A shackle 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. Shackles are used as a connecting link in all manner of rigging systems, from boats and ships to industrial crane rigging. As an automotive term, a shackle refers to a link connecting the leaf spring to the frame. Pin shackles, threaded shackles, snap shackles, d-shackles, headboard shackles, twist shackles, bow shackles are various types of shackles.
Pin shackles are 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-shackles have a narrow shackle shaped like a loop of chain, usually a pin or threaded pin closure. This is probably the most common shackle type, and most others are a variation on the theme. The small loop can take high loads primarily in line. Side and racking loads may twist or bend this shackle. Also referred to as a chain shackle.
Headboard shackles are longer version of a d-shackles are used to attach halyards to sails, especially sails fitted with a headboard such as on bermuda rigged boats. Headboard shackles are often stamped from flat strap stainless steel, and feature an additional pin between the top of the loop and the bottom so the headboard does not chafe the spliced eye of the halyard.
Twist shackles are usually somewhat longer than the average, and features a 90° twist so the top of the loop is perpendicular to the pin. One of the uses for twist shackles include attaching the jib halyard block to the mast, or the jib halyard to the sail, to reduce twist on the luff and allow the sail to set better.
Bow shackles have a larger "o" shape to the loop, with this shackles 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.
In threaded shackles, 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 the salt 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 shackles, as the name implies, are a fast action fastener which can be implemented single handed. Snap shackles use a spring activated locking mechanism to close a hinged shackle, and can be unfastened under load. Snap shackles are potential safety hazards, but can also be extremely useful at times.
