness and should generally be incorporated in this type of ship.

10.4.2 Fatigue

Fatigue can result in the growth of cracks under the cyclic loads to which a ship is
subjected by the bending moments imposed by waves and vibration. The cracks
may not be dangerous in themselves but can lead to brittle fracture if let go too far.