Dfyjkt

A boom blocking the River Foyle during the Siege of Londonderry

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.^[1] Booms could be military in nature, with the goal of denying access to an enemy's ships; a modern example is the anti-submarine net. Booms could also be used, especially along rivers, to force passing vessels to pay a toll.^[1][2]

Description

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 Londonderry, 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, 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

• 
  

  

  
  

  
  

  Dutch 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

• 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
  

See also

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

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