There are two principal types of vessels in the Great Lakes fleets: bulkers, which do not have any onboard equipment for unloading cargo and must rely on shore-based cranes, and self-unloaders, which are equipped with a sophisticated system of conveyor belts used to discharge cargo. Self-unloader technology has been in use on Great Lakes vessels since the 1920s. The current system — known as the C-loop — was developed and refined in the 1970s. Cargo is stored in six separate holds, each with a retractable door on the bottom.
During unloading, the doors are opened one at a time and the cargo falls onto conveyor belts that run some 183 metres from the bow to the stern along the floor of the hull. They carry the material to the rear of the ship where a set of belts lift it approximately 23 metres to the deck and the third part of the system — a boom some 76 metres long that houses another conveyor belt. The boom can be raised 45 degrees and rotated close to 180 degrees. The cargo falls on to the boom belt, which carries it to the end of the boom and dumps it onto a pier, into an adjacent ship or into a hopper. These systems are capable of unloading a fully loaded ship in as little as six hours, whereas a bulker may spend a day tied up while shore-based cranes empty its cargo holds.
This interactive demonstration describes how a vessel’s self-unloader operates. Users can select to see an animated description of how it works, or click to see a schematic view of the operation.