(i) Hogging – If the wave crest is amidships, the buoyancy amidships is increased and the buoyancy at the ends is reduced, the ship will hog. It is equivalent to a beam supported at the centre and loaded at the ends.
When a ship hogs, the deck structure is in tension while the bottom shell/platting in compression.
(ii) Sagging – If the wave trough is amidships, the buoyancy amidships is reduced and the buoyancy at the ends is increased, the ship will sag. It is equivalent to a beam supported at its ends and loaded at the centre
When a ship sags, the bottom shell/plating is in tension while the deck structure is in compression.
Forces/ Loads acting on Transverse Structure
Racking – When the ship rolls there is a tendency for the ship to distort transversely. This is known as Racking. It is similar to a picture frame collapsing.
Other Forces/Loads acting on the ship’s Structures
Pounding – When ship meets heavy weather and commences to heaving and pitching, the rise of the fore end of the ship occasionally synchronises with the trough of a wave. The fore end then emerges from the water and renters with a tremendous slamming effect known as pounding. This can cause damage to the bottom of the ship forward. To prevent buckling the shell plating must be stiffened. Pounding also occurs aft in way of cruiser stern, but has very less effects
Panting – As the waves pass along the ship they cause fluctuations in water pressure which creates an in-and-out movement of the shell plating. The effect of this is greatest at the ends of the ship, particularly at the fore end, where the shell is relatively flat. These movements are known as panting and if unrestricted could eventually lead to fatigue of the material. To prevent the movement of the shell, the structure at the ends of the ship is stiffened.