2. The ship at sea or lying in still water is
constantly being subjected to a wide
variety of stresses and strains.
These forces may initially be classified into:
STATIC FORCES
DYNAMIC FORCES
3. STATIC FORCES
• These are the forces due to difference in weight and
buoyancy which occur at various points along the length
of the ship
5. Dynamic Forces
• A ship is free to move
about six degrees of
freedom.
Three linear and
three rotational.
•
• Various forces acting
on ship are constantly
varying in degree and
frequency.
• This movement of the
ship introduces
dynamic forces which
result in stresses on
the ship’s structure.
6.
7.
8. Forces produce stresses in the ship’s
structure which may be divided into two
categories:
•Global stress – affects the whole ship
•Local stress- affects a particular part of a
ship
9. HOGGING
• If the buoyancy
in midship is
greater the ship
will hog.
11. RACKING
• When a ship is rolling,
the accelerations on
the ship’s structure
are liable to cause
distortion in the
transverse section.
• Greatest effect is
under light ship
condition.
12. STRESSES DUE TO
WATER PRESSURE
• Water pressure
acts
perpendicular to
the surface and
increases with
depth
13. STRESSES DUE TO
DRY-DOCKING
• Tends to set the keel
upwards.
• Due to the up-thrust
of the keel blocks.
• Tendency for the
ship’s sides to bulge
outwards.
• Bilges tend to sag.
14.
15. SLAMMING OR POUNDING
• While heaving or pitching, the forward end leaves and re-enters the water with
a slamming effect.
• the slamming down of the forward region on to the water is known a
pounding.
• Flat areas of plating are brought into violent contact with water at a very acute
angle.
• There is a loud bang and the ship shudders.
• Momentum of the ship receives a check and energy is imparted to the ship
girder to make it vibrate.
• This is also known as pounding and is a result of Dynamic
forces.
16. PANTING
• This is a stress,
which occurs at the
ends of a vessel
due to variations in
water pressure on
the shell plating as
the vessel pitches
in a seaway. The
effect is
accentuated at the
bow when making
headway
DMS-DO
17. Arrangements to resist
Panting
• In the forepeak side stringers
are fited.
• The side singers meet at the
fore end.
• beams are fited at alternate
frames
18. Stresses caused by localized loading
Localized heavy
loads may give rise
to localized
distortion of the
transverse section.
Such local loads
may be the
machinery (Main
engine) in the
engine room or the
loading of
concentrated ore in
the holds.
forces within the ship result from structural weight ,cargo machinery and running of machinery.
External forces include:-
=Hydrostatic preassure of water on hull
=Action of wind and waves
to withstand the above forces the ship is built in such a manner that at all times ship is able to resist all stresses/strains throughout the structure.
These are due to
Internal forces resulting from structural weight, cargo and machinery weight.
External static forces including the hydrostatic pressure of the water on the hull.
Dynamic forces result from ship motion and action of wind and waves.
Heave is the motion of the ship when the ship have being up by a wave or sea.
Sway is the swing of a mast or bow of a ship from side to side as the vessel progresses in a heavy sea.
Surge is the movement forward as the bow of a ship rises and dips when it encounter waves which are strong enough to life it. The ship surge up the side of a wave, often shipping water as it passes through the crest, then dips down the other side of the wave.
Pitch is the motion of a ship in rising the crest of a wave then descending into the following trough.
Roll is the motion of a ship from side to side as she moves through the water.
Yaw is where the bow of a ship falls away or sways erratically from side to side as the vessel moves through the water.
the static and dynamic forces create transverse and loca; stresses in the ship structure. longitudinal stresses are greatest in magnitude and result in bending of the ship along its legth.
Hogging is when the ship bends upwards longitudinally. This occurs when there is more weight concentrated at the ends due to uneven cargo distribution or when the vessel rides a wave crest in its middle, causing excessive buoyancy.
Sagging is the reverse of hogging when the ship bends longitudinally in the downwards direction. This occurs when there is more weight concentrated in the mid length of the vessel due to uneven cargo distribution or when the vessel rides a wave trough in its middle causing excessive buoyancy at the ends.
when the ship is rolling, it is accelerated and decelerated, resulting in forces in the structure tending to distort it. this condition is known as Racking and its greatest effect is felt wen the ship is in light or ballast condition.
to resist this condition Brackets, beam knees joining horizontal ad vertical items of structre are used.
Water pressure increases
with depth and tends to set
in the ship's plating below
the water line.
Transverse section of a ship
is subjected to static pressure
from the surrounding water.
Considerable distortion of
structure can occur in
absence of adequate
stiffening.
This is a result of Static
forces.
This is a result of Static
forces.
additional stiffening must be fited in pounding region to reduce the possibility of damage to the structure.
to reduce pounding effect additional structural strength must be provided forward perpendicular aft for 25-30%of the ships length.
the shell plating in either side of the keel is increased in thickness, depending upon ships minimum draught.
frame spacing is reduced, full and half hieght intercoastal side girders are fitted and solid floors are installed in every frame space.
with longitudinal framing, longitudinal spacing is reduced.
intercoastal side girders are fited and tranverse floors are installed at alternate floors.
the movement of waves along a ship causes fluctuation in water preassure on the plating this tends to create in and out movement of shell plating kwn as panting.
this requirs special strenghtining in forepeak and aftpeak regions to counteract the stress.
The structure of a ship is strengthened to resist the effects of panting for 15% of ships length
in the forepeak side stringers are fited to the shell plating at intervals of 2m below the lowest deck
the side stringers meet at fore end which in many ships a horizontal stringer is fited to the collison bulkhead in line with each shell stringer, this forms a ring around the tank channel.
beams are fited in alternate frames in linw with the stringers and connected to frames by bracket.
