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 above is the principal and enduring meaning of 'block ship', but in the mid-19th century the term blockships was applied to mobile sea batteries developed by the Royal Commission on Coast Defense. They were made around 1845 by converting old ships of the line into floating batteries, equipped with a steam/screw propulsion system. Also called "steam guardships",^[1] these were conversions of smaller ships of the line cut down into floating batteries, 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, Edinburgh and their sisters. 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.

• Sondhaus, L. (2001) Naval warfare, 1815-1914, Warfare and history series, London : Routledge, ISBN 0-415-21477-7
• Brown, D.K. (1983) A century of naval construction : the history of Royal Corps of Naval Constructors 1883-1983, London : Conway Maritime Press, ISBN 0-85177-282-X<