These stresses are measured when the ship is not under way. They are often caused by a poor longitudinal distribution of mass. Even if the ship’s total weight is balanced by the total force of buoyancy, these forces may not be distributed evenly along the full length of the ship.

• Hogging: If the forces of buoyancy are concentrated around the section amidships and the ends are loaded, the ship will tend to move downwards at the bow and stern while the section amidships will tend to move upwards. In this situation, the deck’s structural members are being subjected to tensile stress while the bottom structure is under compressive stress. This phenomenon can be compared to a beam supported in the centre and loaded with weights on the ends.

Hogging

• Sagging: If the forces of buoyancy are concentrated under the bow and stern of the ship and the section amidships is loaded, the ship will tend to move upwards at the ends and trough amidships. In this situation, the deck’s structural members are under compressive stress while the bottom structure is being subjected to tensile stress. This phenomenon can be compared to a beam that is supported at both ends and loaded with weights in the middle.

Hogging and sagging can be amplified by the movement of waves passing along the hull. A crest of waves at each end of a ship combined with a trough amidships will amplify sagging, while a crest amidships combined with a trough at both ends will amplify hogging.

The stresses caused by these situations can be calculated using the load curves table, the stress and sheer curves table, and the bending moments table. Manual or electronic calculators also exist to find the value of the stresses on the hull. The maximum permissible stress values can be found in the ship’s stability book.

Dynamic stresses and constraints

When a ship is under way, some situations create additional stresses. They are caused primarily by the effect of waves on the hull in rough seas. Two of these are pounding and panting.

• Pounding: When a ship sails in heavy seas, it pitches. It can happen that the bow rises over the crest of a wave and emerges completely out of the water. When the bow comes back down on the water, it can be subjected to a major impact, which is pounding. The hull plating at the bow end of the ship must be reinforced to avoid bending of the plating. This stress can also occur at the ship’s stern, but to a lesser degree.
• Panting: When waves hit the bow and stern of a ship, they create variations in pressure that tend to push the plating in and out. This is panting. The framing at the ship’s ends must be reinforced to prevent exaggerated movement of the hull plating.