Sheers (Shears, Sheer legs, or Shear legs) are a form of two-legged lifting device, that were used by sailboats and dockyards for tasks such as lifting masts and heavier parts of the rigging on board.[1]
Unlike in a gyn, which has three legs and is thus stable without support, stability in sheers (and in a derrick) is provided by a guy.[2] Sheers comprise two upright spars, lashed together at their heads and their feet splayed apart. The heels of the spars are secured by splay and heel tackles. The point at the top of the sheers where the spars cross and are lashed together is the crutch, to which a block and tackle is attached. Unlike derricks, sheers need no lateral support, and only require either a foreguy and an aftguy or a martingale and a topping lift. Being made of two spars rather than one, sheers are stronger than a derrick of the same size and made of equivalent materials. Unlike the apex of a gyn, which is fixed, the crutch of a sheers can be topped up or lowered, via the topping lift, through a limited angle.[3]