jaw

A jaw is a semicircular metal or wooden fitting that clamps around a mast, allowing a spar (boom or gaff) to slide up and down while remaining perpendicular or at a controlled angle to it. The jaw is bolted or riveted to the inner end of the spar and hugs the mast like a collar, with the hollow side facing inward. This design lets the spar rotate and move vertically without binding, which is essential during sail handling and trim adjustments.

Jaws come in several forms depending on the spar type and rig. A boom jaw must accommodate the spar's length and the mast's diameter while providing enough bearing surface to handle the compression and bending loads from the sail and rigging. Gaff jaws are similar but often smaller. On larger vessels, bronze or iron jaws are cast or forged; on smaller craft, wooden jaws with metal bands are common. The internal diameter is typically bored or shaped to sit snugly on the mast without excessive play, which would cause chafe and wear.

Function and wear patterns

The jaw must slide smoothly as deck crews raise or lower the boom during maneuvers, and it absorbs loads from the downhaul, sheet, and vang. Over time, the jaw's hollow wears and can develop flat spots or lose its circular grip, allowing the spar to slip or rattle. Salt water accelerates rust in iron jaws, leading to corrosion that increases friction and can jam the spar altogether. Wooden jaws swell and shrink with moisture, and their metal bands can loosen if bolts are not regularly checked.

The term "jaw" reflects the fitting's resemblance to the hinged mouth of a clamp or vise. It is distinct from a gooseneck, which allows a boom to pivot at a fixed point, or a sliding goose-neck, which combines rotational and vertical movement but uses a different mechanical principle. A properly maintained jaw is invisible to observers but critical to smooth sail handling; its failure forces a crew to use tackles or slacken rigging, compromising sail set and vessel control.