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.

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.