Very interesting set of 4 photo-postcards of the famous 2 only manufactured merchant submarines during WW1: "Deutschland" and "Bremen". 3 of them are from "Deutschland" and depict the first trip of the submarine and the crew and the other is from the first trip of "Bremen".A merchant submarine is a type of submarine intended for trade, and being without armaments, it is not considered a warship like most other types of submarines. The intended use would be blockade running, or to dive under Arctic ice.Strictly speaking, only two submarines have so far been purpose-built for non-military merchant shipping use, outside of criminal enterprises, though standard or partly converted military submarines have been used to transport smaller amounts of important cargo, especially during wartime, and large-scale proposals for modern merchant submarines have been produced by manufacturers. Criminal enterprises have also built transport submarines to avoid authorities, such as narcosubs.Deutschland was a blockade-breaking German merchant submarine used during World War I. It was developed with private funds and operated by the North German Lloyd Line. She was the first of seven U-151-class U-boats built and one of only two used as unarmed cargo submarines.After making two voyages as an unarmed merchantman, she was taken over by the German Imperial Navy on 19 February 1917 and converted into U-155, armed with six torpedo tubes and two deck guns. As U-155, she began a raiding career in June 1917 that was to last until October 1918, sinking 120,434 tons of shipping and damaging a further 9,080 tons of shipping.Bremen was a blockade-breaking German merchant submarine of World War I. Developed with private funds and operated by the Norddeutscher Lloyd Line, she was one of the first of seven U-151-class U-boats built and one of only two used as unarmed cargo submarines.