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]}

Description [ edit ]

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.

Historical uses [ edit ]

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.

Gallery [ edit ]

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

Examples [ edit ]

Historical [ edit ]

• The entrance to the Cothon at Carthage was protected by a chain.
• The chain at Fort Blockhouse , protecting Portsmouth Harbour from 1431 to 1539.
• The Leonine Wall included a chain blocking the Tiber
• A chain spanned the Golden Horn
• A chain and boom blocked the River Medway during the Raid on the Medway
• Hudson River Chain
• The chain blocking the Parana River during the Battle of Vuelta de Obligado
• A chain was placed from Columbus, Kentucky across the Mississippi River to Missouri in order to block Union ships during the American Civil War ^[8]
• Between the A Palma Castle  [ gl ] in Mugardos and Saint Philip Castle  [ gl ] , in ria of Ferrol , to defend the city and naval base.

See also [ edit ]

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

Notes [ edit ]

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 .

References [ edit ]