A sturdy, horizontal spar hinged to the lower part of a mast that extends the bottom edge, or foot, of a fore-and-aft sail. The boom keeps the sail flat and allows the crew to control its angle to the wind using a mainsheet. It swings across the deck during tacks and gybes, requiring the crew to keep clear to avoid injury.
Heard on the job
Watch your head, the boom is going to swing across hard when we gybe.