The boom is the horizontal pole attached to the mast that holds out the bottom edge of the mainsail. It swings from side to side as the boat changes course, often at head height, so always know where it is.
The boom is a spar, usually aluminium, that runs aft from the mast along the foot of the mainsail. It pivots at the mast on a fitting called the gooseneck, and it lets the mainsail be set at the right angle to the wind. The mainsheet is attached to the boom, and pulling it in brings the boom towards the middle of the boat.
Several controls act on the boom:
- The mainsheet sets its angle to the wind.
- The traveller, a track across the boat, moves the bottom of the mainsheet from side to side.
- The kicker or vang holds the boom down, which controls how much the top of the mainsail twists open.
- The outhaul pulls the clew of the sail out along the boom to flatten the sail.
- The topping lift, on many yachts, holds the end of the boom up when the sail is lowered.
The boom swings across the boat every time you tack or gybe, and on many yachts it does so at about head height. In a tack it comes across fairly gently. In a gybe, and especially in an accidental gybe while running with the wind behind you, it can swing across very fast and hard enough to injure someone or break the rig.
Some simple habits keep everyone safe: keep your head below the boom whenever the boat is turning, listen for the calls “Ready about” and “Ready to gybe”, don’t stand or sit in the path of the mainsheet, and when running downwind steer carefully to avoid the wind getting behind the mainsail. On a long run many crews rig a preventer, a line that stops the boom crossing by accident.
More in the book: Chapter 1, The sailing boat