Wiktionary
alt. (context nautical English) A form of derrick, consisting of three poles and a block and tackle, used to hoist and lower heavy weights, especially the masts of sailing ships. n. (context nautical English) A form of derrick, consisting of three poles and a block and tackle, used to hoist and lower heavy weights, especially the masts of sailing ships.
Wikipedia
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. They are sometimes temporarily rigged on sailboats for similar tasks.
Usage examples of "shear legs".
Don't know what we'd have done if it weren't the shear legs was right there from slewing the log on target.
As a boy he'd helped Uncle Stacey change a fusion bottle by manpower, pulleys, and a jury-rigged shear legs because another ship was already in the small dry-dock where all Bergen and Associates' power equipment was built.