The bigger sheerlegs usually have their own propulsion system and have a large accommodation facility on board, while smaller units are floating pontoons that need to be towed to their workplace by tugboats. Sheerlegs are commonly used for salvaging ships, assistance in shipbuilding, loading and unloading large cargo into ships, and bridge building. They have grown considerably larger over the last decades due to a marked increase in vessel, cargo, and component size (of ships, offshore oil rigs, and other large fabrications), resulting in heavier lifts both during construction and in salvage operations.
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Steam ships could move below these sheerlegs in order to conveniently (and cheaply) lift machinery in and out. One of the harbors had a big roof covering it against rain. Below it big ships could be finished and painted. At the moment the sea steamship Pylades of 190 feet and 200 hp was in the covered harbor.
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Forms of derricks are commonly found aboard ships and at docking facilities. Some large derricks are mounted on dedicated vessels, and known as floating derricks and sheerlegs. The term derrick is also applied to the framework supporting a drilling apparatus in an oil rig. The derrick derives its name from a type of gallows named after Thomas Derrick, an Elizabethan era English executioner.
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Overseas Tankship had a ship, the Wagon Mound, docked in Sydney Harbour in October 1951. The crew had carelessly allowed furnace oil (also referred to as Bunker oil) to leak from their ship. The oil drifted under a wharf thickly coating the water and the shore where other ships were being repaired. Hot metal produced by welders using oxyacetylene torches on the respondent's timber wharf (Mort's Dock) at Sheerlegs Wharf fell on floating cotton waste which ignited the oil on the water.
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Sheerlegs mounted on an M32 tank recovery vehicle Shear legs, also known as sheers, shears, or sheer legs, are a form of two-legged lifting device. Shear legs may be permanent, formed of a solid A-frame and supports, as commonly seen on land and the floating sheerleg, or temporary, as aboard a vessel lacking a fixed crane or derrick. When fixed, they are often used for very heavy lifting, as in tank recovery, shipbuilding, and offshore salvage operations. At dockyards they hoist masts and other substantial rigging parts on board.
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1600 ton maximum lift capacity sheerleg Taklift 7 of Smit Internationale Fixed shear legs are most commonly found on floating cranes known as floating sheerlegs. These have heavy A-frame booms and vary in lifting capacity between 50 and 4,000 tons, and are used principally in shipbuilding, other large scale fabrication, cargo management, and salvage operations. Temporary sheers comprise two upright spars, lashed together at their heads and their feet splayed apart. Unlike in a gyn, which has three legs and is thus stable without support, stability in sheers (derricks, and single-legged gin poles) is provided by a guy.
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Two legs, called cheeks, are bound together as in the sheerlegs, with the third spar, called the prypole and is fixed under the cheek lashing to form the apex of the tripod. Alternately, a tripod lashing may be used to form the tripod, with the heel of the center spar pointing in the opposite direction of the cheeks to ensure a solid apex when raised. Only four tackles are required; three as 'splay tackles' to prevent the legs of the tripod from spreading, with the fourth tackle as lifting purchase. A timber hitch, six figure-of-eight turns, and a finishing clove hitch lash the cheeks into a crutch but not too tight because the cheeks need some room to spread their heels.
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If the hull were raised in the traditional way, there was no guarantee that it would have enough structural strength to hold together out of water. Many suggestions for raising the ship were discarded, including the construction of a cofferdam around the wreck site, filling the ship with small buoyant objects (such as ping pong balls) or even pumping brine into the seabed and freezing it so that it would float and take the hull with it. After lengthy discussions it was decided in February 1980 that the hull would first be emptied of all its contents and strengthened with steel braces and frames. It would then be lifted to the surface with floating sheerlegs attached to nylon strops passing under the hull and transferred to a cradle. It was also decided that the ship would be recovered before the end of the diving season in 1982.
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