Hogging is one of the two fundamental bending conditions a ship’s hull experiences — the middle of the vessel bending upward relative to the bow and stern.
Why Hogging Happens
Hogging occurs in two main circumstances:
- Wave-induced hogging — when a wave crest sits directly amidships while the bow and stern extend into the relatively lower wave troughs on either side, the buoyant support is concentrated in the middle, tending to push the middle up relative to the ends
- Loading-induced hogging — when cargo/weight is distributed more heavily toward the bow and stern than amidships, gravity pulls the ends down relative to the middle, producing the same hogging shape even in calm water
Hogging vs Sagging
- Hogging — middle bends upward relative to the ends
- Sagging — middle bends downward relative to the ends (the opposite condition, typically occurring when a wave trough is amidships with crests supporting bow and stern, or when weight is concentrated amidships)
A vessel experiences both conditions repeatedly as it moves through a wave train — the hull genuinely flexes slightly with each wave cycle, which is exactly why longitudinal strength (girders, deck and bottom plating) is calculated to withstand this continuous bending stress.
Why This Matters for Loading
Cargo and ballast distribution directly affects whether a vessel’s static condition (before considering wave action) tends toward hogging or sagging — stability and loading calculations check that bending stress, whether from loading condition alone or combined with expected wave action, stays within the vessel’s structural limits as specified in its approved stability/loading manual.
Real Structural Consequence
Excessive hogging or sagging stress, if not properly managed through correct loading, can cause genuine structural damage — this isn’t a purely theoretical calculation, but a real limit that loading officers must respect, particularly on vessels carrying unevenly distributed cargo (heavy cargo concentrated at the ends, for instance).
Exam Relevance
Hogging, its causes, and its relationship to sagging and longitudinal strength calculations are core, heavily-tested Ship Stability topics in deck officer competency exams.