A boom vang is a device that is attached to a sailboat’s boom, the horizontal beam that runs along the bottom of the sail. Sometimes referred to as a kicking strap or martingale, the boom vang is usually a strap, but it could also be a metal pole, rope, or pulley system. Its purpose is to pull the boom down and in toward the mast to ensure that the sail is getting a full and proper amount of wind.
Generally, the structure of the boom vang is arranged so that it connects the bottom portion of the mast to a point on the boom that is about one third of the way out from the mast. The resulting shape is a right triangle, and the vang strap forms the hypotenuse. This configuration allows for the best control of the boom while still optimizing the space in the boat so that the crew can move freely.
The boom vang is typically used when sailing downwind, where the wind is coming from behind the boat. During this situation, the wind will often billow the sail, lifting it slightly upwards with larger gusts of wind. This lifting motion can sometimes create inefficiencies because it causes the sail to luff and lose wind. The main sheet has much less involvement in controlling the boom’s motion when sailing at this angle, reducing its normal stabilizing downward pressure on the boom.
On downward points of sail, which include broad reaches and runs where the wind is at least 45° behind the boat from the centerline, sailors can tighten the sail and boom using the boom vang. This reduces the luffing motions, keeping the boom at a more horizontal angle so that it catches the maximum amount of wind. Sailors occasionally will tighten the boom vang while sailing on a beam reach, where the wind is directly perpendicular to the sail. This practice is usually best if the sailboat lacks a traveler, which is a device that can adjust the sheet’s position along the boom.
When sailing on light wind days, the boom vang should be loosened on downwind points of sail because the wind is less likely to lift the sail upward. The added wind in the sail is usually a greater benefit than the smoother motions of the boom. Similarly, heavy wind days can create extra pressure on the sail, so the vang should be loosened to release the added tension.