Boom (navigational barrier)

[1] 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.

[2] By raising or lowering a chain or boom, access could be selectively granted rather than simply rendering the stretch of water completely inaccessible.

[4] 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.

[5][6][7][8]A Frequently, however, attackers instead seized the defences and cut the chain or boom by more conventional methods.