Self-Unloading Vessels
Self-unloading vessels can discharge dry-bulk cargo at an unimproved dock without assistance from any shoreside equipment or personnel. Because they do not require any land-based assistance, these vessels can literally arrive in the middle of the night, discharge their cargo, and depart before daybreak. They are capable of transporting and unloading almost any free-flowing, dry bulk commodity, including iron ore, coal, limestone, salt, sand, gypsum, potash, and grain. "Self-unloaders" range in length from 500 feet to 1,000 feet, can transport anywhere from 5,700 to 70,000 tons per trip, and can self-discharge cargo at rates up to 10,000 tons per hour. This not only means that customers can avoid investing in expensive, shoreside unloading equipment, but that their cargo is discharged in an incredibly short period of time, quickly freeing the dock.
Courtesy of The American Steamship Company.
The graphic above demonstrates a self-unloader in action. The cargo is unloaded using a system of conveyors built into the ship. The cargo holds are "hopper-sloped", or slanted on their sides, so that the cargo will flow down through gates located at the bottom of the holds. The cargo drops onto a tunnel conveyor belt, which carries the cargo to one end of the ship and transfers it onto a loop or incline conveyor belt system. This system carries the cargo up to the main deck of the ship where it is then transferred onto the boom conveyor belt. The boom conveyor can be lifted and swung hydraulically left or right to position the cargo on the dock or into a receiving hopper as specifically desired by the customer.
