Hogging is the stress a ship 's hull or keel experiences that causes the center or the keel to bend upward . Sagging is the stress a ship 's hull or keel is placed under when a wave is the same length as the ship and the ship is in the trough of two waves . This causes the middle of the ship to bend down slightly , and depending of the level of bend , may cause the hull to snap or crack . Sagging or dynamic hogging may have been what sank the '' Prestige '' off Spain on 19 November 2002
Prestige
Prestige oil spill
Hogging and sagging