Convert from shackle to ell [Scotland]

Unit Definition (shackle)
The shackle is a traditional unit of length used for measuring the lengths of nautical cables and chains, especially anchor chains. Anchor chains are formed by using shackles to join short lengths of chain. When the anchor is dropped and the chain runs out, a seaman counting the number of shackles can report the total length of chain deployed. The size of the unit therefore varied somewhat, depending on the length of the short chains used. In Britain, the unit became standardized in the 16th century at 12.5 fathoms (75 feet or 22.86 meters). In 1949, the Royal Navy adopted a length of 15 fathoms, which is 90 feet or 27.432 meters; this brought the British shackle in line with the U.S. unit, which is usually called the shot.

Unit Definition (ell [Scotland])
See the English El (or Ell). This Scottish length is shorter than the English El. It may reflect an old practice of cloth merchants in giving an extra inch with each yard, to allow for any irregular cutting at the ends of the piece. However, the English cloth ell is definitely longer than the yard; it seems to be the distance from the shoulder to the fingers of the opposite hand. This reflects a practice of cloth merchants of holding the cloth at the shoulder with one hand and pulling the piece through with the opposite hand. This cloth ell was used with a similar length in France, where it was called the aune. The Dutch el and German elle are a little more than half the English ell; they may represent "arm's-length" units like the Italian braccio, the Russian sadzhen, and the Turkish pik