Page 138 - Introduction to Naval Architecture

124 STRENGTH

The study of ship strength is progressive in the sense that the
designer first considers the situation in still water and then goes on to
consider what happens in waves.

NATURE OF THE SHIP'S STRUCTURE

Some ships are made from glass reinforced plastics but the vast majority
are of steel with possibly some aluminium in the superstructure areas.
The following remarks relate to metal ships although GRP ships obey
1
the same general principles . The complete structure is composed of
panels of plating, normally rectangular and supported on the four
edges. They are subject to normal and in plane loads. Together with
their supporting stiffeners in the two directions, a group of plating
elements become a grillage which may be nominally in one plane or
curved in one or two directions. Grillages are combined to create the
hull, decks and bulkheads, all mutually supportive. Additional support
is provided by pillars and strong frameworks, for instance hatch
coamings.
Since, as will be seen, the major forces the hull must withstand are
those due to longitudinal bending, the ship structure must be such that
much of the material is disposed in the fore and aft direction. That is, the
hull is primarily longitudinally structured, whilst taking account of
transverse strength needs. The principal longitudinal elements are the
decks, shell plating, inner bottom all of which are in the form of grillages,
and additional longitudinal strengthening to these. The plating itself is
relatively thin and the spacing of the stiffeners must be such as to prevent
buckling. The transverse stiffening on decks, the beams, and on the side
shell, the side frames, is usually by a variety of rolled sections. Transverse
stiffening in the bottom consists of vertical plates, known as floors,
extending from the outer to the inner bottom. Longitudinal stiffening of
the bottom is by rolled section or plating called longitudinal girders or
simply longitudinals. The central longitudinal keel girder is one of
considerable importance. It is continuous fore and aft, extending from
the flat keel to the tank top or inner bottom. Sided longitudinal girders
are intercostal. That is, they are cut at each floor and welded to them. The
resulting 'egg box' type construction of the double bottom is a very
strong one and is capable of taking large loads such as those during
docking and of resisting the loads caused by running aground.
Most ships now use a longitudinal system of stiffening. Most warships
have used it for many years. It was adopted in some merchant ships
quite early, for example in the Great Eastern, but then gave way to
transversely framed structures. It was then adopted on a large scale in
tankers and was known as the Isherwood System?, It consists of stiffening
decks, side and bottom by longitudinal members the spacing being