In late 1943 Deutsche Arbeitsfront Director, Otto Lafferenz, proposed the idea of a towable watertight container which could hold an A4 rocket. This suggestion progressed to the design of a container of 500 tons displacement to be towed behind a U-boat. Un-manned and unpowered, it was to be towed within range of its target, set up and launched. Three of these vessels were ordered in late 1944, but only one was built, and no trials of the practicality of the system were carried out. Once in firing position, the containers would be trimmed to bring them vertical for launch. The project was dubbed Projekt Schwimmweste and the containers themselves referred to by the codename Prüfstand XII. Work on the containers was carried out by the Vulkanwerft, and a single example was completed by the end of the war, but never tested with a rocket launch.
However, Allied intelligence came to know of these projects, and the US Navy developed a counter-measure, known as Operation Teardrop. This operation was actually carried out in early 1945 when a group of U-boats were detected heading for the US east coast. Most of these submarines were quickly pounced on in Mid-Atlantic and destroyed in the massive anti-submarine operation, though post–war analysis showed no credible missile threat had existed.