A blockship is a ship deliberately sunk to prevent a river, channel, or canal from being used. It may either be sunk by a navy defending the waterway to prevent the ingress of attacking enemy forces, as in the case of HMS Hood at Portland Harbour; or it may be brought by enemy raiders and used to prevent the waterway from being used by the defending forces, as in the case of the three old cruisers HMS Thetis, Iphigenia and Intrepid scuttled during the Zeebrugge raid in 1918 to prevent the port from being used by the German navy.
The practice goes back for several centuries. Notably an early use was in 1667, during the Dutch Raid on the Medway and their attempts to do likewise in the Thames during the Second Anglo-Dutch War, when a number of warships and merchant ships commandeered by the Royal Navy were sunk in those rivers to attempt to stop the attacking forces.
The above is the principal and enduring meaning of 'block ship', but in the mid-19th century the term blockships was applied to two groups of mobile sea batteries developed by the Royal Commission on Coast Defence. The first batch of four were obtained from around 1845 by converting old sailing 74-gun two-deckers, all of them Vengeur class ships of the line, into floating batteries, equipped with a steam/screw propulsion system. Also called "steam guardships", these conversions involved cutting down to a single deck, with ballast removed, and a jury rig installed with a medium 450 hp (340 kW) engine for speeds of 5.8—8.9 knots (11–16 km/h). These ships, converted in 1846, were Blenheim, Ajax, Hogue and Edinburgh. Although these ships were intended for coast defence some of them were used offensively, notably in the Baltic Campaign of 1854 and 1855, where they were an integral part of the British fleet. A second batch of five were similarly obtained from around 1855 by converting other elderly 74-gun ships; these were the Russell, Cornwallis, Hawke, Pembroke and Hastings.
The most recent known use of blockships in warfare was during the 2014 Russian military intervention in Ukraine. On 6 March 2014, the Russian Navy towed and purposely scuttled the decommissioned Ochakov at the entrance to Donuzlav Bay in western Crimea, to prevent remaining Ukrainian navy vessels from leaving port.  Less than 24 hours later, on March 7th, another blockship the former Black Sea Fleet rescue/diving support vessel BM-416 (VM-416) was scuttled near the Ochakov.
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