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A bulk carrier is a type of ship designed to carry bulk materials such as iron ore, corn and sugar. Some of the bulk carrier designs are also used to move liquids, however, these are typically classified as tankers, with the rare exception being a dry/liquid bulk combination ship that is used to move both. Some types of bulk carrier are self-unloading with large, deck-mounted cranes, while others are dock-unloading versions that use dock cranes to remove the cargo from the hold of the ship. Other designs are known as bulk in, bag out (BIBO). These ships transport materials as bulk and then bag the product into individual bags as the materials are unloaded.
Some of the earliest bulk carrier ships were wooden sailing vessels that transported coal and grain. Many of these early ships needed to assemble makeshift bins within the hold to transport the bulk material. This created difficulty in unloading, and many of the ships sank due to the absorption of water through faulty hatch seals. Sandbags and seawater were commonly used as ballast in the early bulk carrier ships to counterbalance the weight of the loose cargo materials. Once in port, it could take several days for a crew to unload the cargo from the cramped and congested hull.
Some modern bulk carrier ships are known as self-loading and self-unloading due to the implementation of large, overhead cranes mounted onto the deck of the ship. The cranes make the task of unloading much easier and are also used to remove the hatch covers on many ships. The size of the cranes make it possible to lift a bulldozer and set it in the hold. The bulldozer is used to push the cargo, such as grains, coal or sugar, into a pile in the center of the hatch opening. This makes it possible for the cranes to easily remove all of the cargo from the bulk carrier.
Some BIBO vessels carry cargo in the hold as a bulk material and then bag the cargo as it is off-loaded. This is done to speed the loading process, provide a more stable cargo as well as avoid certain taxes and restrictions imposed on ships containing packaged materials. Large conveyors are used to move the bulk cargo to a bagging area within the ship, and the bagged materials are then palletized and off-loaded. In some instances, the bulk carrier transports some liquid as well as some dry cargo in a single vessel. This is especially true with sugars and sweetening syrups.