A Tackle System Boomvang for small boats

A boomvang is a tool that helps you adjust the shape of your mainsail if you cannot control the shape of the sail with just the main-sheet tension. The wind pressure when sailing on your sail when sailing off the breeze causes your boom to rise up and this rise degrades the shape of the sail making it less efficient. A boom-vang is a mechanical means of holding the boom down to help maintain sail shape.

Sailboats that have mid-boom sheeting may need a boom-vang to help keep the boom from rising. Smaller boats can use the main-sheet tension to hold the boom down or a simple tackle can work well on the smaller boats. These soft vangs can prevent the boom from rising when the main-sheet is eased.

There are different types of vangs for the sailor. There is the tackle system that uses a line and blocks, rigid vangs, flexing vangs, and hydraulic vangs. Basically the size of the boat determines the vang used.

A simple tackle system that is used for the smaller boat usually has a 4:1 or a 6:1 ratio.  The boomvang goes from the base of the mast and to the boom to form one side of a triangle made by the mast, the boom, and the vang. You can tighten the boomvang to get downward pleasure on the boom.

The Vang is basically two blocks and a jam or cam cleat to keep the line in a set position This takes much of the upward pull pleasure off the main-sheet and it makes for easier sailing. 

A boomvang can make sailing off the breeze easier. It can also serve as a "preventer" to keep the boom from accidentally jibing.  Of course, enough force can also cause any of the components of the vang to fail (i.e. shackles, the line, block, etc.) so your usual care and caution needs to be used when sailing dead downwind and keep the watch for an accidental jib.