A boom vang (or kicking strap) is a sailboat control system that applies downward force on the boom to control mainsail shape, specifically twist and leech tension. It consists of a tackle, rod, or hydraulic piston connecting the base of the mast to the boom, preventing the boom from rising when sailing off-wind.

Key Features and Components
Purpose: Primarily controls sail twist and flattens the mainsail to depower in stronger winds. It is crucial for keeping the boom down when sailing downwind or on a reach.

System Types:
Line-based Tackle: Uses a system of blocks and pulleys (often 4:1 to 16:1 or higher) to increase mechanical advantage, allowing the sailor to apply high tension.

Rigid Vang: A piston-supported rod that not only pulls the boom down but also holds it up, often eliminating the need for a topping lift.

Hydraulic Vang: Used on large boats, it uses hydraulic pressure to control the vang tension.

Attachment Points: Runs from the lower part of the mast (base) to a point about one-third of the way out on the boom.

Components: Includes a cascade or block/tackle system, control line, and attachment arms.

Alternatives: A Boomkicker can be used to support the boom, as seen on West Marine.

Operational Benefits
Controls Leech Tension: Tightening the vang restricts the upper leech, reducing sail twist.

Stability Downwind: Prevents the boom from lifting and causing an accidental jibe or excessive rolling.

Depowering: By pulling the boom down, the top of the sail is opened up, reducing the heeling force.