1 A U-shaped iron closed with a pin across the jaws and used for securing such things as halyards to sails, other parts of standing or running rigging where required, anchors to their cables, and joining lengths of chain cable, etc. Shackles used in the rigging normally have a threaded pin which is screwed into one of the jaws; those used to join lengths of cable are of two kinds. The old-fashioned type of joining shackle was closed by a bolt flush with the lugs of the jaws which was kept in place by a tapered pin. This was driven into place with a hammer and punch and secured by means of a leaden pellet or ring which, when hammered in, expanded into a socket cut round the inside of the hole. The modern joining shackle is made in two parts with a fitted stud, the stud being kept in place by a steel pin which runs diagonally through the stud and both parts of the shackle. The advantage of this type of shackle is that it is the same size and shape as an ordinary link of the cable and so fits better into the snugs of the cable-holder when veering or weighing anchor. A universal form of shackle in yachting is the snap shackle. This is shaped like a hook with the gap closed by a spring-loaded pin. The pin opens under pressure and closes when the pressure is taken off.
2 Before 1949 a shackle of cable supplied to the Royal Navy was 12.5 fathoms (75 ft/22.8 m) long, but was later made in lengths of 15 (90 ft/27.4 m) and 7.5 fathoms (45 ft/13.7 m), called ‘shackles of cable’ and ‘half-shackles of cable’.
3 When used in the plural it is a term for bilboes. It can also be used as a verb: two chains can be shackled together and a person is said to be shackled when put in irons.