Preventer

As a result, the uncontrolled boom will swing across the boat potentially inflicting injury or knocking crew members overboard.

The preventer with the most mechanical advantage is a line, from the end of the boom, led outside the shrouds and a long way forward - perhaps right up to the bow - through a block, back to the cockpit and secured within reach of the mainsheet.

This rig can also be used as a boom vang without taking up space under the mast that may be essential to the cruising sailor for dinghy stowage and other uses.

Another form of preventer is the boom brake, which, when sailing downwind, can also be used to jibe the mainsail in a slow, measured action.

It is important to maintain at least a turn or two around the preventer's cleat the whole time ready to catch an early jibe during this stage of the manoeuvre.

When the boom is as near as possible to midships (near to running fore-and-aft along the boat's centreline), the working preventer is slackened, the lazy one tightened, and the mainsheet made very secure.