Hogging and Sagging
Hogging
When the beam of a vessel is loaded or otherwise is subjected to external forces such that the beam bends with the ends curving downwards it is termed as hogging stress.
For a ship, improper loading as well as in a seaway when riding the crest of a wave the unsupported ends of the ship would have a tendency similar to the beam shown below.
Sagging
In this case the beam is loaded or other wise subjected to external forces making the beam bend in such a way that the ends curve upwards. this is terned as sagging.
Similar with a ship, if improper loaded or when riding the trough of a wave – with crests at both ends then the ship is termed to be sagging.
For Hogging the ship ends to curve downwards would mean that the weight/ load amidships is much less than at the end holds/ tanks.
For Sagging the ship would have been loaded in such a manner that a greater percentage of the load is around the midship area.
In a seaway the hogging and the sagging stresses are amplified when riding the crests and falling into the troughs. Thus especially for large ships there are two conditions in the stability software, Sea Condition and Harbour condition.