Abstract: An extensible boom has a base section that is pivotally connected to a machinery platform and at least one section that is movable axially relative to the base section. A cable system is provided for extending and for retracting the movable sections. The boom sections can be jammed together in either an extended position or a retracted position to resist compression axially of the boom, and to resist bending transversely of the longitudinal axis of the boom in a generally vertical plane. A live mast is offset from the base end of the boom in a vertical plane. Boom carrying ropes extend between the distal end of the base section and the live mast, while a boom pendant extends between the live mast and the distal end of the tip section of the boom. Support of the boom can be transferred by alternately tensioning and slackening the boom carrying ropes and the boom pendant.