But the boom vang has a specific interest: Apart it is increasing the bent of the mast, it allows us to keep a specific profile to the sail when we sheet out and open the angle between the sail and the axis of the boat. (University of Hawaii)

The Wichard boom vang s designed with both jobs in mind. The 4:1 Wichard system utilizes two fiddle blocks to provide the purchase power necessary in a boom vang. (wichard-usa.com)

This is what you get when you order the new boom vang bail from Flying Scot, included are two hammer rivets, the S hook, a twist shackle, and the bail for the boom. (sailingtexas.com)