An auxiliary rope to strengthen a mast.
A rope added for the purpose of supporting or restraining
This is a safety line, rigged to back up a guy, or used, for example, to restrain a main boom forward so that it will not crash across the boat in the event of a gybe. It “prevents” the boom from crashing across the vessel.
line and tackle which limits the movement of the boom, usually for the purpose of preventing accidents by preventing being swept overboard in severe conditions.
Analogous to a boom vang, a line which holds the boom straight down to the edge of the boat and is rumored to 'prevent' gybes...a good sail trimming tool in light air, because it holds the boom still. Dangerous when used in windy conditions because (a) it instills a false sense of security in the crew, because they think the boom can't gybe, (b) if the mainsail backs, the boat will gybe far more violently than if the boom gybed without the preventer, resulting in the boat laying on its side with mast horizontal on water, boom sticking straight up in the air...boat will remain in this position until boom is allowed to rotate down onto the water, (c) either the preventer, or worse, the boom will then break where the preventer is attached.
A line rigged to hold something in place, usually used to keep the boom from changing sides unexpectedly. See jibe.
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line run forward from the boom to a secure fitting to prevent the boom from swinging violently when running.
A preventer is a mechanical device on a sailing vessel which limits the boom's ability to swing dangerously across the boat during an accidental jibe.