STEAMSHIP LINES: see SHIPPING LINES AND GROUPS. STEAM SHOVEL or NAVVY. A form of excavator which drags a bucket or shovel, at the end of a pivoted arm, from a surface position up to the top of a cutting or other place to be excavated, filling the bucket meanwhile. The mouth has steel cutting teeth or tines for easy penetration. When the bucket is filled, discharge is made into wagons or else where by opening a door at the bottom. The machine is a very powerful modified type of jib crane, specially constructed for fast and severe duty, with a great amount of steel construction, and extra hard parts of manganese steel for those subjected to excessive friction and exposure to grit. The pivoted arm either has a fixed swing from its pivotal point about half-way up the jib, or the swing may be varied by racking the arm to or fro. As the crane stands still dur ing a succession of cuts the depth of penetration has to be regu lated to suit the capacity of the bucket. With the fixed arm design this is done by varying the radius of the jib, and with the racking arm by the movement of the latter alone. Bucket capacity varies from cubic yard to about 15 cubic yards. The drive is by steam (the oldest system) but alternatively may be by petrol or paraffin motor, by electric motors, or Diesel-electric. The last mentioned possesses the advantage of flexibility similar to the steam drive, e.g., it will give a sudden access of power (not possi ble with a direct Diesel drive) due to the high torque character istic of an electric motor. Shovels are either mounted on plain wheels, or on railway wheels; or caterpillar tracks are fitted, to transfer the machine as required on any ground. Some of the largest shovels are built for "stripping" or removing a deep over burden of earth covering ironstone or coal. The biggest machine
yet built, a Marion, for this duty weighs 1,400 tons, and carries a 15 cubic yard bucket at the end of an arm 120 feet long. Eight caterpillar tracks sustain the shovel, with hydraulic devices to compensate for uneven ground. The three motions, hoisting the bucket, crowding or thrusting it into the cut, and rotating, are performed by separate electric motors. This is similar to the practice in steam shovels, which carry separate engines for the three functions.
Shovels can be adapted to work grabs when this provision is necessary for handling piles of material and loose excavation. The dragline excavator is either a machine solely used for this special kind of excavating, or is a modified shovel, the change being made rapidly. Instead of thrusting a bucket, it drags it towards the machine, being suitable for reclamation, canalization, drainage, foundation work, etc. The skimmer scoop acts in a hori zontal direction, the jib of the navvy being set parallel with the ground, and the bucket dragged along under it, so that the surface of a road can be torn up. Also the loose debris may be scooped up and loaded into a lorry. A back-acting trencher has the bucket arm pivoted at the top end of the jib, the latter being lowered to a suitable distance above the ground. As the bucket is reversed to the normal direction for a steam navvy it is dragged towards the machine excavating a trench, and the machine retreats as the work progresses. Narrow and deep trenches can be cut rapidly with this adaptation of the navvy. See EXCAVATION. (F. H.) STEAM TURBINE : see TURBINE, STEAM.