In 1945, Germany surrendered to the Allies a submarine fleet so advanced that no practical defense against it existed at the time. None of these radically new boats had seen combat, yet they were so nearly ready for action that two were on their way to patrol areas when the war ended, while 20 more were poised at Bergen, Norway, preparing for sea. Nearly 100 were in German home waters, awaiting deployment.
Until the submarines of other navies were modernized after the war, these German Type XXI U-boats were the world’s most advanced submarines. Although they are now obsolete, many of their features are still found in operational submarines, since most of the world’s submarines built or modernized since 1945 have incorporated ideas first tested and proved in Type XXI.
Three major characteristics set the class far ahead of all other submarines of that period: increased battery power, which gave greater submerged speed and endurance; a streamlined hull and superstructure, contributing to underwater speed; and a snorkel, the first to be included in the original design of a submarine. In addition, Type XXI represented a notable production achievement.
Type XXI was the German answer to Allied antisubmarine warfare, which had won the Battle of the Atlantic in May, 1943. To stage a comeback in the Atlantic, the Nazis needed a submarine which could stay submerged and avoid being detected by radar. This submarine also had to be able to close a target from long ranges while submerged, and to outrun surface ships which might attack it.
For years, submarines had been wedded to the surface. Fresh air must be obtained periodically for the crew, and the diesel engines need large quantities of air while running. Even during the war, in spite of their paramount need for invisibility, many submarines spent more time on the surface than submerged. The snorkel, a retractable air pipe, which could be raised above the surface like a periscope, changed all this. With a snorkel, all the air that was needed could be had without surfacing.
In the same way, submarines had, throughout their history, been forced to lie in wait for their targets, because they could not go fast enough to chase them. After shooting, the slow speed could be fatal to a submarine caught by an antisubmarine ship. The extra battery power in Type XXI reduced the speed handicap to an acceptable level by producing a submerged speed of about 15 knots—slightly higher than the boats’ maximum surface speed.
With a length of about 250 feet, Type XXI was nearly 60 feet shorter than our own fleet type submarines. Space for the large batteries was provided by making the hull very deep, leading to a shape unique in submarine design. Almost two-thirds of the boat, from the bow to the forward end of the engineering compartment, consisted of two intersecting cylinders, one below the other. In cross section, the hull looked like an inverted figure eight. The rest of the pressure hull was cylindrical and tapered toward the stern. The extra space given by the figure-eight hull permitted mounting one battery above the other.
Since these boats had been designed for continuous submerged operation, it would be unnecessary to surface on patrol, except in emergencies; since the Allies had firm control of the sea, it would be extremely foolish to do so. Consequently, no effort was made to design the boats to ride comfortably on the surface. Instead, seakeeping qualities were sacrificed in favor of improved submerged performance. To reduce the hull resistance while submerged, the hull and superstructure were streamlined and all exterior projections either removed or rounded. In doing this, structural shapes that might protect people on the bridge from waves or spray were eliminated.
Because the boats might have to surface at sea, they were given a low silhouette for protection against being sighted. The main deck was only four feet above the waterline, while the highest portion of the superstructure was only 17 feet above water.
Type XXI had no stern torpedo tubes and was the first class of German submarines to have six torpedo tubes in the forward nest. The torpedo room was arranged for rapid reloading of the tubes. A second salvo of six torpedoes could be loaded, ready to fire, within five minutes after firing the first, while the third salvo could be ready in 20 minutes. Although the full torpedo capacity was 23 torpedoes, only 20 were to be carried on long patrols. This loading provided three empty racks, so the torpedoes carried in the tubes could be removed for periodic servicing.
Although guns are not needed by a boat that is not going to surface, they were included in Type XXI to satisfy a faction in the German Admiralty which insisted on them. A pair of 3-centimeter guns was mounted at each end of the bridge fairwater, protected by heavy armor plating.
The engineering plant was simple and rugged. Two 1,050 horsepower diesel engines were the basic source of power. These could be connected to the propeller shafts through a series of clutches and reduction gears. The main motors, rated at 1,840 kilowatts, also doubled as generators for charging the batteries. When used for propulsion, they drove the shafts through the same reduction gears as the engines.
In order to conserve battery power on long patrols, creeping motors were also provided. These 83-kilowatt motors could drive the boats at speeds up to 6 knots, transmitting their power to the shafts through V-belts. For quieter operation, and to reduce the load on the creeping motors, the main motors and reduction gears were disconnected from the shafts while the creeping motors were providing propulsion power.
Type XXI was the first German submarine class to make extensive use of hydraulic power. The rudder, diving planes, torpedo tube doors, periscope hoists, and gun mount training were all operated hydraulically, and one periscope could be trained hydraulically. This shift to hydraulic power was possibly the most serious design error in these boats, since the Germans were relatively inexperienced in hydraulic design. Although they had used hydraulic periscope hoists in earlier boats, and had confidence in their ability to cope with new problems, the decision to apply this untried power source so widely was probably the main reason the Allies never had to fight the Type XXI U-boats.
Type XXI was also the first U-boat class to have full air conditioning—that is, a system which could cool, dry, or heat the air. Despite this, the crew could not be allowed to smoke below decks. Since the battery ventilation was kept at the minimum possible, the risk of a hydrogen explosion was too great to permit indiscriminate use of matches.
Information on the depth these boats were designed to reach is vague, with published reports ranging from 400 to 1,000 feet. Judging by the test specifications for various systems that would be exposed to sea pressure, a 600-foot operating depth would have been reasonable.
The depth capability, the ability to fire 18 torpedoes in 20 minutes, and the submerged speed of 15 knots combined to make Type XXI a potent threat to the Allies. Although a submarine is less effective as an attacker when deep, she is also much less vulnerable to attack. The high speed gave the Type XXI boats the ability to outrun their attackers, a situation which will give nightmares to any antisubmarine team.
By the end of the war, 119 of these boats had been built, the first one having come off the production line 13 months before the cease fire, yet not one Type XXI got into action. To see why the Allies never had to cope with them, it is necessary to go back to the German position in 1942 and 1943.
During the first years of the war, the U- boat campaign very nearly won the Battle of the Atlantic. As a result of the rapid development of new weapons and tactics, however, the Allied antisubmarine forces were becoming effective by mid-1942. Admiral Doenitz, then in command of the U-boat campaign, became worried about this as early as November 1942. At that time, at a meeting in Paris, he asked for submarines which had less need to surface, and greater underwater speed.
A naval architect at the meeting proposed adapting an experimental hull design to meet this need. With a snorkel and high capacity batteries, the suggested design offered promise of meeting Doenitz’ needs, and it would take very little development work before going into production. Doenitz agreeing, preliminary design work was begun soon after the meeting, work that was to result in Type XXI.
Admiral Doenitz was appointed Commander in Chief of the German Navy at the end of January 1943. The U-boats were still striking hard; they reached their peak of effectiveness between January and March of 1943. In this first quarter of the year, Allied ships were being sunk faster than they could be replaced, while German submarine losses were not much higher than they had been previously.
With victory so near in the Atlantic, no strenuous measures were being taken to improve the boats then in service. Major modifications, it appeared, could be postponed until after the war, so as not to divert men and materials from war production. The increasing effectiveness of the Allied antisubmarine forces, which had worried Doenitz the previous November, did not now seem to call for immediate action, since design work was already in progress on Type XXI, with the rough plans scheduled for delivery in June.
Allied antisubmarine measures caught up with the U-boats, however. Forty-two boats were lost in May. There was fierce fighting that month; wolf packs of from 30 to 50 boats struck at convoys and were fought off with heavy German losses. Not all the boats were lost in convoy battles, however; seven of the 41 were lost in the Bay of Biscay, en route to or from their operating areas.
Admiral Doenitz blamed the defeat mainly on Allied possession and use of radar. While radar played a large part, many other weapons contributed. Whatever the reasons, the U- boats had lost the Battle of the Atlantic only two months after reaching their peak of effectiveness.
In June, all Doenitz could do was withdraw his forces from the North Atlantic, giving them areas where opposition—and targets—would not be so dense. Hitler and Doenitz both believed the U-boats were performing a valuable function in keeping huge forces tied up in convoy duty whether they sank any ships or not. To give up submarine warfare would release these forces to other duties, such as the invasion of Europe. Still, the withdrawal from the North Atlantic had its effect. In June, Allied ships capable of 15 knots or better were permitted to cross the Atlantic without escort.
The withdrawal was intended to provide a breathing spell while awaiting the new boats, and to permit modifications to be made to the submarines already in service. Defensive modifications were obviously needed before the U-boats could return to action. While awaiting the new-design boats, snorkels and antiaircraft weapons were added to existing submarines.
The guns were a mixed blessing for the U- boat crews. Giving them better ability to defend themselves while on the surface, the guns also tempted them to remain surfaced longer than was wise in the face of air attack.
Snorkels had been devised before the war; the Dutch were using them in 1940 for ventilation while submerged. The British and Germans had both done development work on them in 1940, but neither had considered the snorkel to be vital at that time, since submarines had always been able to surface for ventilation and battery charges. In 1943, the picture had changed for the Germans: they decided to put snorkels on all active boats.
Snorkels helped, but were not the ultimate answer. They reduced the danger of detection, but also slowed the boats considerably on the long trips out to where the targets were to be found. Snorkels provided security at the expense of mobility.
Doenitz had hoped that the first Type XXI boats would be ready in December. Since he had not called for completion of the rough plans until June, rapid construction was necessary. He proposed that the new boats be prefabricated: built in sections at inland factories and then assembled and launched at yards with shipbuilding experience.
Prefabrication of submarines was not a new idea. Simon Lake had used the method in 1905 to build five submarines for Russia. The Germans themselves had built some 250-ton submarines in 1935 by prefabrication techniques. Still, none of the previous efforts had been on the scale of the Type XXI building program.
Prefabrication would bring new plants into submarine construction, and permit drawing from a larger labor pool, while reducing the effect of bombing by decentralization of submarine construction. Production could be speeded up by using assembly-line methods in specialized plants and reducing the time each boat spent on the building ways. In spite of these obvious advantages of prefabrication, Doenitz had to reorganize the shipbuilding procedure of the German Navy in order to push through his proposal.
Until 1943, the Admiralty had prepared contract plans for each ship, and the contracting shipyards drew the building plans from them. For successful prefabrication, however, one agency would have to prepare a set of uniform building plans to be used by all contractors.
An advisory committee of shipbuilders, Hauptauschuss Schiffbau, or HAS, had been formed in 1942; Doenitz had to work through this committee. HAS, composed of conservative shipbuilders, turned down the proposal, claiming that it was impractical, that no ships could be built by this method before November 1944, and that full production could not be reached until early 1945.
This pessimistic appraisal did not please Doenitz. Reorganizing HAS, he placed it under Otto Merker, an automobile production expert, said never to have seen a ship, much less a submarine. After the new HAS approved of prefabrication, the program gained momentum rapidly.
HAS was put under the Speer Ministry for War Production, and integrated into the over-all German war production program. Speer’s main function was to insure the allocation of steel, labor, and other priorities to submarine production.
A central design agency, Inginieurburo Glueckauf, or IBG, was organized as the submarine design and production section of HAS, and placed under a German naval constructor. IBG headquarters were located near Blankenburg, in the Harz mountains, to reduce harassment by Allied bombing. Submarine experts from all the main building yards were sent to IBG, and by November 1943, over 1,000 persons worked there. This agency either prepared or approved all plans for Type XXI.
The boat was designed in eight main sections, plus a conning tower and superstructure. To keep within the capacities of the cranes to be used in assembly, no section weighed more than 165 tons. Construction took place in three stages: manufacture, fitting out, and final assembly.
Manufacture took place at section yards, mostly steel fabricating plants without previous submarine building experience. Here, the steel plates were cut and shaped, then welded into the basic sections. Large foundations and hull fittings were also installed in this phase. Each section was built by at least four plants, in locations convenient to fitting-out and assembly yards. Most of the plants were located near water, to take advantage of the German inland waterways, though in a few instances the sections were shipped by rail or truck instead of by barge.
Fitting-out yards usually had some experience in submarine construction. Here were installed most of the wiring, auxiliary machinery, main motors, steering and diving systems, piping, and furniture. The only equipment that was not installed in the fitting-out yards was either too heavy or had to be aligned through two or more sections. Otherwise, the sections leaving the fitting-out yards were complete, even to the final coat of paint. In some sections, main bulkheads were located near the ends; where natural structural support was not provided, temporary spiders were used to help support the section.
The sections were placed on carriages which moved past stations at which the same operation was performed on each section. Jigs and dollies were developed to take advantage of the ability to install equipment through the ends of the sections, rather than through hatches or holes cut into the hull.
At the assembly yards, the sections were moved onto the ways in the correct sequence, aligned, and welded together. To get the proper alignment, small holes were drilled through the bulkheads and spiders at precise spots. With correct alignment, a light in the center section could be sighted through these holes. When the sections were lined up, four welders laid each seam, working at even intervals around the hull. Since there were small, bolted access plates over each battery tank, battery installation could take place after each boat had been welded into one piece. All other heavy installations had to be completed before the seams were welded.
With the boat welded together, the main power cables, batteries, propeller shafts, and propellers were installed and the periscopes and snorkel fitted. At this point, the boat was ready for dockside tests. When the shipyard tests were completed, a trial crew took each boat to sea for thorough operational tests, after which she was turned over to the operating crew.
By carrying out as much work as possible before the sections were brought together, time on the building ways was kept to a minimum. With conventional methods of construction, this time would have been seven to nine months. Only 60 days were scheduled for the assembly of a Type XXI boat. Within eight hours after a launching, the sections of the next boat were usually in position on the ways.
Type XXI went into production without benefit of prototype tests. This was a crash program and it had to work right the first time. Steel was ordered for the hulls in September 1943, and became available three weeks later, pushed by the Speer Ministry. The first rough sections were ready in November and December, by which time a full- scale mockup had been built. The first sections were being fitted out in January and February 1944, and in April, U-2501 was launched, the first Type XXI boat off the production line.
As the techniques of prefabrication became perfected, the production rate increased, but the production quota of 30 a month was only attained for a short period. Section yards were given contracts, plans, and materials almost in the same shipment, and fell behind quota from the start, because they had been given such a short time in which to prepare for the new work.
Plans were prepared in great detail. Since some of the section yards had no welding experience, IBG prepared complete instructions on fabricating, welding, and erecting procedures. Strict tolerances of two millimeters were prescribed in hull diameter, to insure matching of adjacent sections, but the inexperienced structural plants could not produce sections within tolerance. The assembly yards, trying to shuffle sections to get a better match, introduced confusion and inefficiency into their operations. Changing the tolerance to five millimeters, about one-fifth of an inch, helped very little. In final assembly, the ends of the sections often had to be stretched, shrunk, or split and patched to get sections to match.
In spite of close attention, the program was continually delayed. Each yard was expected to solve its own production problems, assisted by consultants from IBG. Even so, incomplete sections were sent out, in order to have them leave the yard on schedule, though this disrupted the schedule of the next yard. Bombing raids caused further delays. One devastating raid on Hamburg held up production in that area for six weeks.
The three assembly yards, located at Danzig, Bremen, and Hamburg, were to be augmented and eventually replaced by a bombproof plant on the Weser river, near Bremen. This assembly plant, designed to produce in spite of any air raids the Allies could send against Germany, was started early in 1943 but was still incomplete at the end of the war.
An ambitious project, the building was to be 1,375 feet long, varying in width from 220 to 318 feet, and in height from 74 to 82 feet. The reinforced concrete walls were nearly 15 feet thick, and the roof was 23 feet thick in some areas.
Sections were to be brought to the plant by barge and rail. Inside, they would move along on an assembly line, with the completed submarines coming off the line inside the building. At the end of the line, the boats were to enter a lock chamber, where they would be launched by flooding the chamber. After engine trials the boat, still inside the building, would proceed to the river end of the lock for her initial trim dive. After these trials the boats were to go out into the river, thence to sea.
Within the building, there was to be space for 24 sections plus 13 assembled boats. The building schedule called for one boat to be completed every 56 hours, but the plant never went into production.
By the autumn of 1944, three of the new boats were being tested. Doenitz had planned a renewal of the U-boat campaign during the last half of 1944, using the Type XXI boats. His campaign had to be postponed because of the invasion of Europe, production delays caused by Allied bombing, and the nonavailability of the new boats. When the campaign did renew, it was carried out by the older boats, equipped with snorkels.
As mentioned, a major difficulty with the Type XXI was her clumsy hydraulic system. The main source of trouble in the hydraulic system was in the controllers. The rudder and diving planes were designed so that the operator had only to position his control to indicate the position desired. A follow-up system took over to do the actual positioning. This type of control was so delicate and complicated that it gave continuous trouble and finally had to be removed. In the system which was substituted, the operating lever itself controlled the movement of the rudder or planes, and the operator then had direct control over their position.
The control trouble was not the only problem, however. The cylinders containing the operating pistons for the rudder, diving planes, and gun mounts were outside the pressure hull. Leakage into and out of these cylinders resulted in contamination of the hydraulic system by salt water, as well as leakage of oil into the sea. Water getting into the system would cause corrosion, while the oil leaking out would leave slicks on the surface.
There were other sources of trouble. High quality materials, needed to build a superior submarine, were in short supply in Germany. As a result, many valves, fittings, and piping systems could not be built of the preferred non-ferrous materials. Because of this, any valve which was not operated at least once a day was likely to jam in the position in which it was left.
Although testing of the boats continued while troubles were being corrected, it was impossible to ready them for offensive action before April 1945. The year between the launching of U-2501 and the end of the war was taken up by tests and trouble-shooting.
Type XXI U-boats were quite complex by German standards, and the Admiralty felt that only the most experienced submariners should be assigned to them. Even these men needed a three-month training period before they could be sent on patrol, and top quality personnel were hard to find at that stage of the war. Training was complicated by loss of control of the Baltic, where the major training areas were located.
The operations section of the Admiralty kept pace with the material section, and had prepared an operational guide long before any boats were ready for sea. This set of patrol instructions began on an optimistic note:
Type XXI is a boat possessing exceptionally strong underwater fighting capabilities which are capable of offsetting, to a large extent, the lead attained by the enemy antisubmarine defenses by means of their air superiority and surface location ability. With this boat and other types, it will be possible to begin a new and successful U-boat war.
Later in the book, however, a less rosy picture was painted:
The enemy has air supremacy from the moment boats leave their bases, and there is no spot in the North Atlantic where, at any time of the day or night, except for weather, the appearance of land or carrier-based aircraft is not possible.
When convoys are scattered, our lack of aerial reconnaissance and the small visual range of a U-boat make the finding of the enemy the most difficult problem. It is therefore correct to speak of our problem as being the “expanse of the Atlantic.” . . . for the enemy, there is no such thing as the expanse of the Atlantic, For him, it is merely a pond.
Despite this gloomy prospect, the instructions went on to give the basic principles the Admiralty wanted the U-boat captains to follow. Encouragement was scattered throughout the book:
Type XXI is the ideal long range fighting boat.
Type XXI is not a submersible, but a submarine proper. Its strength lies in its comparatively high underwater speed and endurance . . .
Remember that the U-boat war has failed with the previous fighting boats—which were not submarines proper, but generally operated on the surface and were only submarines that occasionally attacked underwater.
Near the first of May 1945, U-2511, commanded by Korvettenkapitan Schnee set out from Bergen, Norway, on the first Type XXI war patrol. He had been assigned the Caribbean area, and his personal patrol instructions included the requirement that he also test the boat thoroughly by making the fullest demands on her. The war ended before he sighted any enemy ships, and U-2511 was surrendered at Bergen in accordance with the Allied instructions.
Korvettenkapitan Mausek was said to have also commenced a war patrol in U-3008 before the war ended, also without result.
After the war, the Allies divided the German fleet among themselves, each taking examples of various types of ship for study. The United States was awarded two Type XXI submarines, U-2513, and U-3008. These two boats were placed in service and operated by our Navy until 1948.
Although Type XXI was never proven in combat, the basic ideas for postwar modernization of our own submarine force were taken from her. Our guppy submarines include all three of the major features of Type XXI: the increased battery power, the streamlining, and the snorkel.
The shortsightedness of the Germans in failing to have a continuing submarine improvement program to combat our own increasing antisubmarine effectiveness cannot be judged too harshly, since we were guilty of similar shortsightedness. Not until after the war did we make any progress toward being able to combat these improved submarines. Had the Germans been able to get Type XXI to sea a year earlier, these submarines would probably have had a serious effect on our ability to keep our allies supplied with the necessary materials of war.
Although the German prefabrication program failed to produce the weapon in time, the main delays occurred after the actual construction and were not caused by the prefabrication process. Only by this means did the Germans have a chance of sending Type XXI to sea in time to achieve results.
With due regard for our own production record during World War II, the German effort to produce Type XXI is admirable. That rough plans were ready within seven months after the idea was conceived is remarkable in itself, but the achievement of building the first boat of a completely new class in ten more months, while undergoing disrupting bombing raids and indoctrinating new producers into submarine building is phenomenal. The production of 119 of these boats in a 13-month period despite shortages of labor and material, and under continual bombing, is a record we can respect.
A graduate of the U. S. Naval Academy in the Class of 1946, Lieutenant Commander Glennon did graduate work at the Navy’s Postgraduate School, the University of California at Los Angeles, and the Armed Forces Staff College. His principal duty has been in submarines. He is now on the staff of Commander Operational Test and Evaluation Force.