A turnbuckle (or bottlescrew) is a crucial sailboat rigging device used to adjust the tension and length of shrouds and stays (wire ropes). Consisting of a central threaded frame (barrel) and two end fittings with opposing left- and right-hand threads, it tightens or loosens rigging by rotating the body, ensuring the mast is properly secured and tuned for optimal performance.

Key Aspects of a Sailing Turnbuckle:
Function: They secure the standing rigging (shrouds/stays) to the deck, providing the necessary tension to keep the mast upright and in column.

Mechanism: By rotating the center barrel, both end fittings are screwed in or out simultaneously, which adjusts the tension without twisting the attached wires.

Components: Typically consist of a body/barrel, two end fittings (which can be Jaw/Jaw, Eye/Eye, or Jaw/Eye types), and locking nuts to prevent accidental loosening.

Types of End Fittings:
Jaw: A U-shaped clamp with a clevis pin for securing to chainplates or rigging points.

Eye: A closed loop connecting to chainplates or rigging using shackles or pins.

Maintenance: Turnbuckles can seize due to salt water and debris, requiring regular inspection, lubrication, and sometimes penetrant oils to maintain adjustability.