Numerous forces act on a ship’s structure, some are static forces and some are dynamic forces.
Static forces are due to the difference in weight and support which occurs throughout the ship.
The dynamic forces are caused by the motion of the ship at sea and the action of the wind and wave.
These forces make 3 type of stresses in ship structure
> Longitudinal stress
> Transverse stress
> Local stress
Longitudinal stress
> The forces are two in number, the weight of the ship and all that it carries acting downwards and the vertical component of the hydrostatic pressure.
> Depending upon the direction in which the bending moment acts the ship will Hog or Sag.
Must Read: Explain Hogging And Sagging In Ship
Transverse Stress
Racking
When a ship is rolling in a seaway or is struck by beam waves, the ship’s structure is liable to distort in a transverse direction as shown.
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, frames and web frames provide very great strength to resist racking.
Local Stress
Panting
> It is an in and out motion of the shell plating caused by fluctuation in water pressure because of water waves .
>To prevent this classification societies are given extra strength in the form of beam, brackets, stringer plate etc.
The structure at the fore end is strengthened to resist panting to a distance of 0.15L aft of the forward perpendicular. Forward of the collision bulkhead, ‘panting stringers’ are fitted not more than 2.0m apart. The stringers are bracketed to the shell frames, and panting beams are fitted on alternate frames under each panting stringer. Pillars are fitted on the centreline (usually a wash bulkhead) to tie the panting beams together. Deep plate floors are fitted on each frame station and are flanged on their upper edges. Between the collision bulkhead and 15%L aft of FP, intercostal stringers are fitted in line with the panting stringers.
Pounding or Slamming
>At the time of heavy weather the ship may heaving and pitching, so the fore end emerges from the water and re-enters with a slamming effect called pounding.
>Extra stiffening is required to reduce the damage
To resist pounding, the forward bottom structure is strengthened for between 25% and 30% of the length, depending on the ship's block coefficient. Plate floors are fitted on each frame station (transverse framing) or alternate frame stations (longitudinal framing) with intercostal side girders not more than 2.2m apart. The four strakes of shell plating either side of the keel are generally increased in thickness in the pounding region.