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Inside German WW2 submarines
The Type VIIC submarine
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German submarine innovations of the Second World War
The 895 vessels of the types VII, IXB and XIV completed between 1936 and 1945 were the workhorse of the German submarine
offensive which took place in the Atlantic during the whole Second World War. But even if these submarines were designs
from the interwar period the very largest part of them would be built throughout the war, arriving too late for
achieving a prompt and decisive victory in the sea. Antisubmarine aircraft and radar devices progressively nullified
the tactic in which the U-Boot made use of their notable speed in surface for attacking the convoys. For the purpose of
preserving their offensive capabilities and ensuring their survival it was necessary to improve their underwater
prestations as much as possible. This required to develop a propulsion plant which could work independently from the
atmospheric air and also to remove as many protrusions as possible from the hull, to render it adequate for the high
underwater speeds required. The Type XVII was a fundamental albeit transitory stage in this new conception of the
submarine weapon.
The key of this conception relied in the closed-cycle propulsion system developed by Hellmuth Walter, who had
reasoned that an engine powered by a fuel which were already rich with oxygen would not require an external supply
of oxygen. His propulsion system was essentially based on the descomposition of concentrated hydrogen peroxide in
presence of a catalyst. The consequent reaction would generate an expansive mix of water steam and free oxygen
at high temperatures, which could be used to actuate a conventional turbine. To improve this system, fuel could
be injected onto the high-temperature mix to produce a high-pressure gas which could move the turbine with increased
power. The weak point of this principle was the possibility of some impurity acting as a catalyst and triggering
the process prematurely, which would have disastrous consequences.
Two prototypes of the Type XVII were built and these demonstrated the feasibility of the new system. However, the
high electric consumption forced to build small submarines fitted with a single propeller, which at cruising speed was
actuated by a conventional Diesel-electric propulsion plant, and so the hydrogen-peroxide propulsion plant would be used
only for attacking or withdrawing. The external hull had a smooth surface with protrusions reduced to a minimum and an
eight-shaped transversal section, for it was formed by two superimposed cylindrical pressure hulls of different
diameter. In practice, the length to beam relation was too high and this caused hydrodynamic resistance to be higher
than expected; therefore, the top speed of 25 knots, which was theoretically possible by means of two Walter turbines
geared to a single shaft, was never reached.
Only four exemplars of the Type XVII were built, and the subsequent Type XVIIB, of which three exemplars were built,
was a version fitted with a single turbine. A final variant was the Type XVIIK, which would have used closed-cycle Diesel
engines and oxygen tanks instead of hydrogen-peroxide turbines, but this project was cancelled before the construction
stage. The research on submarines propelled by hydrogen-peroxide turbines continued with the Type XXVI, a project which
was interrupted by the end of the war before a single submarine could be completed. Unlike the Type XVII, the Type XXVI
was a class of oceanic submarines, and one which was expected to eventually outclass any class of Diesel-electric submarines,
including the Type XXI. However, it is probable that the proven qualities of the Type XXI would have rendered the Type XXVI
a complement rather than a replacement of that class.
In the following cross section of a Type XXVI submarine the numbers indicate: 1) Immersion tank; 2) Silent propulsion device;
3) Aft fuel-oil tank; 4) Walter turbine room; 5) Diesel engine; 6) Snorkel; 7) Periscope; 8) Conning tower; 9) Torpedo room
and crew quarters; 10) Trimming tank; 11) Lubricating oil tank; 12) Regulation tank; 13) Ingolin (hydrogen peroxid) tank;
14) Battery; 15) Fore fuel-oil tank; 16) Commander's cabin; 17) Hydrophone room; 18) Radio room; 19) Maneuvering room;
20) Sideward torpedo tubes (aiming backward).
Inspection Window
In any case, even if it was a very efficient propulsion system, the hydrogen-peroxide turbine designed by Walter was not
a proper solution for the problem of building an advanced or "true" submarine, mainly because it was too expensive to
operate due to the high cost of hydrogen peroxide, and also because it was prone to breakdowns and serious accidents.
Some foreign nations tested the hydrogen-peroxide propulsion system during the postwar years, until the development of
the first nuclear reactors in the 1950s put an end to that research.
More fortunate was the solution adopted in the Type XXI oceanic submarines and in the Type XXIII coastal submarines.
By increasing the capacity of electrical batteries and the power of electric motors, apart from rendering more hydrodynamic
the surfaces of the external hull, Diesel-electric submarines would be able to move faster while in immersion and to stay
submerged during longer times. Besides the main electric motors there were smaller electric motors to perform silent
maneuvers at low speed. The new shapes given to the hulls would allow not only to achieve better prestations in immersion
but also to reduce the acoustic signature. The hull was composed of eight prefabricated sections which were built in
different cities and then assembled in a shipyard by means of welding. The ambitious Type XXI program intended the
construction of 1500 submarines at a pace of three per week.
Another important German invention (or the perfectioning of an old idea) was the "snorkel", a ventilation device which
allows Diesel engines to get air from the exterior while the submarine navigates in immersion. The submarines of the
Type XXI were fitted with a snorkel from the beginning, because they should always operate while staying submerged. The snorkel
would allow to keep the Diesel engines running and therefore to recharge the batteries. Also other older classes of submarines
were fitted with snorkels. Thanks to this device these vessels became the first true submarines, in contrast with the former
submarine vessels, which were just submersibles. Also the conditions of habitability were significantly improved and this
included facilities for air conditioning and regeneration. The armament comprised two antiaircraft twin mountings installed
atop at each end of the conning tower and six torpedo tubes located in the bow. The electronic equipment included an
echo-goniometer (active sonar) and hydrophones (passive sonar) which allowed to attack a target without having to use the
periscope.
In these blueprints of a Type XXI submarine the different internal areas are highlighted with colors:
red for the engine room, blue for the control and command rooms, green for the crew quarters, yellow and orange for the
battery storerooms and purple for the torpedo room.
The Type XXIII was a class of coastal Diesel-electric submarines which were technically similar albeit much smaller than
those of the Type XXI. These small and agile submarines could operate in shallow waters and, like their oceanic counterparts,
they were fitted with high-capacity battery banks for achieving a high speed in immersion. The submarines of the Type
XXIII were even smaller than those of the Type XVII and, like those, they were propelled by a single shaft, but one fitted
with a larger and thus more powerful propeller. The hull was prefabricated and divided in four sections. The lower part of
the hull housed the batteries, the trimming tanks and the fuel tanks. The external non-resistant structures around the
pressure hull were reduced to a minimum. The buoyancy was therefore very low, with a difference of only 24 tonnes between
surface and underwater displacements. This allowed these submarines to effectuate immersion in an exceptionally short time
of ten seconds.
The submarines of the Type XXIII were armed with only two torpedo tubes and, due to the lack of space, no spare torpedoes
were carried; besides, torpedoes had to be introduced into the launching tubes from outside, after having doven the stern
of the submarine. The lack of reserves meant that the attack had to be accurate on the first attempt, which in turn meant
that the submarine had to get very close to the target and thereafter flee from the area at high speed. The effectiveness
of this tactic was demonstrated in the last attack effectuated by a German submarine in European waters, when the U-2336
sank in the Firth of forth two merchant ships which were part of an escorted convoy. In that time (7th May 1945) there
were already 62 units of the Type XXIII in service and their losses were caused only by aircraft.
[1] The U-3, belonging to the Type II, was one of the first submarines commanded by Joachim Schepke,
one of the submarine aces. [2] The Type XVII, formed by a small number of units distributed among many versions, was not
successful because the Walter turbine entered service hastily. The G version, which had twelve units planned, was
cancelled before the first exemplar were commissioned. It was expected that these vessels reached a speed of 25 knots in
immersion. [3] In the Type XXIII the whole section abaft the conning tower was occupied by the propulsion plant and the
fore section housed the torpedo room.
Albeit the aforementioned innovative designs were intended for an extensive production and a rather large amount of
units was built, only a few exemplars effectuated operational patrols. Ironically, the true beneficiaries of this
technical progress were the enemies of Germany. The Whiskey and Narval classes (Soviet and French, respectively)
built during the postwar were, basically, improved versions of the Type XXI which were introduced while those countries
prepared their own designs.
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Units in service
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Displacement
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Torpedo tubes
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Type XVIIA
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9
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Wa 201: 277 / 309 t
Wk 202: 236 / 259 t
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2 (2 reloads)
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Type XVIIB
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3
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312 / 357 t
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2 (2 reloads)
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Type XXI
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145
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1671 / 1819 t
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6 (17 reloads)
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Type XXIII
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58
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234 / 258 t
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2 (no reloads)
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Midget submarines against the Tirpitz
Since January 1942, when she arrived from the shipyards, until her eventual destruction by an aerial bombing in November 1944,
the battleship Tirpitz remained in Norwegian waters almost all of the time. Even if she navigated only a few times, she was a
real threat for the northern convoy route, and one which was very difficult to eradicate. The British thought that whichever
could not be achieved by means of a direct assault could be achieved by means of small 35-ton submarines, known as X boats
and designed solely for destroying the German battleship. In April 1943, half dozen of these vessels were deployed in
a remote lake in Scotland, where they effectuated intensive exercises regarding the penetration of antitorpedo nets and other
submarine defenses, the utilization of frogmen and the precise placement of two 2-ton explosive charges, which constituted their
main armament. Simultaneous attacks were planned upon the warships Tirpitz, Scharnhorst and Lützow, which were protected in their
anchorages at Altenfjord, in the North Cape.
On the night from the 11th to the 12th September 1943, six patrol submarines left the lake to start a long travel to the north,
while carrying each of them one X boat manned by a single man. On the night from the 20th to the 21st September, after a travel
of nine days on which two X boats were lost, and after having embarked the attack crews to replace the tired navigation crews,
the four remaining X boats left their mothership submarines. They navigated in the surface to avoid the coastal mine field and then
they penetrated into the long fjord. The attack upon the Tirpitz did not begin until the following day, when the X boats arrived to
Kaafjord. They managed to force their way through the nets, but not without difficulties: the X6 suffered a failure in her compass
and damages in her periscope. This boat was detected by the German personnel only when she was too close to the battleship for being
hit with something else than hand grenades or small firearms, and consequently she was able to deploy her explosive charges under
the battleship. Thereafter the X6 emerged to the surface and the crew, before being captured, proceeded to scuttle her.
The German improvised a quick defense against the X7, which penetrated inside the nets and managed to place an explosive
charge at each end of the battleship. Both charges exploded, causing severe damages to the Tirpitz. In turn, the X7 was
severely damaged as well, because she had not been able to cut through the nets in time to avoid the explosions. She eventually
managed to escape but only to finally get sunk, causing the death of two crewmen. Nothing was known about the X5 after the
initial approach and it is probable that the German sank her by cannon fire, as they claimed around thirty minutes after the
main action. Finally, also the X10, which was tasked with destroying the battlecruiser Scharnhorst, suffered the loss of her
compass and her periscope. After having tried to repair the breakdowns without success, her commander decided to abort the plan
and return her to her mothership submarine. Later the X10 had to be scuttled as well.
Midget submarines of the Axis
Due to their own nature, midget submarines enjoyed a romantic "cloak and sword" fame which led to overestimate their
effective capabilities. They were used by the Royal Navy as well as the navies of the Axis and, despite sporadic
successes, their achievements did not compensate the considerable efforts deployed in their design and construction.
The theories of employment of midget submarines differed regarding the diverse belligerent nations.
The Italians, as the British, considered them to be instruments capable of attacking targets located in berths of
difficult access, to where they could transport groups of specialists on risky subaquatic raids. To the British X
class of 16 meters in length corresponded the Italian CB class, of which numerous exemplars were transferred by land
to the Black Sea, where they managed to destroy two Soviet submarines. The units of the CA class were intended for
carrying either torpedoes or frogmen. There were also prototypes of midget submarines of greater dimensions, the
classes CC and CM of 33 meters in length, which did not reach the production stage.
The Japanese, initially, had the ambitious idea of using midget submarines alongside the larger naval units during
naval combats, after having transported them to the vicinity of enemy units onboard surface ships or mothership
submarines. However, this idea soon proved to be impractical, mainly because of the insufficient speed and
operational range of these small submarines, which were relegated to defensive missions against the enemy landing
formations. About 400 vessels of different classes were built, but only the Type A units of 24 meters in length
had an effective employment, even if their actuation during the attack upon Pearl Harbor was not a very convincing
one. After the British occupation of Madagascar, which took place six months later, a Japanese seaplane sighted an enemy
naval formation and, shortly after, the battleship HMS Ramillies and a petrol tanker were torpedoed in protected
waters at Diego Suárez. The battleship suffered severe damages. The attack had been effectuated by some Type A
units, which had been transported to the operational area, along with the seaplane, by three cruiser submarines.
Near the end of the war, the Japanese built a type of vessel, denominated "Kaiten", for being used in suicide attacks.
Being very similar to a torpedo, it was deployed from surface ships and submarines alike. However, the steering
system of the "Kaiten" revealed to be as unrealiable as the suicide spirit of the largest part of the pilots
assigned to these precarious means.
The German built a wide range of midget submarines armed with torpedoes, intended for destroying the enemy landing
craft. The Neger and the Marder carried one 533-millimeter torpedo each, whereas the Biber, which had an operational
range of 240 kilometers at a speed of 6 knots, carried two. These small vessels, which were built in large numbers and
transported by land to the operational area, were deployed in Anzio, in Normandy and in the stuary of the Scheldt, but
they faced an aggressive and efficacious reaction from the numerous escort units deployed by the Allies. Consequently,
many of these midget submarines were destroyed. The larger vessels of the Type XXVIIA ("Hecht") and Type XXVIIB
("Seehund"), which had a length of 10.5 and 12 meters respectively, were theoretically more effective; the latter
type carried two torpedoes and had an operational range of 550 kilometers, which was enough for reaching the eastern
coasts of Great Britain.
The Kriegsmarine, like other important navies during the Second World War, resorted to midget submarines for carrying out
special operations. In this cross section of a German single-seater submarine - of Biber type armed with two external
torpedoes - the numbers indicate: 1) Immersion tank; 2) Electric motor; 3) Endothermic engine; 4) Oxygen tank; 5) Snorkel;
6) Periscope; 7) Reflection needle; 8) Battery; 9) Immersion tank.
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Units in service
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Displacement
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Torpedoes
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Type XXVIIA
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3
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11.75 t
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2 external
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Type XXVIIB
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5
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15 t
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2 external
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Neger
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200
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5 t
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1 external
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Marder
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300
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5 t
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1 external
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Biber
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324
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6.25 t
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2 external
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