A backhoe dredger is a machine mounted on a ship or barge that is used to excavate a seabed. This is done to keep the waterway navigable by larger ships, and the backhoe dredger is only one of many machines that can accomplish this task. This type of dredger looks similar to a backhoe excavator that is used on dry land, and in many cases, it is possible to make backhoe dredgers by mounting a land excavator onto a ship or barge; most dredgers are much larger, however, as the bucket needs to be able to reach the bottom of deep waterways.
The ship or barge will very often feature the backhoe dredger as a permanent structure. This is done for added stability and function as well as safety. The boom arm tends to be quite large, and the bucket at the end of the boom arm is substantially larger than that of a land excavator in many cases. The boom arm can be extended outward away from the ship and down beneath the water; the bucket is pulled back toward the ship, and sediment at the bottom of the waterway is removed by the backhoe dredger. This sediment can be moved to another location or otherwise disposed of to ensure the waterway is kept clear for boat passage.
Like an excavator used on land, backhoe dredgers often feature diesel engines to power the unit, and the house, or body of the excavator, can rotate to dig materials in various locations without having to re-position the ship. The boom arm is operated using hydraulics, which means cylinders filled with liquid — usually some sort of oil — are pressurized to extend the boom and depressurized to retract it. This allows extremely heavy loads to be supported by the boom arm and bucket. An operator will sit in the cockpit mounted on the house to control the boom arm as well as the position of the house.
Some of the largest backhoe dredger ships in the world are capable of dredging to a depth of over 98 feet (30 meters). Most backhoe dredger models are not this large and cannot dig to those depths, but they are still capable of reaching the bottom of deep waterways quickly and easily. The bucket is capable of removing several times as much material as a land excavator in many cases, though for smaller waterways, a smaller backhoe dredger may be used.