Hogging and sagging are the two conditions of longitudinal bending of a ship's hull.

Hogging:

• The ship is supported by buoyancy at the ends (bow and stern) while the middle is relatively unsupported, or the weight is concentrated amidships.
• The ends tend to rise and the middle tends to droop, so the keel is convex downwards (curves upward at the ends).
• This occurs when the ship is light in the middle and heavy at the ends, or when the wave crest is at the ends and the trough is amidships (ship on a wave crest at bow and stern).
• The deck is in tension (stretched) and the bottom is in compression.

Sagging:

• The ship is supported by buoyancy amidships while the ends are relatively unsupported, or the weight is concentrated at the ends.
• The middle tends to sag and the ends tend to rise, so the keel is concave downwards (curves downward in the middle).
• This occurs when the ship is heavy in the middle and light at the ends, or when the wave crest is amidships and the troughs are at the ends (ship on a wave crest amidships).
• The deck is in compression and the bottom is in tension.

Structural members affected and the stresses they are subjected to:

• Deck plating and deck girders (stringers, longitudinals): in hogging the deck is in tension; in sagging the deck is in compression.
• Bottom shell plating and bottom longitudinals, keel: in hogging the bottom is in compression; in sagging the bottom is in tension.
• Side shell plating: subjected to shear stresses, with the maximum shear stress occurring near the neutral axis (approximately mid-depth of the hull).
• Longitudinal bulkheads and girders: share the bending stresses, in tension or compression depending on their position relative to the neutral axis and the bending condition.
• Transverse frames and bulkheads: resist the distortion of the cross-section (racking) and transmit the loads.
• The neutral axis (approximately at mid-depth) is the level where the bending stress is zero; members above the neutral axis are in tension in hogging and compression in sagging, and vice versa for members below.

The maximum bending moment occurs when the ship is on a wave of length approximately equal to the ship's length, with either a crest or trough amidships, and the resulting hogging or sagging stresses are the largest longitudinal stresses the hull experiences.