Sinking and Floating: Buoyancy
What happens to the buoyancy of the ship when cargo and passengers are added?
Hippopotamuses spend over half their day in the water. In order to eat, the hippo, which can reach almost 3 meters (10 feet) in length and weigh 10,000 pounds, must sink to feed off the vegetation that grows on the bottom of rivers. The hippo, however, has one major problem: his low density forces him to float at the surface, and he is not agile enough to quickly dive down to the bottom and come back up again. In order to reach the bottom, he needs to increase his density, so the buoyant force cannot supply a large enough force to keep the animal afloat. To do this, the hippo exhales, reducing the air in his body to increase his density.
When cargo and passengers are added to a ship its weight increases. As the ship’s weight increases is sinks further into the water, displacing a greater weight of water. If it sinks so far that water can spill into the ship, increasing its weight even more, the ship sinks to the bottom of the water.
The amount the ship sinks in the water as a result of cargo and passengers can be critical for navigation and maneuverability. Large cargo and cruise ships have numbers on the bow of the ship that indicate how far the distance is between the water line and the bottom of the ship. This distance is called the ship’s draft. If the ship has a 6-kilometer (20-foot) draft and the water is only 5.5 kilometer (18 feet) deep, cargo and passengers must be unloaded to allow the ship to rise.