A preventer is a safety rigging system on a sailboat that secures the boom to the forward deck or chainplates to prevent accidental, violent jibes when sailing downwind. It consists of a line extending from the aft end of the boom, led through a forward block, and back to a cockpit cleat for control.

Key Components & Setup
Preventer Line: High-strength, low-stretch line (e.g., Dyneema) attached to the boom end, which is the safest attachment point to avoid breaking the boom.

Attachment Point: A pad-eye, bail, or dedicated bridle on the outboard end of the boom.

Forward Turning Point: A block or low-friction ring located near the bow cleat, chainplate, or toe rail to turn the line back to the cockpit.

Control Line/Clutch: The line is secured in the cockpit (typically on a winch or cleat) for easy adjustment or quick release.

Functionality and Safety
Preventing Accidental Jibe: It holds the boom in place even if the wind shifts suddenly or the boat yaws.

Safety Warning: Never attach a preventer to the middle of the boom, as this can cause the boom to break under load; it should be attached to the boom end.

Removal: It must be released or eased during a deliberate jibe.