A boom vang is used on a sailboat to pull down the boom on off-wind points of sail when the wind in the mainsail would lift the boom. Proper use of a boom vang helps keep the sail full and drawing well.
What a Boom Vang Does
When a sailboat sails downwind, the mainsail is let out for best sail trim, and the boom is typically 50 to 80 degrees from the centerline out to the leeward side. Because of this angle, the mainsheet has little downward pull on the boom, which freely rises and falls with wind changes and when the boat rolls on waves from behind. When the boom rises, the mainsail billows out, twists, and spills wind, then may snap back - over and over. This motion makes the sail less efficient.
A boom vang prevents this motion by pulling downward on the boom regardless of its position in relation to the centerline. A traditional block-and-tackle vang (photo) mounts between the base of the mast and mid-boom. The control line is typically led back to the cockpit, where pulling the line exerts force to pull the boom down.
A number of commercial rigid boom vangs are also available. This is essentially an adjustable pole that mounts similarly to hold down the boom. While more expensive than a block-and-tackle vang, rigid vangs have the added function of also holding the boom up when the mainsail is lowered, such that a topping lift is not needed.
How to Use a Boom VangWhen to tighten the vang:
- When sailing downwind on a broad reach or a run, tighten the vang to keep the boom down and minimize mainsail twist that spills air from the sail.
- If the boat does not have a traveller, which is used to pull the boom down on a beam reach, use the vang even on a beam reach (the wind directly over one side).
- When sailing downwind in light air, the mainsail generally has better shape if the vang is not tight. Let the sail bag out some.
- When sailing downwind in a strong wind, ease the vang to depower the mainsail by allowing the boom to rise and the sail to spill air high up (mainsail “twist”). Bring in the mainsheet if needed to prevent chafing of the sail against the shrouds and spreaders.
- After dropping the mainsail, raise the boom to give more headroom in the cockpit.
Boom Vang as Preventer
If your boat lacks a boom preventer, an important piece of safety gear, it may be possible to improvise with the vang as a preventer when needed, if its lower connection can be easily released and moved forward of the mast. See How to Rig a Preventer.