Type XXI U-Boat
Diesel-Electric Attack Submarine
The Type XXI diesel-electric U-boat of World War 2 was ground-breaking by any measure but it was ultimately doomed by the wartime conditions suffered by Germany heading into 1945.
Authored By: Staff Writer | Last Edited:
The Type XXI series of U-boats developed by the German Navy during World War 2 (1939-1945) were revolutionary on many levels and were looked upon by the service as its war-winning design - intended to take back control of the vital Atlantic Theater. The submarine became the first to operate faster under water than on it and opened the doors to a new chapter in submarine warfare and design - one that would set the roots of submarine work in the Cold War period (1947-1991).
The Type XXI was designed specifically for lengthy submerged operation and this centered on high-powered electrical machinery with density cells built directly into the hull. This allowed the boats to produce more power submerged than the vessel could create when on the surface - a decided advantage when compared to the diesel-electric-powered counterparts of the day with their closed-cycle turbine engines. Due to its heavily-dependent electric design, the Type XXI series was also known as the "Elektroboot" ("Electronic Boat").
German Admiral Karl Donitz worked with his staff to plan for the advanced Type XXI to become the new working backbone of the German submarine fleet - such was the challenge laying ahead for the design. Germany had been at war since September of 1939 and the foundation for the new class was laid down in November 1942 as Donitz met with engineers in Paris, France with the end-goal of devising an all-new, more powerful attack submarine to supersede the Type IIC class boats in both capability and performance. With this in mind, thought turned to design work being led by Dr. Walter Hellmuth but his submarine was so advanced that it would prove impractical to design, develop and construct under wartime so it was decided that his hull work could be applied to an interim boat design which could immediately be set in motion and be placed in the hands of German submariners in as little time as possible.
The original tear-drop-shaped hull was mated to a conventional mixed (diesel-electric) propulsion system and this structure incorporated extensive streamlining. Internally, the pressurized hull utilized figure-eight cross-section and the upper reaches were given a greater span than the lower. Externally, the dive planes were made retractable and only brought into play when needed. The powerpack lay at the rear of the boat with the torpedo room in the bow. The center-section would be reserved for the command center and crew areas. Aboard would be up to 57 crewmen and such creature comforts as a improved berthing, showers, integrated air recycling and air conditioning systems and freezer storage spaces for food.
One of the biggest changes to German submarine design in the Type XXI was the streamlining of the sail - gone were the many protrusions and platforms common to submarines of the period and, in its place, was erected a low-profile sail set over midships. All of the usual protrusions (periscopes, snorkel, antenna) were contained at the top of the sail with the snorkel being of particular note for it utilized a double-tube form with valves in place to allow the scope to be raised above the water line with the boat completely submerged - in turn allowing the boat to maintain its stealthy advantage while still expelling dangerous CO2 gasses as it lay near (not at) the surface of the water. The valves also worked to keep seawater from coming into the scope but the danger still lay in the valve shutting unnoticed - as the diesel would continue to operate if left unchecked and potentially kill the crew in the process of drawing on the internal air of the submarine.
Instead of the deck guns common to attack boats of the war, the Type XXI would rely on a pair of Anti-Aircraft (AA) turrets (2 x 20mm) were fitted to the forward and aft facings of the conning tower. Both were remotely-controlled from within the boat, no longer requiring the gunnery crew to be exposed to the elements to fend off aerial attackers (the greatest threat to a World War 2 submarine). No traditional deck gun was fitted per se though the tower was originally intended to carry a 30mm cannon for surface work.
The boat was given an emergency steering mechanism in its aft-end which acted as insurance should the main steering compartment be compromised in some way. Installed radar comprised of two working parts (along with their sub-parts): an FuMB Ant 3 Ball radar detector (coupled with an antenna fitting) and a FuMO 65 Hohentwiel U1 radar with a Type F432 D2 series transmitter device. The passive component of the sonar installation was the Gruppenhorchgerat (GHG) passive sonar which was installed at the keel. The active component became the advanced "Unterwasser-Ortungsgerat Nibelung" non-line-of-sight system.
At the bow was the torpedo room worked in conjunction with the sonar fit as it was to receive targeting data from the sonar room with the data inputted into the new Lageunabhangiger Torpedo. A semi-automatic hydraulic system was intended to aid the crew in the reloading of the torpedoes - providing a reload time of just 20 minutes to fire three salvos of six torpedoes each. The older U-boats required about ten minutes just to load a single torpedo(!) The boat's kill probability was therefore quite high for its time as few vessels (if any) could react in time to such an attack. The boat was armed with 6 x 533mm torpedo tubes, all bow-facing, and carried up to 23 reloads though this total could be reduced to make room for TMC naval mines. The boat had no rear-facing torpedo tubes which was common practice for period submarines.
Power to the Boat
As it stood, the Type XXI design was becoming into a very stealthy underwater beast. Its powerplant involved 2 x MAN M6V40/46KBB 6-cylinder supercharged diesel-fueled engines developing 3,900 horsepower for surface-running and 2 x SSW GU365/30 double-acting electric ("crawling") motors offering 4,900 horsepower. For silent running, the boat relied on 2 x SSW GV232/28 electric motors but could manage just 5.5 knots in this mode. The battery pack constituted a 62-cell 2 x 21 MAL 740 series fit. The collective propulsion scheme made the Type XXI faster submerged than on the surface where it could reach 17.2 knots and 15.7 knots (respectively). Operational range was also of note, the boats able to reach out to 15,500 nautical miles (when traveling at a consistent speed of 10 knots).
Like any diesel-electric boat, the Type XXI was still required to charge its batteries (and expel dangerous CO2 gasses) by surfacing which made any boat of the period highly-susceptible to attack by the enemy. However, the ingenious snorkel design alleviated this some as it kept the boat submerged and allowed the battery pack to be recharged in as little as four hours every few days.
As built, the basic boat has a surfaced displacement of 1,620 tons and a submerged displacement of 1,820 tons. Dimensions included a length of 251.7 feet, a beam of 26.2 feet and a draught of 20.8 feet.
The Future of Submarines
The Type XXI was revolutionary in its design in many ways - it ran quieter than anything else in the water, was able to achieve greater depths and could range out farther than anything seen prior. In addition to this, performance was excellent and firepower was more than satisfactory. Its importance to the German cause was such that construction was immediately switched from the current Type VII boats to the untested Type XXI boats in the middle part of 1943. No prototype was ordered of the new Type XXI to help prove certain design components sound and this came back to haunt the program by war's end.
Another of the revolutionary traits of the Type XXI was in its construction process - eight separate sections, constituting the hull, were built prior to final assembly. This aided in production but complicated delivery and ensuring that certain tolerances were met (precision was key in hull construction for obvious reasons). As such, the manufacturing process involved over thirty German companies and required eleven total yards before the hull sections could even be joined. The German Navy moved ahead an contracted for 118 of the boats from 1943 into 1945 with the initial target delivery date set in April 1944 - but this was without the benefits of testing an active prototype so actual deliveries did not occur until late-July. Even then there stood the challenge of properly training crews and the mechanical components proved themselves unreliable. Furthermore there was the issue of the Allied bombing campaign which wreaked havoc on German war-production facilities - including submarine facilities building the Type XXI. Final assembly of the Type XXI submarine was handled by the German concerns of Blohm & Voss, AG Weser and F. Schichau shipyards.
The target production goal was to have some 1,500 examples in service - a number which no doubt would have devastated Allied navy and marine efforts in the region - and a target production goal of three submarines per week was envisioned, again aided by the boat's modular design.
Production and Numbers
Viable strength for the class was not reached until the end of 1944 and, by this time, much ground was already lost by the German Navy. Heading into 1945, there stood about sixty-two Type XXI submarines but these could seldom be viewed as combat-ready for there was still work to be done. By the end of hostilities in May 1945, the German Navy could claim just four active Type XXIs - hardly enough to make a difference in the outcome of the war. Only U-2511 and U-3008 conducted actual war patrols but neither ever claimed any Allied tonnage. The vast fleet of Type XXIs never materialized for Donitz and much of the promising technology making up the boats fell into Allied hands, providing the impetus for a global revolution in submarine design that led into the Cold War period.
The Type XXI U-boat series began with U-2501 and ended with the U-3530. Two major variants were ultimately proposed in the "Type XXIB" and "Type XXIC" models with the former having an increased number of torpedo tubes to twelve while the latter would have seen its torpedo tubes increased to a an optimistic eighteen. The increase in forward armament would have no doubt necessitated the lengthening of the hull. In the end, however, neither form was evolved.
Some Type XXIs saw service after the war, and these being spoils of war at that. France commissioned U-2518 as "Roland Morillot" and this boat served into 1967 before being scrapped. The Soviet Union received four boats of the class and these became B-27, B-28, B-29 and B-30 in Soviet Navy Service. U-3017 fell to the British who resurrected it as HMS N41 to serve in the research role (it operated until late-1949). The United States received U-2513 and U-3008 and this pair was tested extensively in operating conditions around the Atlantic. The former was later sunk as a target in 1951 and the latter was scrapped in 1956.
All of these navies benefitted immensely from the work the Germans had done (and were attempting to do). The American GUPPY modernization program was a direct result of the capture of the German technology in the post-war period. Likewise, several Soviet Navy submarine classes were directly influenced by their experience with the German boat - such was the legacy of the Type XXI. Only U-2540 (Wilhelm Bauer) was preserved as a museum ship, its location at Bremerhaven.