angle of the shear legs. Stiffleg The Stiffleg derrick, also known as a Scotch derrick, is a derrick with a boom similar to a guy derrick, but instead of using guy wires to secure the top of the mast, it uses two or more stiff members, called stifflegs, which are capable of resisting either tensile or compressive forces. Sills may also be used to connect between the bottom ends of the stifflegs and the base of the mast. There may be a counterweight to place on the sills behind the stifflegs in heavy load lifting. Stiffleg derrick on a vessel is called a floating stiffleg derrick. Derricks are especially useful for high rise rigging, jobs that cover a long period of time, or jobs when the impact to the street/pedestrian traffic is a concern. Oil derrick Another kind of derrick is used on oil wells and other drilled holes. Both the structure itself and the complex set of machines associated with it are referred to as a Derrick. This is also used on some offshore oil and gas rigs. Patent systems Hallen The boom is connected with the lower part of the mast which is shaped like a "Y" or a bipod and therefore it is a single swinging derrick. On the cross trees, two guys are fastened using swivel outriggers which are stayed vertically and horizontally. In order to maintain a good controlling angle between guys and derrick, the outriggers cannot pass the inboard parallel of the centerline. Looking at the illustration, one can easily see that the right outrigger stays in the centerline and the left outrigger has moved outboard. This derrick will lower or heave cargo as both guys are veered or hauled. Three winches, controlled by joystick, are necessary to operate the Hallen Derrick; two for the guys and one for the purchase. To avoid an over-topping or over-swinging limit-switches are used. However, the limits can be modified if a different working range or a special vertical stowage is required. The safe working load (SWL) of the Hallen is between 10 and 80 tonnes. In a Hallen Universal derrick, which has no Hallen D-Frame, the halyard has an extended length since it runs through further blocks on the centerline. The Universal Hallen derrick, replacing the D-frame option, is a kind of traditional topping lift. The Hallen D-Frame is a steel bracket welded on the mast in