boom

A boom is a rigid spar, typically made of wood, aluminium alloy, or steel tube, that extends from a mast or ship's structure to control the foot (lower edge) of a sail or to serve as a mooring point for vessels alongside. On a sailing vessel, the boom holds the foot of a fore-and-aft sail (usually the mainsail) at the correct angle and tension; on a ship at anchor or in harbour, the boom projects outboard to keep lifeboats, tenders, and other small craft secured clear of the hull.

The sailing boom is hinged at its inboard end (the gooseneck) to allow it to swing across the deck as the vessel changes course. The sail's clew (aft lower corner) is lashed or clipped to the boom's aft end, while the foot of the sail is supported along its length. Tension is maintained by a tackle or hydraulic system called a boom vang, which prevents the boom from rising under the upward pull of the sail. On larger vessels, the boom may carry a sliding car or traveller that distributes the mainsail's foot tension evenly.

Mooring and handling booms

The mooring or davit boom extends horizontally or at an angle from the ship's rail and is fitted with blocks, shackles, and ring bolts to which boat painters, line handlers, and securing gear attach. These booms are often hydraulically or manually articulated so they can be swung inboard when the ship is underway. The boom's length and SWL (safe working load) are stamped on the spar itself; typical working loads range from 1 to 5 tonnes depending on the boat complement and boom construction.

Damage to booms is common in heavy weather or poor seamanship. On a sailing boom, the main failure modes are buckling under compression when the sail is overtrimmed or struck by a sudden gust, fatigue cracking at the gooseneck due to repeated swinging, and delamination in wooden booms caused by water ingress and rot. Mooring booms corrode at their welds and bolted joints if not properly maintained, and their guy ropes and preventer lines must be regularly inspected for chafe and UV damage.

The term boom derives from the Dutch word boom (tree), reflecting the timber origins of the spar. In modern yards, booms are still sometimes called gaffs on older vessels, though a gaff is technically a different spar that supports the head (upper edge) of a sail. The boom's prominence in small-boat handling has also made the term familiar ashore: a crane's horizontal beam is also called a boom, and the usage has migrated into other trades entirely.