When the Nazis released their new Type XXI-class U-2511 submarine it was one of Hitlers “Wonder Weapons” that he promised the German people would win the war, and represented a genuine leap in the technology of submarines. Its engine was advanced for its time, allowing it to operate while submerged underwater, at high speed for considerable distances while most of the other submarines at that time operated mainly on the surface and would only dive for short periods either during an attack or to escape.

XXI Type Submarine

Kriegsmarine, as the German Navy was called, relied on their U-boats at the start of the war to strangle the UK with a blockade against her shipping traffic. By 1942, Germany had almost succeeded in isolating Britain from its colonies. They had hundreds of submarines surrounding the British Isles and all but cut them off from everyone.  Merchant ships were being sunk by U-boats faster than shipyards could replace them.  Their tactic was successful in the beginning. In October, their U-boats sank 56 ships solely in the passage between Greenland and Iceland.

U-2540 in wartime configuration and exhibited at the Maritime Museum in Bremerhaven. It is the only floating example of a Type XXI U-boat. (SicherlichCC BY-SA 3.0, via Wikimedia Commons/Retouched by Fotowerkstatt)

The success did not last long, as the new convoy tactics, radar, and anti-submarine patrol planes caused problems with their Type VII submarines. The allies had also broken the Enigma Code that gave them pretty god information on U-boat movements in the Atlantic. That meant that their existing submarines were vulnerable to detection and attack in huge numbers. The electric engines could not hold a charge for more than a few hours when recharged with diesel on the surface. In May of 1943, the Allied forces managed to destroy 43 of these U-boats, about 25% of Germany’s entire operational submarine forces. As Marcus Jones, an associate professor at the US Naval Academy, wrote,

No amount of willpower or doctrinal ingenuity on the basis of existing boat types could overcome the collective effects of the countermeasures the Allies employed so well by 1943.

The Germans realized that if they wanted to get ahead, they should develop a new kind of submarine that would change the tide of the water war. With that, propulsion engineer Helmuth Walter designed the Type XXI with a unique figure-eight interior. Its larger electric battery bay meant that the submarines of this type could run submerged for more than two days and raise a snorkel while still below the surface to run her diesels and recharge the batteries. The sub had two sets of electric motors, one set for ultra-quiet operations and another set for bursts of speed in an emergency allowing it to move at 17 knots plus while submerged. These were  Siemens-Schuckert GU 365/30 double-acting electric motors that provided more than 36kW of output, and two Siemens-Schuckert silent running GV232/28 electric motors with 166kW of output

Most convoy escorts at the time sailed were only going about 10 knots underway to travel long distances.  This was vastly superb to the older Type VII which could not travel faster than eight knots underwater and for only short periods.

It was also fast enough to keep up with convoys while running submerged. It could run silent for 60 hours at five knots. It could also pick up the pace, traveling for an hour and a half at a breakneck speed of 18 knots. This would allow her to approach a target unseen, fire her torpedoes and then dash away at high speed and depth before the escorts could react. It could also dive to nearly 1,000 feet in an age when the most advanced US Submarine, the Tambor Class could only dive to 400 feet and manage two days submerged at 2 knots.


The U-2511 was a Type XXI submarine, a German diesel-electric submarine specifically designed during World War II. Four of the 118 submarines of this type were combat-ready, two of which were put into active service and patrols, although none were used in actual combat.

The U-2511 was more than 251 feet long and had a displacement of 1,621 tonnes when at the surface and 1,819 tonnes while submerged. She had a beam of 8 meters and a draught that measured 6.32 meters. It was armed with six torpedo tubes that were loaded hydraulically and could carry 17 torpedoes or 12 mines.

Failed the Kriegsmarine

It’s easy to say that the U-2511 submarine would have been an invulnerable ship sinking, war-winning weapon of destruction, except it wasn’t. In fact, it was rushed into production with some serious design problems.

Only two of the submarines were functional, and they encountered several technical issues that the engineers had to work many hours to resolve. Their diesel engines had superchargers that required a great deal of maintenance and care that was beyond the ability and training of the average german submariner.  As a result, the superchargers often broke down cutting the surface speed of the sub by nearly half. The breathing snorkel that would allow the diesels to run while the sub was submerged so she could recharge her batteries was prone to suck in water in the waves of the Atlantic which would shut down the diesels.  Unless the seas were relatively calm the sub would just about have to broach the surface to the run the diesels, making her far less stealthy. The hydraulic steering system was prone to leaks and wouldn’t work properly, and the hydraulic torpedo loading systems were defective at first too, turning the submarines into “Pier Queens” that were constantly at the dock being fussed over. They had 32 different factories all working on sections of the sub separately so the fit and finish were off, things didn’t quite fit together right.  They were certainly advanced in their technology but it was perhaps overbuilt with too many untried systems that each had its own problems to overcome.

The Germans eventually sorted out most of the issues to make them safe enough to put to sea with, but by the time they did, Germany was already past the point where she could build enough of the Type XXI boats to turn things around. Allied bombers were wrecking everything above ground making it impossible to build submarines in the open and Germany was running short of the raw materials it needed to even make them. The steel used to make just two of them could have made as many as 5,000 tanks, which Germany would have found much more useful at that period in the war.

It should also be remembered that the Allies were able to develop tactics to neutralize the threat of Germany’s first submarines and by 1944 had years of experience successfully killing them, they would have come up with new tactics to locate, identify, track and attack the new Type XXI boats too. It may have taken months, but it would have happened.

Following the war, one of  Type XXIs, the U-2513 came into American hands with its crew where it was extensively tested and evaluated.  You can read the full report on that here.

Here is what the navy engineers and sub-officers said about it,

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“This type of vessel is a radical departure in hull form and in certain mechanical and electrical respects from earlier types of German submarines, for the purpose of increasing submerged speed, and permissible submergence depth.

The changes have been made at the expense of surface speed and other surface characteristics.  Further, the design was not completely thought out before the beginning of construction, and has a number of shortcomings as a result, as described in the pertinent sections of this report.

Nevertheless, the results obtained indicate the need to exploit the possibilities of the type to the maximum.”

Navy specialists marveled at the hull shape, designed to travel faster underwater than on the surface, and also discovered that the submarine was originally going to have an engine that ran on a hydrogen peroxide fuel that created its own oxygen as it combusted. The engine technology was very new and the fuel was incredibly flammable making it very dangerous to handle.  This gave the US navy experts a chill.  The Germans were about six months away from creating a true submersible submarine.  One that could run underwater all the time without the need to surface except for food and refueling. One that could make its own fresh water and oxygen for the crew while underway.

The navy took the lessons learned and “exploit the possibilities of this type to the maximum,” the navy built a new program around it of their own, the first nuclear submarine, the first true submersible was called the USS Nautilus which set sail just 9 years after the end of WWII.  You can see in the photo of her below in New York harbor the similarities it shares with the Type XXI.


USS Nautilus SSN 571 entering New York harbor (USA) on 25 August 1958, after her voyage under the North Pole. 25 August 1958. Official U.S. Navy photo