''That's why they call it the "Boom"!''[[note]] DontExplainTheJoke--For the sound it makes when it hits you in the head. Not really, but a old sailing joke.[[/note]]

On a small, single-masted ship, the sail will be large and the deck so small you have to watch for the boom -- it's the spar under the sail. So when the sail moves, it's likely to swing suddenly across the deck and knock the sailor unaware in the head. Sometimes this results in unconsciousness. Sometimes the sailor is knocked into the water. And sometimes, the sailor is knocked ''unconscious'' into the water.

In some cases, the target of the boom will get a warning that it's coming... but usually the warning comes a mere second before the impact, doing the target no good whatsoever.

Very much TruthInTelevision. When the boat is jibing (changing the sail orientation by moving the stern into the wind, as opposed to tacking, turning the bow into the wind), the boom can very suddenly swing across the deck at high speed, which can lead to a concussion (or worse) for anyone not paying attention. Keeping your head down is one of the first things taught to beginning sailors.

Compare PlankGag.
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!!Examples

[[foldercontrol]]

[[folder: Film ]]

* ''CheaperByTheDozen'' - Happens to the father.
* ''OneCrazySummer''.
* ''ThePiratesOfPenzance'' (the 1983 film) - The Pirate King runs into the boom during the Pirate King number (but it doesn't knock him overboard, though he does that later on his own).
* ''PiratesOfTheCaribbean: Curse of the Black Pearl'' - Jack Sparrow does this intentionally to Will Turner.
* ''Film/{{Sabrina}}'' - Happens to Linus Laraby.
* ''JoeVersusTheVolcano'' - Happens to Patricia (Meg Ryan's third role) during a storm, knocking her out. Joe jumps in to save her, hero that he is.
* Done deliberately at the end of ''Film/WildThings''. The victim is then left to drown.
* This is how [[KrakenAndLeviathan the Leviathan]] actually kills [[HornyVikings the Viking captain]] in an infamous deleted opening for ''Disney/AtlantisTheLostEmpire''.
* Played for laughs in the JohnCandy movie ''Summer Rental''.

[[/folder]]

[[folder: Literature ]]

* In the novel ''Man of the Century'', 15-year-old Woodrow Lowe stows away aboard the yacht belonging to the prominent Balfour family, to try and get close to his love, their daughter, Amy. Almost as soon as he tries to venture on deck, [[LoveHurts he is hit smack in the face with the boom]], which knocks him clean off his feet and right into the harbor.
* In KurtVonnegut's novel ''God Bless You, Mr. Rosewater'', Eliot Rosewater accidentally kills his mother when he takes her sailing on his small boat and tacks. The boom swung across, knocked her off the boat where she sank like a stone.
* Subverted in ''[[DragonridersOfPern Dragondrums]]'', in that Sebell, nautically inexperienced, doesn't react when warned to get out of the way of the boom. Fortunately for him, Menolly yanks him out of the way just in time.

[[/folder]]

[[folder: Television ]]

* ''CSIMiami''. Played For Drama in an episode where a woman died as a result of getting struck with a boom, rupturing her breast implants. No, really.
* ''Series/DoctorWho'', "The Curse of the Black Spot" - Happens to Rory.
* Happens to Pierce in ''Series/{{Community}}'' while in a boating class. Only since the college is nowhere near water, the boat is in the middle of the parking lot, yet the instructor insists that they act as if they were at sea, and Pierce is essentially left to die.
* ''TopGear'': Happens whenever the trio are tasked with turning cars into watercraft. Inevitably, James May decides that making a sailing car is so much easier than a motorboat-car (which it is) and neglects this trope. Cue the boom smacking into his head every few minutes.

[[/folder]]

[[folder: Web Original ]]

* There is a minor piece of Polish internet lore which is a [[http://www.humorkowo.net/txt/zeg.php humorous dictionary]] of nautical terms. The dictionary mentions that the boom's main purpose is to sweep landlubbers, annoying crewmembers and miscellaneous litter from the deck.

[[/folder]]

[[folder: Western Animation ]]

* ''WesternAnimation/TheSimpsons'' - TreehouseOfHorror XXI - It's how they kill Dr. Series/{{House}}.
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