Six Type IIB U-boats operated in the Black Sea from a base at Constanza Romania. They made up the 30th U-boat flotilla. They were brought to the Baltic coast, disassembled, floated on barges via canal, driven via flatbed trucks down Autobahn, barged down the Danube to Constanza and then re-assembled. They began to operate in the Black Sea in October 1942, 15 months after the start of Barbarosa. They did not sink any large number of ships or tonnage, but they did help to suppress Soviet movement by sea in that area. There of the U-boats were put out of action by Soviet air raids on Constanza in 1944. After the Red Army captured Constanza, the other 3 U-boats had no base where they could return. They were scuttled off the northern coast of Turkey.

On orders from grand Admiral Raeder, suitable ships were transferred overland to the Black Sea from North and Baltic Seas. In August 1942 the U-Boats U 9 (Lt. Cmdr. Petersen), U 18 (Lt Arendt), U 20 (Lt. Grafen), U 23 (Lt. Arendt) and U 24 (Lt. Lenzmann), previously stationed in Gydnia, put to sea for Keil, where the submarines’ diesel engines, electric motors and batteries were dismantled and their towers removed by oxyacetylene cutters. This was the first stage of a 1,500-mile journey overland. According to a plan conceived by a Mr. Baumgarten, an engineer and departmental manager in the shipbuilding offices of the Deutsche Werke Keil AG, the empty hulls of the six U-Boats, weighting 112-138 tons each, were to be tilted by an angle of ninety degrees so that they could be transported under bridges along the autobahn, firmly secured on the heaviest transport lorries available, the Kuhlemeyer trucks. The most powerful traction machines available in Germany, Kablbe tractors and Faun machines, were provided by the Luftwaffe. The U-Boats – tilted and packed in floatable pontoon crates – were pulled by tugs through the Kaiser Wilhelm Canal and along the Elbe as far as Ubigau near Dresden. From this point onward, at a speed of five miles per hour, the journey proceeded three hundred miles along the German autobahn, with its 150 bridges, numerous gradients, bends and several hilly stretches, as far as Ingolstadt.

Then the U-Boats were again packed in their pontoon crates and transported on the non-navigable Danube as far as Regensberg. In Linz, the U-Boats were again straightened up, and refitting of the engines and all dismantled parts began. The operational U-Boats were positioned between two large river barges, to disguise the nature of the transport, and towed to Galatz. From here the U-Boats made for Constanza under their own power. After this operation, unique in the annals of the German Navy, the Black Sea Flotilla was formed under command of Lt. Cmdr. Rosenbaum, who, as the commander of U 73, had sunk the British aircraft carrier Eagle in the Mediterranean.

Thirty more S-Boats, twenty-three evacuation craft, fifty naval lighters, twenty-six submarine hunters, eighty-four launches, thirty tugs, eighteen motorboats, four dredgers, two paddle steamers, two ice-breakers and 153 coastal craft were transported on the route over the autobahn and the Danube by summer 1944.