State FOUR terms used to describe the conditions that relate to the distortion of a ship's hull undergoes in heavy seas, stating in EACH case the type of stresses involved and where the stresses occur. (16)
Structured for DG Shipping MEO Class II examination scoring criteria.
Sagging: Sagging deformation are caused by the uneven distribution of weight and buoyancy in the length of the hull. If a ship is supported on either end by the crest of a wave, the forces exerted by the waves on the buoyancy of the vessel would tend to lift the ends, whilst the center of the vessel would suffer a loss of buoyancy and tend to sag. This deformation is known as sagging.
Type of Stress: Compressive stress on deck, tensile stress on keel.
Hogging: When the wave passes the reverse situation of hogging results, i.e. the middle of the vessel is supported on the crest whilst the two ends hang over the crest on either side. This action is known as hogging.
Type of Stress: Tensile stress on deck, compressive stress on keel
Racking: When a vessel is rolling in a seaway the transverse section will try to distort at the corners due to racking stresses.
Type of Stress: Shear stress on transverse bulkheads and hull structure
Torsional forces: If a vessel is subjected to pitching and rolling at the same time, i.e. taking the wave on either bow, the vessel tends to twist longitudinally. These are known as torsional stresses or forces.