German-Japanese Exchange of Information
- The special Significance of Berlin-Tokyo traffic
- Allied interest in the effort toward "Total Underwater Warfare"
- First details of the new type U-boats
- Delay in completion of the new type U-boats
Type XXI U-Boat
1. SummaryTraffic of the Japanese Naval Attache in Berlin was of special value to radio intelligence because it gave full details on technical matters under investigation by the German navy, and also gave an insight into the future broad policy of the Naval High Command. A very large amount of information was furnished on all sorts of technical matters, including U-boats, aircraft, weapons, radar, radar defense, and the extent of Axis knowledge of Allied weapons, devices and procedures. Of particular value to the Atlantic Section was the data on the new type XXI, XXIII, and XXVI U-boats, with which the Germans hoped to regain their former power on the high seas. With the early knowledge of the main feature of the new types, their higher underwater speed, and subsequent fuller details, it was possible to appreciate the proportions of the anticipated new German measures. Of no less importance was the ability of radio intelligence to follow closely the progress in construction of the new type boats.
2. The special significance of Berlin-Tokyo trafficTraffic between the Japanese Naval Attache in Berlin and his superiors supplied certain unique contributions to the general picture which were not available in the administrative and operational traffic of the German navy. Whereas the technical material sent in U-boat traffic was being passed to people who already had a knowledge of the subject discussed through possession of handbooks and other aids, hence presumed a prior knowledge of most details, the Japanese were in much the same position as their enemies in that they had no such extensive prior knowledge. Therefore, when a new device or measure was reported to Tokyo by the Japanese Naval Attache, full details had to be given and were thus made available to the Allies through decryption.
The information to be gained form Japanese Naval Attache and diplomatic traffic was also valuable because, in addition to its full discussion of technical matters, it provided an insight into the line of major policy adopted by the German navy after the U-boat was driven from the North Atlantic in the spring of 1943. The type XXI U-boat, which was to be the
mainspring of that policy, will be discussed at some length in following paragraphs. The selection of new type U-boats as a prime example of the value of Japanese Naval Attache traffic should not, however, lead to disregard of the importance of the information furnished. Of direct Naval interest were details of midget, transport and mine-laying U-boats; new torpedoes, including the turbine torpedo still under development at the time of the German surrender; and location and anti-location devices. Of a more general interest were the details of radar and radar defense; jet-propelled and other aircraft; V-weapons and many other types of enemy weapons; and the extent of enemy knowledge of Allied weapons, devices and procedures.
3. Allied interest in the effort toward "Total Underwater Warfare"It was apparent to the Allies as well as to the Germans that, after the utter defeat to which the U-boat had been subjected in spring of 1943, the German navy would be forced to turn to the skill of its technical departments even more urgently than before. The situation demanded quick, effective and revolutionary measures, and it was a matter of supreme importance for the Allies to find out what those measures were to be in order to plan counter-measures before any large measure of success could be achieved by the enemy. The introduction of the Zaunkonig and the Schnorchel have already been discussed in other chapters. They were part of the general plan to restore to the U-boat its old power.
It was known, however, that other plans were also in the making. On 6 March 1944 the Japanese Naval Attache reported to Tokyo:
Although German submarines were amazingly effective at first the enemy now has invented new defensive weapons and in addition devised great counter-measures which have brought about a continued decrease in the damage inflicted by submarines. Consequently the German navy had determined to improve the submarine and we believe they are making plans for the future, but judging from what we know now, we do not expect any great activity form new type submarines before autumn.
The direction of German thought, as was to be expected in view of past developments, and as was indicated by the introduction of Schnorchel, was toward severing the tie which bound the U-boat to the surface and made discovery by radar possible.
Schnorchel was a step in that direction, but only a step, and it was feared by the Allies that exhaustive research might lead to a U-boat which would not need to go to the surface or even near the surface during the entire period of a cruise. It was thus a great relief to be able to learn from reports of the Japanese Naval Attache that while new type U-boats were indeed to be introduced, their chief advantage over older types provided with Schnorchel would be in their increased underwater speed.
4. First details of the new type U-boatsOn 28 December 1943 (in a message not read until June of 1944), the Japanese Naval Attache in Berlin sent to Tokyo a description of two new types of U-boats, details of which had not yet been officially released to him by the German navy. The information represented the report of Naval Inspector Tomonaga, who had been permitted to view plans and charts at the shipyard and to catch a glimpse of part of one of the vessels. The Attache himself had first viewed the plans the preceding August at the Deschimag Shipyard, and his observations, added to those of the Inspector, provided specifications and other information sufficient to indicate the trend in U-boat construction for the remainder of the war. As was later confirmed by official information, it appeared that the chief feature of the new U-boat would be their increased underwater speed, which would be made possible by large-capacity batteries.
Two new types were described, the large type XXI, designed for long-range operations; and a small type designed for coastal operations. The small U-boat was referred to as type XVIIb but later evidence pointed to the conclusion that the U-boat under construction was actually the type XXIII, concerning which a large amount of information was later gained from Attache traffic. As to the state of construction of the new boats, the Attache considered that large numbers were already in the construction stage, would be completed from about August or September. (JNA #479, 28 December 1943)
5. SpeedsSince the increased underwater speed of the new type U-boats was to be their chief advantage over older types, a dispatch in which pertinent figures were given, Japanese Naval Attache #316 of 13 August 1944, was of more than passing interest. The speeds and ranges quoted were as follows:
(6 battery compartments)
Type XXIII Speed
3 450 2 250 6 265 4 175 8 165 6 113 11 110 8 70 14 45 10 43 top 24 top 22
According to Japanese Naval Attache #420 of 12 September 1944, the tested top speed for type XXI was about 16 knots, and it was hoped that this could be increased to 17 or 18 knots. Also of significance was the type XXI's lowest submerged speed on electric motors, 0.5-1 knot, at which the motor was said to be noiseless as the result of reduction by rubber belts. For type XXIII the tested top speed was 12.5-13 knots. It can be seen from the above table that use of the higher underwater speeds would be severely limited by the short ranges at which such speed could be maintained before exhaustion of the batteries.
6. Delay in completion of the new type U-boatsThe Japanese anxiety over the condition of German striking power led to continual inquires as to when the new type U-boats would be ready for combat. The serious construction difficulties due to Allied bombings and other factors were reflected in repeated postponements in estimates of that date. By the reading of Japanese messages it was possible for radio intelligence to keep informed of construction developments. On 26 July 1944, Ambassador Oshima reported concerning the type XXI boat:
At present it is in process of construction and its effect will first be felt from about September; and by November or December it is said that greater results will be effected by its use in great quantity. (#756)
Again, on 12 August (#621), Dönitz was quoted as saying that the new offensive U-boat warfare would begin "at one swoop" after a considerable number of the new U-boats had been completed, therefore in the winter of 1944-45. But on 3 December (#753) the Naval Attache wrote with regard to combat use of new type U-boats:
Although the German submarine fleet and ministry of military supplies are making feverish preparations, there will probably be no results until about March of next year.
Even in March 1945, the best that could be reported concerning type XXI boats was that they would be ready on a large scale in May or June (#301, 7 March). Actually, only four of the type XXIII's made war cruises, while the only type XXI to leave port for an operational cruise was U-2511 (Schnee) which was outbound on the day of surrender.
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