Boom (navigational barrier)
A boom or a chain 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.
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