The sketch of the Type XX-boat. The vast bulk of cargo should be placed outside the pressure hull in wet storage. Inside the pressure hull, there was only about 220 cubic metre of storage space for 50 tons of cargo.
Three boats (U 1701, U 1702 and U 1703) were laid down at the beginning of 1944 at Germaniawerft Kiel, but not more completed.
During World War II several navies-and one army-developed cargo-carrying undersea craft. The U. S. Navy used submarines to carry ammunition and supplies to the beleaguered garrison on Corregidor Island in Manila Bay as the Japanese overran the Philippines early in 1942. Those submarines, running the Japanese naval blockade, brought out military nurses, communications specialists, and gold bullion. These were fleet-type submarines, with most of their torpedoes removed.
After this experience, President Franklin D. Roosevelt directed the Navy to develop cargo-carrying submarines. The three submarines of the Barracuda (SS 163) class, unsatisfactory as fleet boats, were converted to equally unsatisfactory cargo craft. They were never employed in the cargo role and saw secondary service in U. S. coastal waters during the war.
American forces began their offensive in the Pacific in August 1942 in the Solomons. Soon Japanese-held islands were being bypassed by U. S. forces, and the Japanese Navy began using submarines to carry cargo and to tow cargo canisters to sustain cut-off garrisons. Subsequently, the Japanese Navy and Army developed and built several specialized cargo submarines, some of which saw extensive service.
Germany used submarines to carry cargo to Norway during the assault on that nation in 1940; seven U-boats made nine voyages carrying munitions, bombs, lubricating oil, and aircraft fuel as well as spare parts for a damaged destroyer. Both Germany and Japan used submarines to carry high-priority cargoes between the two Axis nations, with Germany also employing Italian submarines in that role. Also, as Allied armies closed on the surviving German and Italian troops trapped in Tunisia in May 1943, the German naval high command began planning to employ U-boats to carry gasoline from Italian ports to North Africa.
During the war the German Navy developed several specialized cargo submarine designs. The first of these were the so-called milch cows- specialized craft to provide fuel, torpedoes, and other supplies to U-boats at sea to enable them to operate at greater distances, especially off the U. S. Atlantic coast, in the Caribbean, and in the Indian Ocean. The first milch cow was the U-459, a Type XIV submarine built specifically for the U-boat supply role. Completed in April 1942, she was relatively large, although she had no torpedo tubes; she did mount anti-aircraft guns for self-protection. The U-459′s cargo included almost 500 tons of fuel, sufficient to add an extra four weeks on station for 12 Type VII submarines or eight more weeks at sea for five Type IX submarines. Four torpedoes were carried outside of the pressure hull for transfer by rubber raft to other submarines.
The U-459 carried out her first at-sea refueling on 22 April 1942 some 500 n. miles (925 km) northeast of Bermuda. In all, she successfully refueled 14 submarines before returning to her base in France. Subsequent refueling operations became highly vulnerable to Allied attacks, because they concentrated several U-boats in one location, and because the radio messages to set up their rendezvous often were intercepted and decoded by the Allies. Ten Type XIV submarines were built, all of which were sunk by Allied air attacks. In addition, several other large submarines were employed in the milch cow role.
The first German U-boat designed specifically for cargo operations to and from the Far East was the Type XX. This U-boat was to have a surface displacement of 2,700 tons and was to carry 800 tons of cargo, all but 50 tons external to the pressure hull. The first Type XX was to be delivered in August 1944, with an initial production at the rate of three per month. No fewer than 200 of these submarines were planned, but construction was abandoned before the first delivery in favor of allocating available resources to the Type XXI program.
In 1944, when it became evident that the Type XX program was suffering major delays, the Naval Staff considered the construction of Type XXI variants as supply and cargo submarines. No decision was made to pursue this project, although several designs were considered. They are significant because of the role of Type XXI submarines in the development of early U. S. and Soviet submarines of the Cold War era.
The Type XXID was to have been a milch cow and the Type XXIE a cargo submarine. The supply submarines were to carry out replenishment while both U-boats were submerged. The Type XXID1 retained the Type XXI pressure hull and internal arrangement, but all but two torpedo tubes (and all reloads) were deleted, and the outer hull was enlarged (as in the earlier Deutschland). Light antiaircraft guns were to be fitted, and later design changes included the addition of two bow torpedo tubes as a counter to ASW ships. This project was abandoned, however, in favor of the Type XXIV fuel supply boat, because construction changes would put too much of a demand on an already-critical situation in the shipbuilding industry. In the Type XXIV the outer hull configuration of the original Type XXI was retained with the exception of an enlarged upper deck. A large portion of the submarine’s battery was eliminated to provide space for cargo. Still, the design could not come even close to the cargo capacity of the aborted Type XIV.
The Type XXIE1, E2, and T variants also were cargo ships. Their internal arrangements were to remain unaltered except for the torpedo armament; hence there was to be a major enlargement of the outer hull. There were variations in conning towers, gun armament, and torpedo tubes. To use as many standard sections as possible, the Type XXIT and (earlier) Type XXIV gained cargo space by reducing the size of the crew spaces, battery, and torpedo compartment. These changes would have provided for carrying 275 tons of cargo on a return voyage from East Asia-125 tons of rubber, 12 tons of concentrated molybdum, 67 tons of zinc in bars, 67 tons of concentrated wolfram, and 4 tons of miscellaneous cargo. None of these Type XXI variants was constructed before the collapse of the Nazi state.
Details of these U-boat programs and plans were acquired and studied by the American, British, and Soviet navies after the war, and-coupled with their own wartime experiences-would influence submarine developments.