Blockship

A blockship is a ship deliberately sunk to prevent a river, channel, or canal from being used as a waterway. 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 in 1914; 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. 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. An even earlier use are the six 11th century Skuldelev ships in Roskilde Fjord, sunk to protect Roskilde from northern Vikings. They are now on display in the Viking Ship Museum. 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 was 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-horsepower (340 kW) engine for speeds of 5.8–8.9 knots (10.7–16.5 km/h; 6.7–10.2 mph). These ships, converted in 1846, were Blenheim,...