A gybe preventer, preventer, or jibe-guard, is a mechanical device on a sailing vessel which limits the boom's ability to swing unexpectedly across the boat due to an unplanned accidental jibe. During an unplanned accidental jibe (or gybe), neither the crew nor the boat is set up properly to execute a planned jibe. As a result, the uncontrolled boom will swing across the boat potentially inflicting injury or knocking crew members overboard. The mainsheet or traveller can also inflict serious injury. Uncontrolled jibes may also damage the boat itself. Rigging a preventer on a yacht's mainsail is often performed when the wind is behind the beam (i.e. when it's coming from more than 90° off the bow). It can also be useful at other times when there is more swell than wind, a situation when the wind may not have the strength to keep the boom in place as the boat dips and rolls. On any boat that is sailing downwind without a preventer, strict 'heads-down' procedures must be enforced anywhere within the boom's arc. Certain areas of the side-decks and maybe the cockpit also have to be strictly 'no-go' to all crew depending on what the boom and mainsheet could do in unchecked full swing. 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. Many cruising sailors prefer to rig two tackles (port and starboard) that run from the midpoint of the boom to blocks on a track such as the headsail-sheet-block track. These tackles are typically a 2 - 4 part tackles for greater purchase. 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. There is a possibility of breaking the main boom with a preventer rig such as this, but many modern yachts are considered to have short enough booms and be beamy enough to overlook this possibility in normal use. For example, while running with the preventer cleated, a large swell could roll the boat, dipping the boom end into the water, snapping the boom in half. Care should be taken when selecting the rope which