A shackle is a U-shaped, high-strength metal connector secured with a pin or bolt, used in sailing to securely link ropes, chains, blocks, and sails together. They are essential for rigging, allowing quick connection and disconnection of components. Additionally, a "shackle" refers to a unit of anchor chain length, typically 15 fathoms (90 feet).

Key Uses and Types in Sailing:
Purpose: Securely attach halyards to sails, blocks to deck fittings, or anchor chain to an anchor.

D-Shackle: A narrow "D" shaped shackle, ideal for high-load, straight-line applications.

Bow Shackle: A wider, U-shaped (or O-shaped) shackle that allows for multi-directional loads.

Snap Shackle: Features a spring-loaded pin for quick, one-handed release, often used on spinnaker sheets or halyards.

Soft Shackle: A modern, high-strength alternative made from synthetic rope (like Dyneema) instead of metal.

Shackle as Measurement:
In anchor handling, a "shackle" refers to a specific length of chain, commonly 15 fathoms (90 feet or 27.4 meters). An anchor rode is often measured in these units (e.g., "three shackles in the water" indicates 45 fathoms of chain are deployed).