The two main types of forces acting upon a ship's entire hull girder, causing it to bend and shear, especially in waves, are hydrostatic forces and hydrodynamic forces. Hydrostatic forces are those generated by the pressure of the water in which the ship floats, assuming the water is at rest or its changes are very slow. The primary hydrostatic force is buoyancy, which is the upward force exerted by a fluid that opposes the weight of an immersed object. Buoyancy is distributed along the ship's length according to the hull's shape and the volume of water it displaces. While the total buoyancy balances the total weight in a static condition, the distribution of buoyancy along the ship's length rarely perfectly matches the distribution of the ship's weight (from its structure, cargo, and machinery). This inherent mismatch creates what are known as still water bending moments and shear forces. When a ship encounters waves, the local water level around the hull continuously changes. For example, as a wave crest passes under the amidships section, that part of the hull becomes more deeply submerged than the bow and stern, increasing the local upward buoyancy significantly in the middle. Conversely, when a wave trough is amidships, the local buoya....
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