piece of wood in a lake, a stream, or the ocean and it will be seen to roll and bob with the motion of the water. These motions resemble those of an airplane; likewise a ship can move in six different ways: Forward, sideways, vertically (up and down); as well as roll sideways, pitch up and down, and yaw about a vertical axis. Once a vessel has been designed for level floating in calm water, with sufficient freeboard (clearance above the waterline), when it is fully loaded and the load properly distributed so the decks stay high and dry, its roll and pitch are the two characteristics of greatest interest.
The weight of a ship and its cargo may be thought of as a force acting downward through its center of gravity. Opposing this downward force is an upward force—the buoyancy force—that keeps the vessel afloat. When the ship is upright and level, the buoyancy force acts through a point called the center of buoyancy that is aligned with the center of gravity. As the ship rolls, the shape of that part of the hull presented to the water changes, and the buoyancy force now pushes upward at a point away from the center of the ship (more toward the side of the hull), causing the ship to return to an upright position. When the ship rolls, the buoyancy force is directed through a point above the center of gravity called the metacenter. The metacentric height is the distance from the center of gravity to the metacenter. It is usually abbreviated GM.
A vessel with a large metacentric height and weight low in the vessel (a tanker or dry bulk ship) is very stable—that is, it can heel over a great deal and still return to an upright position. Such a vessel is also said to be “stiff,” meaning it responds quickly to sea motions and is uncomfortable to passengers. On the other hand, a vessel with a smaller metacentric height (a passenger ship) will be more comfortable to passengers because of its slower response to rolling. This type of vessel is said to be “tender.” (Remember reading about the SS Waratah elsewhere in this book?)
The cargo in a bulk carrier, container ship, or tanker, when properly stored and evenly distributed, ensures that the vessel rides in the water as the designers intended—not too high, so as to be top heavy, and not too low, so as to be subject to breaking seas. In the empty state, vessels take on ballast to maintain stability.