Question
Describe how a ships structure withstands
the forces acting on it:
When floating;
When in dry dock.
Answer
(a) A ship at sea is subjected to a number of
forces which may be: hogging and sagging; racking; effects of water pressure;
panting and pounding.
Longitudinal stresses created by hogging and sagging can be very severe. These
stresses are resisted by all continuous longitudinal material especially those
parts further from the neutral axis.
Double bottoms (centre girder, side girders/inner bottom and outer bottom
longitudinal, keel and bottom shell, tank top plating); side shell at the top
(the sheer strake); deck stringer plates; continuous decks together with their
longitudinal girders and deck longitudinal.
In tankers, longitudinal bulkheads also give great
strength.
High tensile steels for high stress areas are now
used, especially in large ships.
When a ship is rolling in a seaway, or is struck by
beam waves, the ship's structure is liable to racking stresses. The stress
mainly affects the corners of the ship, i.e. on the tank side brackets and the
beam knees, which must be made strong enough to resist it. Transverse
bulkheads, web frames, frames, or cantilever frames provide very great strength
to resist this stress.
Water pressure acts perpendicular to the shell of the
ship, increasing with depth. The effect is to push the ship's sides in and the
bottom up. It is resisted by frames, bulkheads, floor and girders•
Panting is an in and out motion of the plating which
occurs at the ends of the vessel due to the variation in water pressure as the
vessel pitches in a seaway.
The effect . is accentuated at the bow when making headway. In general, the structure is strengthened to
0.15 L aft of the forward perpendicular to combat panting.
When a ship is pitching, the bows often lift clear of
the water and then slam down heavily onto the sea, subjecting the forepart to
severe pounding. To compensate for -this, the bottom is strengthened to 0.25L
aft of the forward perpendicular.
(b) When the
vessel is allowed to settle on the keel blocks before side blocks are
positioned, transverse stress will be induced in the structure and it will have
the tendency to sag at the bilges.
These stresses are resisted by the double bottom
structure (floors etc.) transverse webs and bulkheads. Tankers having no double
bottoms for most of the ship length, have a deep centre girder with 'docking
brackets'.
Longitudinal stresses are created since the vessel
generally grounds aft first and this creates longitudinal bending stress on the
ship.
Resistance to the stresses will be by all
longitudinally continuous material, particularly those items furthest from the
neutral axis. e.g. upper deck, bottom shell and tank
top together with associated longitudinals, girders
etc. (as for resistance to hogging and sagging)
High pressures are created aft as the vessel grounds
aft first and this increases to a maximum as the water
level drops and the vessel is about to