In sailing

Sailing

Sailing is the propulsion of a vehicle and the control of its movement with large foils called sails. By changing the rigging, rudder, and sometimes the keel or centre board, a sailor manages the force of the wind on the sails in order to move the boat relative to its surrounding medium and...

, a boom is a spar

Spar

In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a pole of wood, metal or lightweight materials such as carbon fiber used on a sailing vessel. Spars of all types In sailing, a spar is a...

 (pole), along the foot (bottom) of a fore and aft rigged sail

Sail

A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...

, that greatly improves control of the angle and shape of the sail. The primary action of the boom is to keep the foot of the sail flatter when the sail angle is away from the centerline of the boat. The boom also serves an attachment point for more sophisticated control lines. Because of the improved sail control it is rare to find a non headsail

Headsail

A headsail of a sailing vessel is any sail set forward of the foremost mast. The most common headsails are staysails, a term that includes jibs and the larger genoa...

 without a boom. In some modern applications, the sail is rolled up into the boom for storage or reefing (shortening sail).

Boom attachment

The forward end of the boom attaches to a mast

Mast (sailing)

The mast of a sailing vessel 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...

 just below the sail, with a joint called the gooseneck

Gooseneck

The gooseneck is the swivel connection on a sailboat by which the boom attaches to the mast. The boom moves from side to side and up and down by swiveling on the gooseneck....

. The gooseneck pivots allowing the other end of the boom to move freely. The clew (back corner) of the sail attaches to the free end of the boom. The entire foot of the sail may be attached to the boom or just the clew. If the foot is not attached to the boom, the rig is known as loose footed.

A boom may be found on small headsails. There the forward end of the boom is attached to the same stay

Stays (nautical)

Stays are the heavy ropes, wires, or rods on sailing vessels that run from the masts to the hull, usually fore-and-aft along the centerline of the vessel...

 as the sail's luff (forward edge).

Lines on the boom

The control lines (ropes) on the boom act in conjunction with the halyard

Halyard

In sailing, a halyard or halliard is a line that is used to hoist a sail, a flag or a yard. The term halyard comes from the phrase, 'to haul yards'...

 and leech line to ensure that the sail is trimmed most effectively.

Two primary sail control lines are attached to every boom:

• The outhaul
  Outhaul
  An outhaul is a line which is part of the running rigging of a sailboat, used to extend a sail and control the shape of the curve of the foot of the sail. It runs from the clew to the end of the boom...
  
   runs from the clew of the sail to the free end of the boom. Hauling in on (tightening) the outhaul increases foot tension in the main sail. Modern loose footed sails are cut so that the outhaul is also able to pull the clew downwards towards the boom.
• The sheet
  Sheet (sailing)
  In sailing, a sheet is a line used to control the movable corner of a sail.- Fore-and-aft rigs:Fore-and-aft rigs comprise the vast majority of sailing vessels in use today, including effectively all dinghies and yachts. The sheet on a fore-and-aft sail controls the angle of the sail to the wind,...
  
   is attached midway along the boom or at the free end, typically by means of a block
  Block (sailing)
  In sailing, a block is a single or multiple pulley. One or a number of sheaves are enclosed in an assembly between cheeks or chocks. In use a block is fixed to the end of a line, to a spar or to a surface...
  
  . The block is typically attached to the boom by means of a bail, which is a U-shaped piece of metal, flattened at the ends to allow attachment with screws or rivets. In smaller boats such as dinghies it is used to control the angle of the sail to the wind on each point of sail. On largest boats this function is largely assumed by the traveller and the main sheet is used to adjust the twist of the sail to present the luff of the sail to the wind all of the way up the mast. Easing the main sheet increases twist and the twist is usually adjusted so that the aft end of the top batten
  Batten
  A batten is a thin strip of solid material, typically made from wood, plastic or metal. Battens are used in building construction and various other fields as both structural and purely cosmetic elements...
  
   in the main sail runs parallel to the boom. The traveller
  Mechanical traveller
  A mechanical traveller is a moving part of a machine, typically a ring that slides between different positions on a supporting rod when the machine goes through its operating cycle. The term may also be used refer to the supporting rod....
  
   is a track running from one side of the boat to the other upon which sits a car to which the other end of the sheet is attached. Moving the car from side to side alters the angle of the boom to the centreline of the boat while minimising the effect on the twist of the sail.

A boom will frequently have these additional sail control lines attached:

• A downhaul
  Downhaul
  The downhaul is a line which is part of the rigging on a sailboat; it applies downward force on a spar or sail. The most common downhaul on a modern sailboat is attached to the spinnaker pole, though this may be referred to as the foreguy in some rigging nomenclature...
  
   may be attached to the boom near the gooseneck to pull the boom down and increase tension on the luff (forward edge) of the sail. If no downhaul is present, the gooseneck is usually fixed vertically to the mast and a cunningham
  Cunningham (sailing)
  In sailing, a cunningham or cunningham's eye is a type of downhaul used on a Bermuda rigged sailboat to change the shape of a sail. Sailors also often refer to the cunningham as the "smart pig"....
  
   may be used to control luff tension.
• The boom vang
  Boom vang
  A boom vang or kicking strap is a line or piston system on a sailboat used to exert downward force on the boom and thus control the shape of the sail. An older term is "martingale"....
  
  , kicking strap or kicker is an intricate set of pulleys (and, on yachts, a hydraulic ram) running diagonally between the boom and the lower portion of the mast. The kicker pulls the boom downwards. When the boat is running away from the wind the sheet will be fully eased and so the kicker becomes the primary means of controlling sail twist.
• The Preventer
  Preventer
  A preventer, or jibe-guard, is a mechanical device on a sailing vessel which limits the boom's ability to swing unexpectedly across the boat due to an unplanned accidental jibe....
  
  , prevents the boom from jibing. This line is run from a point on the boom to a point forward such as a deck cleat or the base of a stanchion. Ideally, the preventer should run from the aft end of the boom to a turning block at or very close to the bow and then back to a cleat convenient to the cockpit. The line does not take tremendous force if used properly, but it prevents the boom from starting a jibe if the wind shifts or the boat rolls. Sophisticated form of preventer is a Boom Brake
  Boom Brake
  A boom brake is a device designed to control the swing of the boom on a sailboat. The boom brake acts as a preventer when sailing downwind, and can also be used to jibe or gybe the mainsail in a slow measured action....
  
  , which not only prevents unwanted jibes, but allows a slow measured jibe by modulating the tension on the brake.
• Reef Lines, are used to tie-off excess sail, when sails are reefed
  Reefing
  Reefing is a sailing manoeuvre intended to reduce the area of a sail on a sailboat or sailing ship, which can improve the ship's stability and reduce the risk of capsizing, broaching, or damaging sails or boat hardware in a strong wind...
  
   (shortened). Some modern systems known as "jiffy reefing" or "slab reefing" have permanent lines running through the boom for purposes of reefing. Pulling on these lines helps to gather the excess sail at the bottom of the boom, and to secure the reef points to the fore and aft of the boom. With a well designed system sailors can reef the sails without leaving the cockpit.

Other lines that may found on a boom include:

• A topping lift
  Topping lift
  The topping lift is a line which is part of the 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....
  
  , holds up the free end of the boom when the sail is lowered.
• Lazy jack
  Lazy jack
  Lazy jacks are a type of rigging which can be applied to a fore-and-aft rigged sail to assist in sail handling during reefing and furling...
  
  s guide the sail onto the top of the boom for furling
  Furling
  Furling may refer to:*Furling , characters in the series Stargate SG-1*Furl , to gather a sail*Furling , to manipulating an airfoil*The Furlings, characters in the 1993 film Once Upon a Forest...
  
   when the sail is lowered.
• A pair of Anti-Jibe Preventer
  Preventer
  A preventer, or jibe-guard, is a mechanical device on a sailing vessel which limits the boom's ability to swing unexpectedly across the boat due to an unplanned accidental jibe....
  
   lines that prevents the boom from rotating during an accidental Jibe
  Jibe
  A jibe or gybe is a sailing maneuver where a sailing vessel turns its stern through the wind, such that the wind direction changes from one side of the boat to the other...
  
   or Gybe
  Gybe
  Gybe may refer to:*Gybe, an alternative spelling of jibe, a sailing maneuver**Chinese gybe, a type of jibe*To deride or tease with taunting words, also spelt "gibe" or "jibe" and done with a Sneer...
  
  .

Boom material and hardware

Traditionally booms, and other spars, were made of wood. Classic wooden hulled sailboats, both old and new, will usually have wooden spars. When aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 became available, it was adopted for sailboat spars. Aluminium spars are lighter and stronger than their wooden counterpart, require less maintenance and generally hold up better to marine conditions. Aluminium spars are usually associated with fibreglass boats, although one can still find a few early fibreglass hulled yachts that were equipped with wooden spars. On very large sailing vessels, the spars may be steel. Modern, high performance, racing yachts may have spars constructed of more expensive materials, such as carbon fibre.

Various hardware is found attached to the boom. The hardware could include fairleads, blocks

Block (sailing)

In sailing, a block is a single or multiple pulley. One or a number of sheaves are enclosed in an assembly between cheeks or chocks. In use a block is fixed to the end of a line, to a spar or to a surface...

, block tracks, and cleats. For attachment, screws are used on wooden booms and screws or rivets on aluminium booms. If the foot of the sail is attached to the boom, there may be hoops from the foot of the sail, around the boom, or there may be a track on the top of the boom into which fittings on the foot of the sail are slid.

In-Boom Furling

There are quite a few variations of in-boom furling available. Generally the boom is hollow with a spindle in the center upon which the sail is rolled (furled). The techniques for turning the spindle vary, but frequently a line is used to spin the spindle and recover or reef the sail. In most cases the sail can be full battened and has virtually infinite reefing options. Some sailors consider this approach safer than in-mast furling, since the sail can be lowered and flaked in the traditional method, in the case of mechanical failure. In most applications, the sail can be lowered or reefed from the cockpit. Most designs will not accommodate a loose-footed mainsail.

Boom Safety Concerns

The second leading cause of death on sailboats is directly attributed to the use of booms. Booms can cause injuries directly, sweep people overboard, and their associated hardware and lines represent tripping hazards. On larger boats, sailors tend to stand on the boom to perform sail maintenance and install or take off sail covers. Falls from the boom onto the deck below occur. Even when stationary, booms represent a hazard since on most boats there is insufficient headroom to walk below them without ducking. According to a German study, "boom strikes were the most common cause of sailing injury overall".

When boom injuries occur far from shore they can require expensive rescues. In 2010 the US Coast Guard and Air National Guard utilized a Lockheed C-130 Hercules aircraft to rescue a man from 1400 miles off the Mexican coast.
Deaths and injuries can occur on boats operating upon lakes and coastal waters.
Boom related accidents can also imperil the remainder of the passengers and crew aboard when a key member is lost as occurred in April 2011.

As a precaution, any sailboat with a low boom should mandate life jackets, and ensure others know how to obtain assistance and operate the craft. In Boston a sailor knocked overboard by the boom died in full sight of the land and other boats and the person left aboard didn't know how to use the radio.

To address the dangers associated with the boom, some designers have raised the boom higher off the deck or applied padding. However, these raise the center of gravity

Center of gravity

In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...

 and increases the chances of capsizing.

Some designers have addressed the issue by eliminating the boom completely and employed a Mast aft rig

Mast aft rig

A mast aft rig, alternately referred to as aft-mast rig, is a sailboat sail-plan that uses a single mast set in the aft half of the hull. The mast supports fore-sails that may consist of a single jib multiple staysails, or a crab claw sail. The mainsail is either small or completely absent...

.

Other boom uses

On an open cockpit

Cockpit (sailing)

In the Royal Navy, the term cockpit originally referred to the area where the coxswain was stationed. This led to the word being used to refer to the area towards the stern of a small decked vessel that houses the rudder controls...

 sailboat at a mooring

Mooring

Mooring may refer to:* Mooring , any device used to hold secure an object by means of cables, anchors, or lines* Mooring mast, a structure designed to hold airships and blimps securely in the open when they are not in flight....

, a tarpaulin

Tarpaulin

A tarpaulin, colloquially tarp, is a large sheet of strong, flexible, water-resistant or waterproof material, often cloth such as canvas or polyester coated with urethane, or made of plastics such as polyethylene. In some places such as Australia, and in military slang, a tarp may be known as a...

 may be run over the boom and tied to the rails to form a tent over the cockpit.

In certain situations on larger boats, the boom can be used as a crane to help lift heavy items like a Dinghy

Dinghy

A dinghy is a type of small boat, often carried or towed for use as a ship's boat by a larger vessel. It is a loanword from either Bengali or Urdu. The term can also refer to small racing yachts or recreational open sailing boats. Utility dinghies are usually rowboats or have an outboard motor,...

 aboard.

During certain sailing manoeuvres

Jibe

A jibe or gybe is a sailing maneuver where a sailing vessel turns its stern through the wind, such that the wind direction changes from one side of the boat to the other...

, the boom moves rapidly from one side of the boat to the other.