As youngsters, you must have sailed paper boats. You watched them float until they got all soaked up and sank. You might wonder why these paper boats could float; after all they were small and only made out of paper. But how about the real big sea vessels that sail on smoothly? How is this possible that despite their sizes and the heavy material they are made of, they do not sink? The famous Greek mathematician Archimedes
supplies us with the answers. Archimedes found out that sea vessel could float because water exerts an upward force called buoyant force
. His principle states states that the buoyant force on solid placed in a liquid is equal to the weight of the displaced liquid. If the total weight of the object is less than the amount of water it has displaced, it will float.
Have you notice that if you allow any hollow object such as a plastic container to float on a basin of water, you will observe that as you place it on the water, part of it will submerged. You will also notice that as it submerges, it gently propels water to the sides. This amount of water displaced is equal to the volume that is submerged in water. The container can only displace as much water as its own size. In principle, if the amount of water displaced is equal to the weight of the container, the container floats; if it is less, the container sinks.
A ship is always built in such a way that its weight is less than of all the water it can displace. Ships will float on smoothly and speedily if they are stable and have a low resistance to the water and air. These feature lie on the design and structure of the ships, particularly the hull
(main body of the sheep). The hull must have bulkheads or steel walls divided into watertight compartment. In case the ship is accidentally hit and gets a hole in the side, only the holed compartment is filled with water and not the whole ship. The part of the ship that serves as a backbone is the keel; which has frame or ribs that run the length of the ship.
What cause a ship to stay upright despite of its buoyancy? A ship is stable when its center of gravity and center of buoyancy are in push and pull motions. The center of gravity is defined as the spot where all the forces pushing upwards appear to act. The center of buoyancy is located somewhere at the middle of the ship. When the ship leans over, the center of buoyancy moves to one side but its force is still upward. The line where this force meets the center of buoyancy is called the metacenter.
Metacenter is the vertical line where the force of buoyancy meets the center of buoyancy. An ideal way to keep a boat steady is to make sure that the center of gravity is low. This can be done by adding extra weight to the ship without overloading.
For submarines, the mechanisms of floating and sinking can be controlled. Submarines have ballast tanks
, which are large containers for water and air to help the submarine go underwater, the ballast tanks are filled with sea water. If it needs to emerge, the water is released from the tanks.
Overloading is one of the causes a ship to sink; navigation aids for ships and other sea vessels are continuously provided for by the government to prevent any sea accidents.