Torsional stresses in ships

Torsion in ships is caused due to forces which do not pass through the sheer centre line axis of a ship’s hull cross section. Torsion basically tends to twist the vessel just like how we rinse a cloth by twisting it.

Torsional moment has two main components namely –

a) static torsion or still water torsion,

b) dynamic torsion or wave induced torsion.

Other forms of torsional moments arise from thevibration of propeller shaft,vibrations due to twinscrew propellers etc.As the name suggests, wave induced torsion is caused due to the unsymmetrical hydrodynamic wave loading on the port and the starboard sides of the vessel. Similarly, still water loading is caused due to the unsymmetrical cargo loading over port and starboard with the ship remaining upright.

A ship heading obliquely to a wave will be subjected to righting moments of opposite direction at its ends, twisting the hull and putting it in ‘torsion’.In most ships these torsional moments and stresses are negligible but in ships such as large container ships with extremely wide and long deck openings they are significant.