Britannica
Sources used in this answer
- naval architecture - Variation of buoyancy and weight along the length of the ship
- ship - Structural integrity
- naval architecture - Hydrostatic and hydrodynamic loads in service
- naval architecture - Strength of ships
- naval architecture - Structural configurations
- naval architecture - Superposition of calm-water and dynamic wave loads
- naval architecture - Scantlings and strength calculations
- ship construction - The naval architect
How does a shipâs hull act like a beam when waves support it unevenly?
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naval architecture
A cargo ship passing the Golden Gate Bridge, near San Francisco.
In naval architecture, a shipâs hull is conceptually treated as a beam designed to support its own weight and the weight of its cargo while resisting external forces, such as uneven buoyancy caused by waves.1
When a ship is supported unevenly by waves, the hull experiences longitudinal bending moments that cause it to deform in two primary ways:
- Hogging: When the ship is supported by a wave crest amidships and troughs at the ends, the buoyancy is greater in the middle. This causes the midship portion to bend upward while the ends droop.23
- Sagging: When the ship is supported by wave crests at the bow and stern with a trough amidships, the buoyancy is lower in the middle. This causes the midship portion to bend downward.23
To maintain structural integrity, the hull is designed as a hollow box girder, where the deck, sides, and bottom plating act as flanges and webs to resist these bending stresses. Naval architects calculate these forces to ensure the structure remains within safe limits, preventing buckling or failure under the dynamic loads encountered at sea.415