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A shackle (also called gyve) is a U-shaped piece of metal secured with a pin or bolt across the opening, or a hinged metal loop secured with a quick-release locking pin mechanism. They are used as a connecting link in all manner of rigging systems, from boats and ships to industrial crane rigging. A carabiner is a variety of shackle used in mountaineering.
Pin shackle
A pin shackle is closed with a clevis pin. Primarily used above the deck, pin shackles used to be the most common shackle used aboard boats. Pin shackles can be inconvenient to work with at times because they are secured using something else, usually a cotter pin or seizing wire.
D-shackle
Also known as a chain shackle, D-shackles are narrow shackles shaped like a loop of chain, usually with a pin or threaded pin closure. D-shackles are very common and most other shackle types are a variation of the D-shackle. The small loop can take high loads primarily in line. Side and racking loads may twist or bend a D-shackle.
Bow shackle
With a larger "O" shape to the loop, this shackle can take loads from many directions without developing as much side load. However, the larger shape to the loop does reduce its overall strength. Also referred to as an anchor shackle.
Threaded shackle
The pin is threaded and one leg of the shackle is tapped. The pin may be "captive", to prevent it from dropping loose. The threads may gall if over-tightened or have been corroding in salty air, so a liberal coating of lanolin or a heavy grease is not out of place on any and all threads. A shackle key or metal marlinspike are useful tools for loosening a tight nut.
Snap shackle
A snap shackle spliced to a line.As the name implies, a snap shackle is a fast action fastener which can be implemented single handed. It uses a spring activated locking mechanism to close a hinged shackle, and can be unfastened under load. This is a potential safety hazard, but can also be extremely useful at times. The snap shackle is not as secure as any other form of shackle, but can come in handy for temporary uses or in situations which must be moved or replaced often, such as a sailor's harness tether or to attach spinnaker sheets. Note: When this type of shackle is used to release a significant load, it will work rather poorly (hard to release) and is likely to have the pin assembly or the split ring fail.