Stress in Still Water and in Seaway
Stress in still water occurs from vertical shear and longitudinal bending. This is caused by the unevenly distributed weight of the ship due to its structure of design and cargo load. The shear force then acts on the ship causing areas that have been heavily loaded to be matched with equal or more upward force to keep the ship buoyant. While areas that has less load are matched with less upward force to match the downward force created by the dead weight of the ship in that particular section. This result in the vertical shear and longitudinal bending which can either cause the ship to hog or sag. Which hogging is the longitudinal downward concave of a ship and sagging is the longitudinal upward concave of a ship.
Stress in seaway occurs from the variation of waves which create different buoyant levels while at sea. The crest and the troughs passing beneath the vessel increases bending moments which increases the shear force and longitudinal bending of the vessel. Depending on where the crest and troughs are formed will determine the type of stress that will act on the vessel. When a crest is formed forward of ship this can cause pounding to occur and also if both the fwd and aft of ship have crest and the amidships has a trough this will create a hogging effect of the ship. A sagging effect can also occur if a crest of a wave is at amidships of the vessel and troughs are at fwd and aft of the vessel. On a whole vessels at sea are exposed transverse, longitudinal and localized stresses which causes the ship to become fatigue.