Hogging and Sagging: On longships, such as very large tankers and bulk carriers, those responsible for loading the ship have to take care to avoid straining the ship’s hull.
If too much weight is placed amidships the vessel will SAG. As the ship cannot submerge her load line mark amidships she will not be able to load her full cargo.
If excess weight is placed at the ends of the ship and not enough in the middle the vessel may HOG. If a ship in such a condition were loaded with a full deadweight cargo, her load line marks amidships would indicate she could carry more cargo.
In the old days, it is said that this was done deliberately. With large modern vessels this distortion can be feet rather than inches, and apart from the obvious strain on the hull and the problems already mentioned, it may also increase the draft which is often critical for large ships getting in and out of port.
Ships are fairly flexible structures and the bending may not do much permanent harm; but if bent severely the ship may become permanently distorted, which is obviously undesirable from many points of view.
To help ships’ officers and those responsible for making the necessary calculations avoid this bending they must, of course, be supplied with the necessary information, gadgets, and calculators. Any such longitudinal stresses will be aggravated by the ship pitching when ends on to the waves.