Boom (navigational barrier)

A boom or a chain (also boom defence, harbour chain, river chain, chain boom, boom chain or variants) is an obstacle strung across a navigable stretch of water to control or block navigation.

In modern times they usually have civil uses, such as to prevent access to a dangerous river channel. But, especially historically, they have been used militarily, with the goal of denying access to an enemy's ships: a modern example is the anti-submarine net.

Booms have also been used to force passing vessels to pay a toll.^[1][2]

A boom generally floats on the surface, while a chain can be on the surface or below the water. A chain could be made to float with rafts, logs, ships or other wood, making the chain a boom as well.

Especially in medieval times, the end of a chain could be attached to a chain tower or boom tower. This allowed safe raising or lowering of the chain, as they were often heavily fortified.^[1] By raising or lowering a chain or boom, access could be selectively granted rather than simply rendering the stretch of water completely inaccessible. The raising and lowering could be accomplished by a windlass mechanism or a capstan.^[3]

Booms or chains could be broken by a sufficiently large or heavy ship, and this occurred on many occasions, including the siege of Damietta, the raid on the Medway and the Battle of Vigo Bay.^{[4][5][6][7]A} Frequently, however, attackers instead seized the defences and cut the chain or boom by more conventional methods. The boom at the siege of Derry, for example, was cut by sailors in a longboat.

As a key portion of defences, booms were usually heavily defended. This involved shore-based chain towers, artillery batteries, or forts. In the Age of Sail, a boom protecting a harbour could have several ships defending it with their broadsides, discouraging assaults on the boom. On some occasions, multiple booms spanned a single stretch of water.

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  Crusaders break a chain protecting the harbour (at left) in the siege of Damietta.
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  Boom towers in Norwich
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  Remains of the great chain that protected the Golden Horn
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  A preserved section of the Hudson River Chain
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  Remains of the Shoeburyness Boom, built to protect the Thames Estuary from Soviet submarines during the Cold War

Wikimedia Commons has media related to Defensive booms.

• Anti-submarine net
  • Boom defence vessel - a vessel charged with laying anti-submarine nets
• Log boom - a boom for collecting logs
• Boom (containment) - a boom for containing oil spills

A.^ Some sources have the chain being dismantled instead of broken by a ship in the siege of Damietta and in the raid on the Medway.