Defensively equipped merchant ship

Defensively equipped merchant ship (DEMS) was an Admiralty Trade Division programme established in June 1939, to arm 5,500 British merchant ships with an adequate defence against enemy submarines and aircraft. The acronym DEMS was used to describe the ships carrying the guns, the guns aboard the ships, the military personnel manning the guns, and the shore establishment supporting the system. This followed a similar World War I program of defensively armed merchant ships (DAMS).

The program was distinct from armed merchant cruiser program, which were warships converted from civilian vessels, operated by the Royal Navy itself.

Background

In the eighteenth and nineteenth centuries, European countries such as Spain, France, the Netherlands and Britain armed their merchant ships to prevent capture by pirates, enemy commerce raiders and privateers when they conducted overseas trade. The most heavily armed were ships carrying valuable cargo back from the Far East. Notably the East Indiamen class of ships were constructed from the keel up with defence in mind, with their heavy armament making some of the most powerful examples equivalent to naval Fourth-rate ships of the line. After the end of the Napoleonic Wars in 1815, these were replaced for some of the balance of the 19th century with faster and lighter unarmed ships such as clippers that, in theory, could outrun any threat when blockade running or carrying smaller quantities of the most valuable cargoes long distance.

Prewar preparations

From the turn of the 20th century, growing tensions between Europe's Great Powers included an Anglo-German naval arms race that threatened the security of merchant shipping. In December 1911 a memo from Winston Churchill, recently appointed as First Lord of the Admiralty, proposed that the utility or otherwise of arming British merchant ships "for their own defence" be ascertained.