Boom

A boom is a pivotal horizontal spar attached to the mast near its base. As noted in Wikipedia's entry on the topic, a boom holds the foot of a fore-and-aft sail flatter, significantly improving the control of both the sail's aerodynamic shape and angle relative to the wind. On typical 30-to-50-foot cruising yachts, booms are constructed from extruded aluminum alloy or carbon fiber with a cross-sectional profile ranging from 100 to 200 mm (4 to 8 inches).

The nautical term "boom" must be distinguished from homonyms such as acoustic booms (sonic boom), containment oil booms, or economic booms. Modern booms house internal reefing lines and outhaul systems. The mainsheet connects to the boom to control sail angle, while the boom vang restrains upward movement to preserve leech tension. Mastering yacht rope handling is essential to adjust these control lines accurately under load.

Safety in the cockpit revolves around awareness of the boom's sweeping arc. During windward and downwind maneuvers detailed in essential sailing maneuvers, particularly gybing, the boom shifts rapidly across the boat under high force. Standard clearance above the cockpit sole is usually 1.8 to 2.2 meters (6 to 7 feet), but heel angle reduces effective clearance. An accidental gybe can cause severe head injury or structural failure; therefore, sailors rig a preventer line on downwind legs to secure the boom.