Rigging extempore gear

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Rigging extempore gear

Sometimes a seafarer needs to improvise when heaving (lifting and moving) cargo. On some occasions a pre-built, permanent derrick can not be used. A seaman can then switch to sheers or gyns (bi-pod and tripod lifing rigs that consist of wooden or steel spars held together by ropes and pulleys called rigs). Teak, oak, elm, ash and beech are the preferred woods for spars.

The topping lift or back guy must not have an angle less than 45°. The best choice of the angle is between 60° and 90°. The legs of the sheers can be further supported by lashing or bolting timbers (fishing). The base of the legs, called heels, are fixed in position by attaching ropes and pulley (tackle) called strops. Attached to the legs just below the strops are rope collars to prevent the strops from slipping; another choice is thumb pieces. To bring out the best handling, the length between the heels should be the same length as the spars. The splay tackles hold the spars a fixed distance apart at the heel. This distance should be one-third of the effective length of the spars for sheers and one-half of the effective length for a gyn. The tackle should be attached as low as possible to provide a minimum of bending-stresses on the spars. The heels of the spars can be set on pieces of wood called shoes to spread the pressure on a wider surface. The shoes consist of two orthogonal layers of 75mm² timber and can be notched to prevent the spars from sliding off. Alternately, wet sand bags can be attached to the heel of the spars to prevent the spars from skidding. Extra heel tackles, which are unnecessary for gyns because the splay tackles already maintain rigidity, can be added to hold the heels and to give multi-directional support.

Sheerlegs
Sheerlegs (also written shearlegs) are a hoisting apparatus with two wooden legs. The two legs are tied together with a fourteen-round-turned timber hitch at the top and spread apart at the bottom. A pulley is suspended from the apex. Six frapping turns are then made through the crutch, as in a seizing, and finally a clove hitch is made around the other spar. Splay tackle attached to the bottom of the legs hold the sheerlegs a fixed distance apart. Heel tackles attached to the bottom of the legs hold the sheerlegs in fixed position. The sheers can be erected when the topping lift or back guy and martingale are rigged. The cargo should not be swung out of the vertical; it may cause a slipping from the shoes. If the deck happens to have a slick surface, the sheerlegs can be secured with additional two forward and back guys on each leg. In this case the sheers are set 20° to toward the load. To land the load, the sheers switch to 40° in the opposite direction. The process can be repeated when the heal tackles are heaved on. The swinging-through process is comparable to a Stülcken derrick.

Gyn
The gyn has three legs and provides more stability than the sheerlegs. Two legs, called cheeks, are bound together as with the sheerlegs described above. The third spar is called the prypole and is fixed under the cheeks. Only four pulleys are required; three as splay tackles and the fourth one as lifting purchase. A timber hitch of six figure-of-eight turns and a finishing clove hitch lash the crutch but not too tightly because the cheeks need some room to spread their heels. The cheeks of the gyn are now ready to spread and to be erected, the cheek splay tackle is hauled tight and then the other adjacent splay tackles can be lashed. At the sides, the gyn is unstable and it is crucial that the cargo is not swung out of the base triangle; consequently the gyn is only for lifting cargo vertically.

See British Navy's "Admiralty Manual of Seamanship" and http://www.central.cadets.ca/events/sea-SheerLeg2006.pdf

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