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.

A boom blocking the River Foyle during the siege of Derry

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.

Historical usesEdit

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.




See alsoEdit


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.


  1. ^ a b Philip Davis (May 7, 2012). "Site types in the Gatehouse listings — Chain Tower". Gatehouse. Retrieved October 17, 2013.
  2. ^ Boom Towers, Norwich
  3. ^ Bob Hind (January 27, 2013). "Filling in the missing links on history of harbour chain". The News. Archived from the original on October 21, 2013. Retrieved October 17, 2013.
  4. ^ Gibbon, Edward. The History of the Decline and Fall of the Roman Empire, Volume 6. p. 510.
  5. ^ "THE DUTCH IN THE MEDWAY - 1667". M.A. de Ruyter Foundation. Archived from the original on October 21, 2013. Retrieved October 21, 2013.
  6. ^ Hervey, Frederic (1779). The Naval History of Great Britain: From the Earliest Times to the Rising of the Parliament in 1779. W Adlard. pp. 77.
  7. ^ Long, WH (2010). Medals of the British Navy and How They Were Won. Great Britain: Lancer Publishers. p. 24. ISBN 9781935501275.
  8. ^ "The Mississippi River in the Civil War Historical Marker".