Keeping a container ship balanced
Cargo plans

Image supplied courtesy of Port of Melbourne Corporation. 

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.
  1. Stack containers in groups that will be removed in different ports; it becomes more expensive if containers are moved around to get to containers at the bottom of the ship.
  2. If there are very heavy containers they are usually best placed at the bottom; this stops the ship becoming top heavy.
  3. The weight of the containers needs to be evenly spread from the front to the back of the ship.
  4. Often the containers are empty, being returned to collect new cargo; empty containers are usually stowed on deck.
  5. The weight of the containers needs to be evenly spread from side to side of the ship.
  6. What is already on the ship and where there are spaces when containers are removed; they will need to group containers that will be offloaded at different ports.