A container ship needs to remain balanced in the water. At times ships will be hit by large waves and strong winds. If the cargo containers are not evenly stacked on a ship, this can change the balance of the ship.
There are two main dangers if the cargo is not properly balanced. If the heaviest load on the ship is at either end then there will be a lot of stress on the middle of the ship when it is hit by waves. Over time, this can crack the middle of a large ship. If the weight of cargo is not evenly distributed on both sides of a ship, it can easily topple over or lean to one side.
Cargo ships including container ships have a cargo plan. The plan shows the location on the ship of every bit of cargo and every container as well as the port where it will be unloaded. The plan is very complicated and is usually worked out using computer programs. Container ships in port will often exchange some but not all of their containers. Some containers will remain and new containers will be loaded. To work out the cargo plan the cargo planners must make sure of the following things.