The length of the vessel may be considered to act like a long girder pivoted on a wave about its centre. In this position the fore and after ends of the vessel will bend downwards, causing compression forces in the keel area and tension forces at the upper deck level.
The condition is brought about by increased buoyancy forces being created at and around the midships point of the vessel. Increased gravitational force, due to the metal structure of the vessel acting vertically downward, occurs at the extremities of the ship.When both forces exist at the same time, e.g. as the vessel is pivoted by a wave midships, a ‘hogging’ condition is present.