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In sailing, a spar along the bottom edge of a fore and aft rigged sail

In sailing, a boom is a spar (pole), along the of a fore and aft rigged sail, that greatly improves control of the angle and shape of the sail. The primary action of the boom is to keep the foot flatter when the sail angle is away from the centerline of the boat. The boom also serves as an attachment point for more sophisticated control lines. Because of the improved sail control it is rare to find a non headsail without a boom, but lateen sails, for instance, are loose footed. In some modern applications, the sail is rolled up into the boom for storage or reefing (shortening sail).

The forward end of the boom attaches to a mast just below the sail, with a joint called 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 as the sail's luff (forward edge).

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

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.

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. As a precaution, any sailboat with a low boom should mandate use of 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 and increase the chances of capsizing and turtling. Some designers have addressed the issue by eliminating the boom completely. Classic types of sail like the square rig or the standing lugsail have always worked without booms. Modern alternatives without a boom are the mast aft rig.

On an open cockpit sailboat at a mooring, a tarpaulin 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 aboard heavy items like a dinghy.

When the foot of a sail is attached along its whole length to a boom, the stiffness of the boom tends to hold the lower part of the sail flat. However, the greatest aerodynamic efficiency of the sail is created when the sail is allowed to curve into an airfoil like shape. Instead of being fastened directly to the boom, the foot of the sail is fastened to fittings that slot into grooves that run transversely across the boom.