What is the purpose of a preventer on a sailboat?

The purpose of a preventer on a sailboat is to stop the boom from swinging while downwind. When sailing with the wind coming from behind, the boom can swing unpredictably, which can lead to dangerous situations, such as accidental jibes. A preventer is rigged to the boom and usually attaches to the bow or transom of the boat, providing a controlled method to limit the boom's movement. This enhances safety by reducing the risk of injury to the crew or damage to the boat.

While securing the sail when not in use is a good practice, this is typically accomplished through other means, such as sail ties or covers, rather than a preventer. Increasing sail power during a race is more about sail trim and geometry, and does not relate to the function of a preventer. Facilitating faster docking involves techniques and equipment related to maneuvering the vessel, but not with the use of a preventer, which specifically addresses the control of the boom.