A permanently rigged preventer is always operational and does not require detachment and reattachment in order to jibe. It prevents an accidental jibe and allows control of the boom throughout a jibe. It can stabilize the boom in rough weather.
Whenever a preventer is not rigged there is risk of an uncontrolled or unintended jibe. When the wind is just by the lee at the very beginning of a jibe the force against the roach of the mainsail is minor, and it takes very little to prevent the boom from swinging over. Once the jibe starts and the boom begins to swing, even in light air, force rapidly builds as the boom accelerates across the boat and at the end of the jibe the force is significant. The mainsheet may be equally dangerous if it crosses the cockpit, cracking like a whip.
The reason for a permanently-rigged preventer:
This CCA Fleet Surgeon was struck with the seriousness of the problem during the 1989 Marion Bermuda Race when a pediatrician at the helm of a fellow neurosurgeon’s boat died within minutes from a head injury by the mainsheet during an accidental jibe at night. The binnacle was also badly damaged.
The following is an anecdotal list of similar fatal accidents in familiar racing venues, and there are many examples among coastal and offshore cruisers.
1979 SORC boom injury
1981 Practice - USCG Sailing Team boom injury
1989 Marion Bermuda Race mainsheet injury
1992 Week mainsheet injury
1996 Antigua Race Week mainsheet injury
1998 Ft. Lauderdale-Key West Race boom injury
2007 Atlantic Rally for Cruisers (ARC) boom or mainsheet injury
2010 Fishers Island Labor Day Race (Mumm 30) boom or mainsheet injury
2011 J World San Diego on (J 124) boom or mainsheet injury
In 1990, a USNA midshipman remained in coma after a head injury during an accidental jibe, and was never able to return to school.
Equipment, set-up and operation of a permanently-rigged preventer
Phil Garland, at Hall Rigging, Bristol, Rhode Island, designed the following permanently-rigged preventer for our Morris 46. It could be easily adapted to any offshore cruising or racing boat. It is simple to operate, inexpensive and was used in four Newport Bermuda Races, two roundtrip transatlantic passages, and coastal cruising.
Preventer lines set-up on a Morris 46 (DIVA)