A boom or a chain is an obstacle strung across a navigable stretch of water to control or block navigation.

Fort Blockhouse is a former military establishment in Gosport, Hampshire, England, and the final version of a complicated site. At its greatest extent in the 19th century, the structure was part of a set of fortifications which encircled much of Gosport. It is surrounded on three sides by water and provides the best view of the entrance to Portsmouth Harbour. It is unique in that it was built over five centuries from its original construction as a blockhouse in 1431 to the final addition of submarine base structures in the mid-1960s. Coastal fortification was abolished nationally in 1956, and the fort has not been used in a military capacity since 2021.