Prevent Accidental Jibes
When sailing downwind, there is always a danger of an accidental jibe due to a wind gust, a wave suddenly turning the boat, or a steering error. To prevent this, use a line to hold the boom in place so that it cannot move across the boat.
This line, called a preventer, can be rigged in various ways depending on the boat. It can be as simple as a dock line tied to the boom and a cleat or stanchion forward of the mast. Permanent preventers can be rigged from the boom on both sides, running forward to blocks at the rail and then back to the cockpit. Such preventers can be left in place, cleated tight in the cockpit on the lee side as needed and released on the windward side until needed.
Jibing Still Dangerous even with a Preventer
A preventer does not keep the boat from turning across the wind—it only prevents the boom from crossing the boat. Should the boat turn across the wind, the mainsail will back and it will be difficult to control or turn the boat, especially in a strong wind. It is therefore important to steer very carefully downwind and, when practical, to sail a broad reach rather than a run to avoid the risk of an accidental jibe.