Hogging: the ship bends so that the ends are lower than the middle - the keel is convex downwards (the deck is in tension, the bottom in compression). It occurs when the ends are heavier than the middle, i.e. when the buoyancy is concentrated amidships and the weight at the ends. This happens when the ship is light in the middle (e.g. empty midship holds) and heavy at the ends, or when the ship is in a wave crest amidships (a wave crest under the middle with troughs at the ends) - the "hogging" wave condition.

Sagging: the ship bends so that the middle is lower than the ends - the keel is concave downwards (the deck is in compression, the bottom in tension). It occurs when the middle is heavier than the ends, i.e. when the weight is concentrated amidships and buoyancy at the ends. This happens when the ship is loaded heavily amidships (full midship holds, empty ends), or when the ship is in a wave trough amidships (a wave trough under the middle with crests at the ends) - the "sagging" wave condition.

Causes: the difference between the weight distribution and the buoyancy distribution along the length. In still water, hogging/sagging arise from the loading pattern; in a seaway, the wave profile changes the buoyancy distribution, and the maximum hogging/sagging moments occur when the wave length is about equal to the ship length and the wave crest/trough is amidships. Hogging is most likely in ballast (light ship, empty midship) and sagging in the fully loaded condition with full midship holds.

Effects: hogging and sagging produce longitudinal bending moments and shear forces in the hull girder. The deck and bottom are the extreme fibres and carry the highest bending stresses - in hogging the deck is in tension and the bottom in compression; in sagging the deck is in compression and the bottom in tension. Excessive bending can cause buckling of the deck or bottom plating, cracking, and fatigue, and can lead to structural failure. The shear forces are greatest near the quarter points. The hull girder must be designed to withstand the maximum still-water and wave bending moments.

Measures:

• Design: adequate section modulus of the hull girder (deep, strong deck and bottom, continuous longitudinal material), high-tensile steel where appropriate, and the hull girder designed for the maximum combined still-water and wave bending moment.
• Loading: distribute cargo to avoid excessive hogging/sagging - avoid empty midship holds in ballast (use ballast in midship tanks), avoid overloading the ends or the middle, and follow the approved loading manual and the maximum still-water bending moment limits.
• Operation: use ballast to reduce the still-water bending moment, avoid heavy weather that increases the wave bending moment, and reduce speed in heavy seas; monitor the hull stress (hull stress monitoring system) and the draft/trim.
• Maintenance: inspect and maintain the deck and bottom structure, and repair corrosion and fatigue cracks promptly.