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A boom vang (US) or kicking strap (UK) is a line or piston system on a sailboat
Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in size are not strictly defined and what constitutes a sailing ship, sailboat, or a smaller vessel varies by region and culture....
 used to exert downward force on the boom
Boom (sailing)

In sailing, a boom is a spar , along the Parts of a sail#The edges of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail....
 and thus control the shape of the sail. An older term is "martingale".

The vang typically runs from the base of the mast
Mast (sailing)

The mast of a sailing ship is a tall, vertical, or near vertical, spar, or arrangement of spars, which supports the sails. Large ships have several masts, with the size and configuration depending on the style of ship....
 to a point about a third of the way out the boom
Boom (sailing)

In sailing, a boom is a spar , along the Parts of a sail#The edges of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail....
.






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Boomvang
A boom vang (US) or kicking strap (UK) is a line or piston system on a sailboat
Sailboat

A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in size are not strictly defined and what constitutes a sailing ship, sailboat, or a smaller vessel varies by region and culture....
 used to exert downward force on the boom
Boom (sailing)

In sailing, a boom is a spar , along the Parts of a sail#The edges of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail....
 and thus control the shape of the sail. An older term is "martingale".

The vang typically runs from the base of the mast
Mast (sailing)

The mast of a sailing ship is a tall, vertical, or near vertical, spar, or arrangement of spars, which supports the sails. Large ships have several masts, with the size and configuration depending on the style of ship....
 to a point about a third of the way out the boom
Boom (sailing)

In sailing, a boom is a spar , along the Parts of a sail#The edges of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail....
. Due to the great force necessary to change the height of the boom while a boat is under sail, a line based boom vang usually includes some sort of a pulley
Pulley

A pulley is a mechanism composed of a wheel with a Groove between two flanges around the wheel's circumference. A rope, cable or belt usually runs inside the groove....
 system. Hydraulic piston vangs are used on larger sailboats and controlled by manual or electric hydraulic pumps.

The term kicking strap is normally shortened to "kicker" whilst sailing. ie. "Hard on the kicker when we get to the leeward mark"

On some sailing boats, such as the 49er
49er (dinghy)

The 49er is a newer one-design class of small dinghy sailing. It is a double handed twin trapeze Boats, meaning that it is sailed by a helm and a crew, the helm making many tactical decisions, as well as steering, and the crew doing most of the sail control....
, rather than a set of lines pulling the boom downwards, a rigid member is used to push the boom down. As this is the reverse of a vang, it is known as a "gnav".

By controlling leech tension, the boom vang is one of the three methods of controlling sail twist
Sail twist

Sail twist is a phenomenon in sailing where the head of the sail is at a different angle of incidence from the foot of the sail in order to change the lift distribution with height....
.

On small sailboats and some cruising sailboats a vang may be omitted. If a vang is not installed, then the sheet
Sheet (sailing)

In sailing, a sheet is a line used to control the movable corner of a sail....
 has to try to control both horizontal and vertical angles of the boom. When the boom is near the centerline, the sheet is nearly vertical, and can exert downward force on the boom. As the sheet is loosened to increase the horizontal angle of the boom and sail, the sheet becomes horizontal and exerts less downward force. A vang works with the sheet to apply the downward force on the boom at all horizontal angles, allowing the sheet to be used to control the horizontal angle of the boom effectively.

While under sail, the opposite force to the vang is supplied by the sail itself. When the sail is furled, a topping lift
Topping lift

The topping lift is a rope which is part of the running rigging on a sailboat; it applies upward force on a spar or boom . The most common topping lift on a modern sailboat is attached to the boom....
 supplies the upward force on the boom. Some line vang systems incorporate a piston to provide the topping lift force and to damp oscillations. Hydraulic vangs can inherently act in the topping lift role.