SHIP CONSTRUCTION

1
SHIP CONSTRUCTION

• Group C
• Basujit Chakravarty
• Harsh Thakkar
• Vimal Raj Menon
• Shantanu Mishra

2
Explain, with the help of diagrams the effect of
dynamic forces on the structure of the ship.
3
DEGREES OF FREEDOM
Six ship motions are illustrated in the diagram.
When these motions are large then very large
forces may be generated. These forces are often
of a local nature, but are liable to cause the
structure to vibrate and thus transmit stresses
to other parts of the structure.
4
DYNAMIC FORCES

• Dynamic Forces result from
• The ships motion in the sea
• The action of the wind and waves
• The effects of operating machinery
• Effects of Dynamic Forces
• Sagging
• Hogging
• Racking
• Torsion
• Pounding or Slamming
• Panting

5
SAGGING
In a heavy seaway, a ship may be supported at the
ends by the crests of waves while the middle
remains unsupported. If the wave trough is now
considered at mid-ships then the buoyancy in this
region will be reduced. With the wave crest
positioned at the ends of the ship, the buoyancy
here will be increased. This loading condition
will result in a bending moment which will make
the ship to sag.
6
HOGGING
If the wave crest is considered at mid-ships then
the buoyancy in this region will be increased.
With the wave trough positioned at the ends of
the ship, the buoyancy here will be reduced. This
loading condition will result in a significantly
increased bending moment, which will cause the
ship to hog. This will be an extreme condition
giving the maximum bending moment that can occur
in the ships structure.
7
RACKING
When a ship is rolling, the accelerations on the
ships structure are liable to cause distortion
in the transverse direction. The deck tends to
move laterally relative to the bottom structure,
and the shell on one side to move vertically
relative to the other side.
This type of deformation is referred to as
Racking. Transverse bulkheads primarily resist
such transverse deformation, the side frames
contribution being insignificant, provided the
transverse bulkheads are at their usual regular
spacing. Its greatest effect is felt when the
ship is in the light or ballast condition. The
brackets and beam knees joining horizontal and
vertical items of structure are used to resist
this distortion.
8
TORSION
A ship traversing a wave train at an angle will
be subject to a twisting moment (torque) and the
structure in torsion The greatest effect occurs
with decks having large openings. In some ships,
a heavy torsion box girder including the upper
deck is provided at the topsides to accommodate
the torsional stresses.
9
POUNDING OR SLAMMING
Heavy pitching assisted by heaving as the whole
ship is lifted in a seaway may subject the
forepart to severe impact from the sea. The
greatest effect is experienced in the lightship
condition. To compensate for this, the bottom
over 30 forward is additionally strengthened in
ships exceeding 65m in length and in which the
minimum draught forward is less than 0.045L in
any operating condition.
10
PANTING
This is a stress which occurs at the ends of a
ship due to variations in water pressure on the
shell plating as the ship pitches in a seaway.
This tends to create and in-and-out movement of
the shell plating, known as panting. The effect
is accentuated at the bow when making
headway. Additional stiffening is provided in the
form of panting beams and stringers.