Boom vangs are primarily used to control the leech of the mainsail by restraining the boom from rising when sailing. This helps eliminate excessive sail twist which slows the boat, and causes an increased tendency to roll in waves. A must on any boat, large or small, a preventer helps keep the boom from unexpectedly, and dangerously, swinging back into the center of the boat in light air. In heavy air, preventers help avoid accidental gybes, which can threaten the entire rig. The strong, investment cast snap shackles of the Schaefer boom vangs offer quick placement of the tackle, allowing the system to be moved to the ideal position on the deck or toe rail for use as either a preventer or a boom vang. Each vang includes 40 feet of 3/8" line.