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Bulk carrier discharging ballast water. Image supplied courtesy of International Maritime Organization
The word draft is used to explain how many metres the hull of a ship is below the surface of the water. The water line refers to where the water comes to on the side of the hull of a ship. Lines are painted on the ship's side to show the maximum draft the ship can be loaded to. These are called load lines. Numbers are painted on the bow and stern to show the ship's draft. They can be in either feet or metres.
Each ship is designed to be stable in the water between specific drafts. When the ship has no cargo, it will float above its lowest draft and the ship's propeller will be out of the water. To keep the ship steady on an even trim and the propeller below the water line pumps are used to place seawater into ballast tanks within the ship.
This makes the ship heavier and keeps it to the correct draft.
Ships have many small tanks for the storage of ballast water. This prevents water moving about, and upsetting the balance of the ship. Oil tankers have ballast tanks in the space between the inner and outer hull and bulk carriers have side ballast tanks. As these ships are loaded, ballast water is pumped out to keep the ship within the load lines. When tankers and bulk carriers unload or discharge their cargo, ballast water is pumped in. Marine pests, which live in the sea, can be carried in ship's ballast water from one part of the world to another. If this ballast water is discharged in a foreign marine environment, it can cause major harm.