Preventer

A preventer, or jibe-guard, is a mechanical device on a sailing vessel which limits the boom's ability to swing unexpectedly across the boat due to an unplanned accidental jibe.

During an unplanned accidental jibe (or gybe), neither the crew nor the boat is set up properly to execute a planned jibe. As a result the uncontrolled boom will swing across the boat potentially inflicting injury or knocking crew members overboard. The mainsheet or traveller can also inflict serious injury. Uncontrolled jibes may also damage the boat itself.

Rigging a preventer on a yacht's mainsail is often performed when the wind is behind the beam (i.e. when it's coming from more than 90° off the bow). It can also be useful at other times when there is more swell than wind, a situation when the wind may not have the strength to keep the boom in place as the boat dips and rolls.

On any boat that is sailing downwind without a preventer, strict 'heads-down' procedures must be enforced anywhere within the boom's arc. Certain areas of the side-decks and maybe the cockpit also have to be strictly 'no-go' to all crew depending on what the boom and mainsheet could do in unchecked full swing.