A thruster is a (typically) transversely mounted propeller used for maneuvering a vessel. The thruster is mounted in a tube or "tunnel" that runs across the ship. This type of thruster can be powered with either a diesel, electric or hydraulic motor. Another variation on this design is the externally mounted thruster which typically consists of one or more propellers mounted below the boat, this type of thruster is typically electrically operated. One other type of thruster commonly seen is the jet thruster. The jet thruster uses a pump to push streams of water to direct the vessel. The jet thruster can be either tunnel mounted or external and if external it can azimuth 360 degrees and serve as a means of back-up propulsion.