Page 71 - Ship Construction.DJ Eyres 6Ed

Ch08-H8070.fm Page 60 Wednesday, October 18, 2006 7:29 AM

8
Stresses to which a Ship is Subject

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The stresses experienced by the ship floating in still water and when at sea
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
may conveniently be considered separately.

Vertical Shear and Longitudinal Bending in Still Water
If a homogeneous body of uniform cross-section and weight is floating in
still water, at any section the weight and buoyancy forces are equal and
opposite. Therefore there is no resultant force at a section and the body will
not be stressed or deformed. A ship floating in still water has an unevenly
distributed weight owing to both cargo distribution and structural distribution.
The buoyancy distribution is also non-uniform since the underwater sec-
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tional area is not constant along the length. Total weight and total buoyancy
are of course balanced, but at each section there will be a resultant force or
load, either an excess of buoyancy or excess of load. Since the vessel remains
intact there are vertical upward and downward forces tending to distort the
vessel (see Figure 8.1) which are referred to as vertical shearing forces, since
they tend to shear the vertical material in the hull.
The ship shown in Figure 8.1 will be loaded in a similar manner to the
beam shown below it, and will tend to bend in a similar manner owing to
the variation in vertical loading. It can be seen that the upper fibres of the
beam would be in tension; similarly the material forming the deck of the
ship with this loading. Conversely the lower fibres of the beam, and likewise
the material forming the bottom of the ship, will be in compression. A vessel
bending in this manner is said to be ‘hogging’ and if it takes up the reverse
form with excess weight amidships is said to be ‘sagging’. When sagging the
deck will be in compression and the bottom shell in tension. Lying in still
water the vessel is subjected to bending moments either hogging and sagging
depending on the relative weight and buoyancy forces, and it will also be
subjected to vertical shear forces.

Bending Moments in a Seaway

When a ship is in a seaway the waves with their troughs and crests produce
a greater variation in the buoyant forces and therefore can increase the