What two main types of forces act upon a ship's entire hull girder, causing it to bend and shear, especially in waves?

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 buoyancy decreases there. This continuous, uneven change in localized buoyancy distribution along the hull, relative to the more constant distribution of the ship's weight, creates significant wave-induced bending moments and shear forces. A ship experiences *hoggingwhen the midship section is supported by a wave crest, causing the bow and stern to drop relative to the middle, leading to compressive stress on the upper deck and tensile stress on the keel. Conversely, *saggingoccurs when the bow and stern are supported by wave crests (or the midship is in a trough), causing the middle to drop relative to the ends, leading to tensile stress on the upper deck and compressive stress on the keel. *Shear forcesare associated with the rate of change of these bending moments along the hull, representing the tendency for one section of the hull to slide vertically past an adjacent section. Hydrodynamic forces are dynamic forces generated by the motion of the water relative to the ship, or the rapid motion and acceleration of the ship relative to the water. Unlike hydrostatic forces, which primarily depend on the volume of displaced water, hydrodynamic forces arise from the dynamic interaction between the moving water and the moving hull. In waves, these forces manifest in several ways. Firstly, the passage of waves itself causes the water particles to move and accelerate, generating dynamic pressure variations on the hull that are distinct from simple hydrostatic pressure changes. Secondly, the ship's own motions in response to waves create significant hydrodynamic impacts. *Slammingis a major hydrodynamic force, occurring when a part of the hull, typically the bow or stern, emerges from the water due to pitching motion and then re-enters the water with a high relative velocity, or when a wave forcefully impacts a relatively flat bottom area forward. This impact generates a very large, impulsive pressure spike on the hull surface. These slamming impacts can excite global vibrations of the entire hull girder, a phenomenon known as *whipping*. Whipping causes additional, rapidly oscillating bending moments and shear forces that are superimposed on the slower-varying hydrostatic and static wave-induced components, significantly increasing the total dynamic stress on the hull. Another hydrodynamic phenomenon is *springing*, which is a continuous, resonant vibration of the hull girder excited by regular wave encounters, typically in head or following seas. Springing results from the continuous interaction between the oscillating wave pressures and the hull's elastic properties, causing fatigue-inducing cyclical stresses. These various hydrodynamic forces, whether impulsive from slamming or continuous from wave interaction, directly contribute to the total dynamic bending moments and shear forces experienced by the hull girder, often leading to higher peak stresses than those caused by hydrostatic forces alone.