Most of us are aware of the danger presented by the boom and mainsheet of a sailing boat.
In most instances, the boom is stable and its movement is predictable. However, when the yacht is sailing with the wind behind it (on a run or broad reach) there is always the danger of an accidental gybe.
A gybe is when the boat changes direction and in doing so, the prevailing wind moves from behind the beam on one side of the boat to behind the beam on the other. In order to stop the boom from slamming violently across the boat, together with the slack mainsheet, and possibly damaging the boat’s rigging or severely injuring crew, the boom is first centred by trimming on the main sheet and then easing it again after the gybe.
To protect the yacht and the crew from an accidental gybe, whilst sailing downwind, we can take measures to stabilize the boom and prevent a gybe. A line is run from the end of the boom to a strong point on the vessel forward of the mast. This is known as a ‘preventer’.
The preventer usually takes the form of a line of similar strength to the mainsheet. It is tied to the boom end furthest from the mast and runs to a strong point forward of the mast, usually a mooring cleat on the bow. More sophisticated preventers may run around a strong point on the bow and then run inboard along the coach roof to a winch in the cockpit. This allows for the preventer to be released quickly and safely when needed and thus improves the maneuverability of the yacht.
It should be noted that preventers can snap under load and should only ever be considered to be a safety net allowing the helm to correct a crash gybe quickly before the boom crosses the cockpit.