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Enigma M1, M2 and M3
3-wheel Naval Enigma - wanted item

M1, M2 and M3 were 3-wheel electromechanical cipher machines, generally known as M3, used during WWII by the German Navy (Kriegsmarine). The machine was compatible with the Enigma I used by the Army (Heer) and Air Force (Luftwaffe). After the Wehrmacht had introduced the Enigma I in 1932, the Kriegsmarine followed in 1934 with the introduction of the M1. Although the machine is compatible with the Enigma I, it has some manufacturing differences that are unique to the German Navy. The M1, M2 and M3 are also known by their designator: Ch.11g.

As an example: the cipher wheels have letters (A-Z) around the rim rather than numbers (01-26) and the machine has a 4V or 6V socket, so that it could be powered from a ship's network.

In total 611 M1 units were built. The M1 was followed in 1938 by the M2, of which 890 units were delevered. Finally, in 1940, it was replaced by the M3, of which approx. 800 units were built [2]. All three machines, M1, M2 and M3, had the same internal designator: Ch. 11g. We will therefore use the name M3 (which refers to the procedure) to identify all three machine variants.
Enigma M3 in U-Boot U-124. Photograph courtesy German Federal Archive [1]. Click to enlarge.

The image above shows an Enigma M3 aboard German U-Boot U-124. The machine is located in a recessed bay at the bottom left of the image. In the enlargement, a power cable is visible in the corner of the bay. It was used to supply 4V directly to the machine. The photograph was taken in March 1941, nearly a year before it was replaced by the M4. Also visible in the picture is a coding table to the left of the Enigma (bigram table?) and a message form (Funkspruch) at the far right.

Initially, the M3 was supplied with 5 cipher wheels that were wired identically to the wheels of the Enigma I. This way the Kriegsmarine was able to exchange messages with the Heer and Luftwaffe. In 1939 however, three more wheels were added (VI, VII and VIII) which were used exclusively by the Kriegsmarine. At the start of WWII, the U-Boot division of the Kriegsmarine also used the Enigma M3, until it was replaced unexpectedly by the 4-wheel Enigma M4 on 2 February 1942.

Extra wheels VI-VIII
Whereas the Enigma machines of the German Wehrmacht (Heer and Luftwaffe) were supplied with 5 cipher wheels, all Naval machines had 8 wheels to choose from. The first five wheels (I-V) were identical to the five wheels supplied to the rest of the German Forces, allowing some level of compatibility, but the additional three wheels (VI-VIII) were used exclusively by the Kriegsmarine.

Of the 8 wheels supplied with the Enigma M3, three would be placed in the machine at any time, subject to the current key settings. Although 3-of-8 wheels theoretically gives a total number of 336 wheel orders (8 x 7 x 6), this was limited in practice by the operational procedures. There were instruction that one of the 3 wheels in the machine had to be a Naval wheel (VI-VIII) and that that particular naval wheel could not be used in the same position on two successive days.

The remaining 5 (unused) wheels were stored in a special wooden box. The image on the right shows such a box with the unused wheels in it.

The box has space for seven wheels but only five positions were used. The remaining positions were probably meant for a future extra wheels or for storing additional gadgets, such as UKW-D.

More pictures of this box below. Click any of the images to enlarge. Note that the serial number of the wheels is present on the box' ID-plate and is also engraved in the lock of the wooden box.
Spare Naval wheel in wooden box

Wheels I to V each have one turnover notch, but the three extra Naval wheels (VI-VIII) each have two notches. This was done to generate more frequent wheel-turnovers and, hence, make wheel stepping less predictable. The problem however, is that the cipher period is reduced, as 26 (letters) is dividable by 2 (notches). As you can see in the images below, these two notches are positioned opposite each other. Details of the wheel wiring can be found in the table below.

The construction of the Naval wheels is different from other Enigma wheels. Rather than using a spring-loaded pin to set the Ringstellung (common on all other wheels), the user has to press two pawls simultaneously. Furthermore, the M3 wheels are marked with the 26 letters of the alphabet (A-Z) rather than numbers (01-26). The overall dimensions of the wheels are identical however and the wheels of a naval Enigma fit and work perfectly inside an Enigma I and vice versa.

Wooden box front view Wooden box with spare Naval Enigma wheels Wooden box with 6 wheels (open) Spare Naval wheel in wooden box Wheel 4 (left) and wheel 7 (right). Note the presence of 2 turnover notches on the rightmost wheel. Two levers for altering the ring-setting Pressing the two levers in order to change the ring-setting
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Wooden box front view
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Wooden box with spare Naval Enigma wheels
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Wooden box with 6 wheels (open)
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Spare Naval wheel in wooden box
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Wheel 4 (left) and wheel 7 (right). Note the presence of 2 turnover notches on the rightmost wheel.
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Two levers for altering the ring-setting
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Pressing the two levers in order to change the ring-setting

Wheel turnover
One weakness of the wheelturnover mechanism is that the wheels move regularly. Only after the rightmost wheel has completed a full revolution, does it cause the next wheel to make a single step. As a result, the 2nd wheel (from the right) will only make one step every 26 characters and the 3rd wheel will hardly ever move. This makes the machine predictable and easier to break. The only machine that did not suffer from regular stepping was the Enigma G (Zählwerkmaschine).

The three Naval wheels (VI, VII and VIII) have two notches each, which causes a more frequent wheel turnover and less regular stepping. This was done as the 3rd wheel (from the right) hardly ever moved during the ciphering process.

Note however, that the wheels have 2 notches which is not a relative prime of 26 (26 can be divided by 2) and that the notches are positioned opposite each other (see the image on the right). The result is that the cipher period is effectively halved, which was yet another weakness of the system and a bonus for the codebreakers.
Wheel 4 (left) and wheel 7 (right). Note the presence of 2 turnover notches on the rightmost wheel.

Furthermore, operational procedures instructed the use of at least one of the extra 3 Naval wheels (VI-VIII) every day, and that such a wheel could not be used in the same position on two successive days. This was known by the code breakers at Bletchley Park and reduced the number of possible combinations, making it easier to guess to order in which the wheels were placed.

During WWII, several attempts were mounted to make Enigma traffic more secure. In January 1944, a field-rewirable reflector, called UKW-D, was introduced by the Luftwaffe (German Air Force). It is little known that a special UKW-D was also developed for the Kriegsmarine (Navy).

The image on the right shows an extremely rare UKW-D with a serial number starting with the letter M, indicating naval-use (Marine). It was developed especially for the M3 and M4 and replaces the existing reflector (B or C).

When in use, the wiring of the UKW-D was changed approx. every 10 days. It provided strong additional security but was disliked by most operators as it was difficult to set up.

 More information


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Keys and locks
The Enigma M3 has two locks in the top cover that can be used to prevent the internal wheel settings from being changed. This was probably done to allow only an officer to change the wheel settings. The Enigma M4 only has one such lock. As far as we know, locks were only present on the Enigma machines used by the Kriegsmarine (Navy). It is likely however that the lock was not used very often, as only a limited number of keys have been found with surviving machines.

That said, the two locks of the M3 with serial number M 522 (see below) were locked when the machine was recovered from the Danish waters by a fisherman in 1992, indicating that, at least early in the war, the two locks were in use.

The image on the right shows an extremely rare key of an Enigma M4. Note that the machine's serial number is engraved in the key. The locks, and hence the keys, of the M3 were of the same make and model. As each machine was supplied with its own set of locks, the key of one M3/M4 generally does not fit another machine's lock.
Key for M7782 (Enigma M4)

If no original key is present, it is possible to have a replica key made from a so-called blank, even whithout removing and disassembling the lock. For this, one would need to call the help of a skilled lock-picker, such as Barry Wels of the Dutch lockpickers organisation Toool who, in 2008, created a smoothly operating key for our M4 in just under 10 minutes, using impressioning.

The lock to the right of the wheels Close-up of the lock The lock seen from the inside The key in the palm of a hand Key for M7782 (Enigma M4) Original Enigma key as found inside the wooden box of an M4 (courtesy Arthur Bauer) A replica key The key inserted into the lock
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The lock to the right of the wheels
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Close-up of the lock
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The lock seen from the inside
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The key in the palm of a hand
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Key for M7782 (Enigma M4)
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Original Enigma key as found inside the wooden box of an M4 (courtesy Arthur Bauer)
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A replica key
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The key inserted into the lock

It is sometimes thought that the name Enigma M3 refers to a 3-wheel machine and that for the same reason M4 was used for the 4-wheel model. However, this is not correct. It was just the name of the key procedure that was in use when the machine was issued. The range started with M1 and ended with M4, the special U-Boot key. Nevertheless, the key names correspond to small manufacturing differences. The following variants of 3-wheel naval Enigma machines are known:

Enigma M1 and M1a
Enigma M2 and M2a
Enigma M3

The internal designator used by the manufacturer was Ch.11g for all three versions, whilst the later 4-wheel machine (M4) was designated Ch.11g4. The table below was produced by Frode Weierud in 2009 and shows how many of each version and variant were made between 1934 and 1941 [3]. At the bottom is the first production run of the later 4-wheel Enigma M4 machine.

Year Quantity Serial numbers Version Designator
1934 401 M 501 - M 901 M1 Ch.11g
1935 60 M 902 - M 961 M1a Ch.11g
1937 150 M 962 - M 1111 M1a Ch.11g
1938 310 M 1112 - M 1421 M2 Ch.11g
1939 580 M 1422 - M 2001 M2a, M3 Ch.11g
1940 800 M 2002 - M 2801 M3 Ch.11g
1941 1011 M 2802 - M 3812 M4 Ch.11g4
Enigma M1
The first 3-wheel naval Enigma machines were introduced with the Kriegsmarine in 1934, along with the first operating procedure: M1, which is why these machines are known as Enigma M1. Only 611 of these machines were built (M 501 - M 1111) before it was succeeded by the M2.

The image on the right shows a good example of an Enigma M1 with serial number M 897 that is on public display as part of the Enigma Gallery at the Bletchley Park Museum (taken in 2014) [4].

This machine was found complete with the original maintenance booklet. Unfortunately, the lamp panel is missing, but all other features are clearly visible. To the right of the cipher wheels is a large circular 2-pin socket that allows the machine to be powered from a 6V source. The lid that covers the wheels has two locks, which is a typical feature of the 3-wheel naval Enigma.
Enigma M1 with serial number M897. Copyright Niels Faurholt [4].

Note that the lamp panel of all naval machines can be removed separately from the wheel cover. This was done to allow the connection of additional devices to the lamp panel, such as an external lamp panel (Lesegerät) and an electric typewriter. In both cases the lamps have to be removed in order to accomodate the large connector that takes over the lamp contacts.

The top lid of the wooden box contains two spare Steckerbrett cables with the old style holders. It also carries an oval shield with the Enigma logo and a rectangular one with the manufacturer's name (Heimsoeth und Rinke), both of which were omitted later. The machine shown here was produced in 1934. A typical feature of the M1-variant is the arrangement of the contacts on the Steckerbrett, which carry both letters and numbers and is ordered like the keyboard (QWERTZ).

The layout of the Steckerbrett of the Enigma M1

On later machines, the Steckerbrett was arranged in the order of the alphabet (ABCDE...), but it is currently unknown when this transisition took place. Some manufacturing changes were applied in 1935, resulting in the M1a-variant, but the differences with the M1 are currently unknown.

Enigma M2
In 1938, the M1 and M1a versions were succeeded by the M2. Although the exact manufacturing differences with its predecessor are currently unknown, the machine with serial number M 1322 gives us a good impression of the production changes that had meanwhile been implemented.

The image on the right shows the machine with serial number M 1322, which is part of the Enigma Gallery at the Bletchley Park Museum (photograph taken in 2014) [4]. Initially this machine was available for the public to touch in the early day of the museum, but given its rareness it has since been placed behind glass.

The machine has the same circular power socket as its predecessor, but a rectangular metal frame has been added to the left of the wheels. This was intended for holding the station call sign. The keyboard now carries the serial number.
Enigma M2 with serial number M 1322. Copyright Niels Faurholt [4].

The holders for the spare patch cables in the top lid of the wooden box are of the new Bakelite type and the shields with the Enigma logo and the manufacturer's name have disappeared. One of the most significant features of this variant is the presence of a hinge above the lamp panel. It allows the lamp panel to be raised without removing it completely, but was dropped again later.

The layout of the Steckerbrett of the Enigma M3

On this version the earlier QWERTZ-order of the sockets on the Steckerbrett has been given up, in favour of the alphabet sequence (ABCDE...). The plugboard still has letters and numbers, but these are now placed together above each socket, rather than around the middle as before. Two additional sockets were added, each marked with a red dot. These were used for testing the patch cables in combination with an extra lamp on the lamp panel (visible after raising the lamp panel).

Enigma M3
The M2 and M2a variants were followed in 1939/1940 by the M3, which was the last series of 3-wheel naval Enigma machines. Like with the other versions, the exact differences between the M3 and the earlier ones are not known, but the M3 with serial number M 2272 gives us some idea.

The machine shown here has serial number M 2272 and is from the collection of the Museum for Communication in Frankfurt (Germany).

This machine is largely identical to the M2 shown above, except for the fact that the hinge between the lamp panel and the wheel cover has been removed again. The lamp panel is separate though, just like on the later 4-wheel Enigma. This photograph also shows the rather strange arrangment of the sockets on the plugboard, with a gap at the bottom center. This typical arrangement was also used on the earlier M2.

The most interesting change to this model however, is the fact that the circular 6V power socket has been replaced by two oval ones: a smaller one that accepts 4V and a slightly larger one for connection to 220V. This means that this version has a built-in 220V/4V mains transformer. The same 4V socket was present on the later M4 machines, but the 220V socket was not seen again.

Recovery of an M1 in Denmark   1992 - 2015
In 1992, a Danish fisherman found a strange device in the waters of a small fjord (firth) named 'Vemmingbund', not too far from the historically important Flensburg Fjord, right on the border between Germany and Denmark. According to the reports at the time, there was no wreckage involved. Instead, it was reported, the device mysteriously popped up from the water one day...

The mysterious machine turned out to be an Enigma machine and was given to the Danish Post and Tele Museum in Copenhagen [6] where it has been stored ever since. In 2015 it was decided to do some further research in the hope to determine the exact model and age, and, if possible, preserve it in its present condition.

The image on the right shows the machine in 2015. Although at first sight it may seem to be in bad condition, it is actually not too bad for a machine that has been in the sea for about 50 years and then in storage for another 23 years.
The mysterious Enigma that was found in Danish waters in 1992 [6]

Looking a bit closer, we see that the machine is largely undamaged and that it was fully intact when it was lost to the waters of Denmark. The patch cables on the Steckerbrett (plugboard) are still present in their sockets and the three cipher wheels are still installed inside the machine.

One of the goals of the 2015 investigation was to determine the exact model and age of the machine. From studying the remains it had already become clear that it could be an Enigma M3, as the wheels have letters on the rim. As it was impossible to open the case due to heavy corrosion, the National Museum of Denmark [5] arranged for the machine to be X-rayed. Despite the fact that the machine is now over 80 years old and that it has been in the water for some 50 years, followed by 23 years of unpreserved storage, the photograph is still amazingly detailed.

It reveals that the machine is largely undamaged and that most of the internal wiring is still intact after all these years. It also shows that the cipher wheels and the UKW are still in place and that both locks on the wheel cover are indeed locked.

After carefully studying an extreme enlargement of the the x-ray photograph below, we found the machine's serial number imprinted in the right­most lock. After flipping the image and applying contrast enhancement to it, we were finally able to determine the serial number of this machine: M 522, as shown in the image on the right.
Serial number M 522, visible after applying contrast enhancement.

According to Frode Weierud's research [3] this means that the machine was built in 1934 and that it is probably from the first batch delivered to the Kriegsmarine, as the range started with M 501. Other features that confirm that this is indeed an M1, are the presence of the 6V power socket at the top right and, more importantly, the typical arrangement of the sockets on the Steckerbrett.

We are grateful to the National Museum of Denmark [5] and the Danish Post and Tele Museum [6] for allowing us to reproduce the X-ray photograph here. Click the image below for a close-up. We should also like to thank Frode Weierud for providing details of his forthcoming publication about research on naval Enigma machines [3], and Niels Faurhold for his help and valuable insights [4].

X-ray image of an Enigma M1 [5][6]. Click to zoom in.

The image above shows an X-ray photograph of the Enigma M1 with serial number M 522 that was found by a fisherman in the Danish Vemmingbund Fjord in 1992. The machine is now part of the collection of the Danish Post and Tele Museum [6] and is currently being preserved (2015).

Wheel wiring
The wiring of the M3 is given in the table below. Wheels I thru V are identical to those of the Enigma I. The same is true for UKW B and C. The three additional wheels (VI, VII and VIII) were used exclusively by the Navy. The rightmost column gives the number of turnover notches.


Differences with Enigma I
As discussed above, the M3 is functionally identical to the compatible Enigma I that was used by the Heer and Luftwaffe. Nevertheless there are a couple of manufacturing differences that are unique to the Kriegsmarine machines. So far, the following differences have been recorded:

  • Letters on the wheels rather than numbers
  • Different ring setting mechanism
  • Separate lamp panel (hinged on the M2)
  • Lockable wheel cover with two locks
  • Removable top lid (of the wooden case)
  • Plugs with longer pins (incompatible with Enigma I plugboard)
  • Lock in top lid of wooden case rather than in the flap at the front
  • Two grips at the sides (for pulling the machine out of a bay)
  • 6V circular power socket (M1, M2), or two sockets for 4V and 220V (M3)
  1. Deutsches Bundesarchiv (German Federal Archive), Bild 101II-MW-4222-02A
    Dietrich, Lorient (France), U-Boot U-124, 9 March 1441.
    Retrieved from Wikimedia Commons.

  2. Frode Weierud, Enigma History
    Personal correspondence, June 2009 (not yet published).

  3. Frode Weierud, Naval Enigma Models
    Personal correspondence, 2009, 2015 (not yet published).

  4. Niels Faurholt, Images of Enigma M1 at Bletchley Park
    Personal correspondence, May 2015.

  5. National Museum of Denmark, X-Ray photograph of Enigma M1
    X-Ray of the M1 from the collection of the Danish Post & Tele Museum [6].
    Retrieved May 2015. Reproduced here with kind permission.

  6. Danish Post & Tele Museum, M1 recovered from Vemmingbund in 1992
    Retrieved May 2015. Reproduced here with kind permission.
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Crypto Museum. Created: Monday 14 September 2009. Last changed: Saturday, 30 September 2017 - 10:08 CET.
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