Boom brake with line. The brake shackles to the bottom of the boom, and the line attaches to the base of the shrouds; tensioning the line actuates the brake.

A boom brake is a device designed to control the swing of the boom on a sailboat. The boom brake acts as a preventer when sailing downwind, and can also be used to jibe the mainsail in a slow measured action. In sailing, a boom is a spar (pole) usually made of aluminum or wood, is connected to the foot of the mainsail and allows the crew to control the angle of the sail to the wind. ... Diagram of Sailboat, in this case a typical monohull sloop with a bermuda or marconi rig. ... A Preventer is a mechanical device on a sailing vessel which limits the booms ability to swing dangerously across the boat during an accidental gybe. ...

The brake usually rides on a line running perpendicular to the boom; when the boom brake is actuated, it grabs the line and either works as a preventer, or slows the boom’s speed while jibbing. The brake is actuated by either tensioning the line upon which it rides or using a second line to tension the brake relative to the main line.