This document was made available by Aaron S. Hamilton

Translation by Thomas Borchardt and Jerry Mason
Considerations for employment of the Type XXI
Click the flag to view the above page from the original German KTB
  Oberkommando der Kriegsmarine Headquarters Korale,  
  2. Abtlg. Skl B.d.U. - Op. - 10 July 1944  
  Serial No,  Gkdos.  4858        AI  
  273 Copies.  
  Secret Official Document ! . . 4. . Copy.  
Considerations for Employment of the Type XXI.

 The Type XXI boat has exceptional submerged combat features, capable of virtually eliminating enemy superiority in anti-submarine defence achieved through air supremacy and underwater detection methods.  With this boat, and other types, it will be possible to begin a new successful U-boat offensive.


This document contains theoretical considerations for the employment of the Type XXI boat in combat against North Atlantic convoys and operating remotely in southern operations areas.  It describes the operational and combat capabilities of the boat for Commanders and training units in the homeland with respect to the prevailing defensive situation in the Atlantic.


Particular operational questions, especially concerning communications during convoy operations, locating strategies and so on, were only touched upon and can only be answered fully after debriefing crews returning from the Front.  It is possible that some previous assumptions of tactics at the Front are later proven to be incorrect.


The main purpose of this document is to give both trainers and Commanders suggestions to the crews on all aspects concerning training and the tactical operation of the boat at the Front.


This active cooperation and continuous feedback of new experiences is one of the prerequisites for the successful employment of the boats on a wide basis.

  I expect all U-boat men, training- , logistic- and testing agencies to apply themselves with all our strength in the service of this task.  
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O r g a n i z a t i o n  .
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I.  Conduct of the Type XXI in transit.
  1.) Strong air surveillance areas.
  2.) Weak air surveillance areas.
  3.) When snorkeling.
  4.) Action on detection by enemy aircraft.
II.  Type XXI as combat boat in the North Atlantic.
  Situation in operational area.
A. Type XXI as lone warrior in the open Atlantic.
  1.) Disposition.
  2.) General conduct of the boats.
  3.) Action on detection by enemy air.
  4.) Action on enemy contact.
    a)  By day, boat is submerged,
    b)  by day, boat is surfaced,
    c)  by night, boat is surfaced,
    d)  by night, boat is submerged.
B. Type XXI as lone warrior close to the coast.
  1.) Outbound transit.
  2.) Conduct in operations area.
III.  Type XXI as combat boat in convoy warfare.
  Basic principles.
A. Group disposition for detection of convoys.
  1.) Disposition in places of traffic concentration.
  2.) Disposition (Patrol lines) in open sea areas (without aircraft bearing signals).
  3.) Disposition (Patrol line) in open sea areas with aircraft bearing signals.
  4.) Patchwork disposition [individual action stations]
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B. Conduct as combat boat in the group.
  1.) Advance and occupation.
  2.) Action on enemy contact (first boat).
    a)  By day, boat is submerged,
    b)  by night, boat is submerged.
  3.) Conduct of remaining boats on receiving enemy report over very low frequency [VLF]
IV.  Type XXI as "Remote combat boat".
(Coastal areas Central-, South Atlantic, Indian Ocean).
  Situation in operations area.
  Suitability of the Type XXI as "remote combat boat".
  1.) Large action radius.
    a)  Cruising range.
    b)  Accessible operations areas.
  2.) Brief advance and retreat periods
  3.) Long length of stay in operations area.
  4.) Improved living conditions for crew.
  5.) Operation in heavy air surveillance areas is possible.
  6.) Good success opportunities through high submerged speed.
V.  Type XXI in attack.
  Basic principles.
A. Day attack.
  1.) Unnoticed attack beyond close escort.
  2.) Unnoticed attack within the screen.
    a)  Unnoticed periscope attack.
    b)  Unnoticed attack without periscope.
    c)  Conduct after attack.
    d)  Comparison of attack without periscope with attack with periscope.
    d)  Comparison of Type XXI with previous combat boats
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  3.) Submerged attack as noticed attack.
  4.) Attack from beneath the convoy.
B. Night attack.
  1.) Surfaced attack.
  2.) Submerged attack.
VI.  Type XXI during depth charge pursuit.
  1.) How have losses by depth charge pursuit occurred thus far?
  2.) Comparison of resilience of Type XXI with previous types.
  3.) Conduct with depth charge pursuit.
    a)  Passive defense.
    b)  Active defense.
VII.  Communications outfitting and operational possibilities of the communications system.
A. Equipment outfitting.
B. Communication links of surfaced boats.
  1.) Short wave.  [SW]
  2.) Low frequency.  [LF]
  3.) High frequency.  [HF]
  4.) Very high frequency.  [VHF]
C. Communications links of "snorkeling" boats.
  1.) Short wave.
  2.) Low frequency.
  3.) High frequency.
  4.) Very high frequency.
D. Communications links of dived boats.
  1.) Very low frequency reception.  [VLF]
  2.) Under water telegraphy.
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I.  Conduct of the Type XXI in transit.
  1.) Strong air surveillance areas.
    Bay of Biscay, transit from homeland into the North Atlantic, transit in coastal vicinity.
    Transit basically submerged.  Surfacing to charge, if snorkeling is not possible.
    Theoretical Values:
  Charging transit on the surface.
  Calculated for:  
  Battery condition: Before diving 90% of total capacity.
    Before charging 40%   "          "
  Cruise duration:   21 hours at 6 knots
       3 hours at 6 knots
  Day's run: 144 nm.  Fuel oil consumption 2 tons.
  Calculated for:  
  Battery condition: Before diving 90% of total capacity.
    Before charging 40%   "          "
  Cruise duration:   20 hours at 7 knots
       4 hours at 6 knots
  Day's run: 164 nm.  Fuel oil consumption 2 tons.
  Charging transit submerged with snorkel.
  Calculated for:  
  Battery condition: Before setting off 90% of total capacity.
    Before charging 40%   "          "
  Cruise duration:   19 hours at 6.5 knots
       5 hours charging period at 3 knots
  Day's run: 139 nm.  Fuel oil consumption 2 tons.
  Calculated for:  
  Battery condition: Before setting off 90% of total capacity.
    Before charging 20%   "          "
  Cruise duration:   17 hours at 7.5 knots
       7 hours charging period at 3 knots
  Day's run: 148 nm.  Fuel oil consumption 2 tons.
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    Submerged transit with highest silent running and charging with snorkel:
    Calculated for:
  Battery condition: Before setting off 90% of total capacity.
    Before charing 60%   "          "
  Cruise duration:   21 hours at 5 knots
       3 hours charging period with snorkel at 3 knots
  Day's run: 144 nm.  Fuel oil consumption 1.5 tons.
  2.) Weak air surveillance areas.
    Most economical transit 11 hours surfaced at 7 knots.
    Calculated for:
  Battery condition: Before diving 90% of total capacity.
    Before surfacing 40%   "          "
  Cruise duration:   11 hours at 7 knots
      13 hours at 7 knots
  Day's run: 168 nm.  Fuel oil consumption 2 tons.
  Calculated for:  
  Battery condition: Before diving 90% of total capacity.
    Before surfacing 20%   "          "
  Cruise duration:   11 hours at 7 knots
      13 hours at 8 knots
  Day's run: 181 nm.  Fuel oil consumption 2 tons.
  Medium transit speed 11 hours surfaced at 10 knots.
  Calculated for:  
  Battery condition: Before diving 90% of total capacity.
    Before surfacing 40%   "          "
  Cruise duration:   11 hours at 10 knots
      13 hours at 7 knots
  Day's run: 201 nm.  Fuel oil consumption 3.5 tons.
  Calculated for:  
  Battery condition: Before diving 90% of total capacity.
    Before charging 20%   "          "
  Cruise duration:   11 hours at 10 knots
       13 hours at 8 knots
  Day's run: 214 nm.  Fuel oil consumption 3.5 to 4 tons.
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  Highest transit speed 11 hours surfaced at 14 knots.
  Calculated for:  
  Battery condition: Before diving 90% of total capacity.
    Before surfacing 40%   "          "
  Cruise duration:   8 hours at 14 knots
      3 hours at 6 knots to charge
    13 hours at 7 knots
  Day's run: 221 nm.  Fuel oil consumption 5 tons.
    Comparison shows that the highest cruising speed is located in the range of 10 knots on the surface, since at maximum speed of 14 knots no charging is possible.
  3.) When snorkeling!
    (Objective:  Snorkel made either with a radar frequency absorbing coating or equipped with Fu.M.B. including decimeter range).
    In areas where no sea U-boat hunting is expected: for example, the Norwegian coast, Iceland - Faroe Islands sea area, inner Bay of Biscay east of 10°W, strictly at night.
            All around listening every 20 minutes or sonar operation every 20 minutes.  Periscope extension at night to recognize aircraft with searchlights.
            In areas where sea surveillance is expected:  In good weather by day with extended periscope.  All around listening every 20 minutes.  In heavy Sea States, where periscope all around sweep does not provide sufficient safety, snorkel at night, all around listening every 20 minutes with diesels stopped.
    Over the next few months, many new experiences concerning implementation of snorkeling will arise from the Front
  4.) Action on detection by enemy air.
            Hard course change, a minimum of 60° from the last course sighted by the enemy before diving, or from the general course assumed by the enemy, depending on the sea area. If the arrival of U-boat hunting groups is to be expected, run from the location at higher underwater speed depending on the battery situation.
    P r i n c i p l e :
    Gain as much distance as possible from the last position known by the enemy and the position designated by him, remain submerged for longer periods.
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II.  Type XXI as combat boat in the North Atlantic.
+ The Type XXI U-boat is equipped with 6 torpedo tubes that can be reloaded in a short time. (Second salvo reloaded after 5 minutes, third salvo after 20 minutes).  Its combat power is, - if it is employed - sufficient to simultaneously destroy the strongest surface enemy or several enemy ships.  However, its staying power is not enough, to force the use of its own torpedo weapons from the required range on the surface, it must proceed unnoticed on the surface or underwater "surreptitiously".
++ In the heyday of the U-boat war, despite numerous U-boats, at times only slight sinking results were achieved because the enemy detected U-boats with locating aircraft and avoided our deployments with its convoys. He demonstrated the same behavior in the control of his independents in the peripheral regions of Central and Southern Atlantic.
  From + and ++ follows:
    Remain unnoticed before the attack in the operations area.  (The Type XXI allows meeting this requirement to an incomparably greater extent than previous combat boats).
Situation in operational area.
  Traffic:  The main hunting ground of the U-boat war is on America-England-traffic routes, therefore the best organized "Traffic Control" by the English is here. More than 95% of all merchant vessels travel in convoys, with a few fast independents of 16 knots, Once the enemy recognizes an imminent hazard from U-boats (only with the Type XXI will he fully recognize this again) he scatters strongly irregularly in latitudes between the southeastern tip of Greenland and the Azores.
          U-boats (also Type XXI) must operate essentially in the open sea, a stay at the starting points of the convoy routes (off the Minch, North Channel, Irish Sea, New York, Halifax) is due to good enemy defense possibilities and thereby reinforced its own immobility and is impossible in the long run with many boats.
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  Due to the lack of own aerial reconnaissance - for the given convoy scattering and the low horizon of the submarine - finding the enemy is the most difficult problem. For this problem, it is correct to speak of the
Our problem:    "Expanse of the Atlantic".
  Defences:  Air.  From leaving own base the enemy has air superiority.  There is no spot in the North Atlantic, where every day or night - with a few exceptional weather times - where the appearance of land or carrier-based aircraft is not possible.
  There are especially heavy enemy operations:
          on our U-boat departure routes off our coasts,
          on the convoy departure routes off his coast,
          on his at the moment convoy routes,
          ahead of and at convoys as ahead, remote and close escort (operation of carrier air as close escort),
          in conducting U-boat hunts by aircraft and supporting the U-boat hunts of its antisubmarine warfare groups.
  Defences:  Sea:  Large numbers of U-boat hunting groups consisting of 2-5 destroyers, frigates to fishing trawlers are scattered in the Atlantic.  Scattering and control of these groups is based on the transit and formation orientation of the U-boats identified by him and on the own convoy routes then recognized as correct by him.
          Larger numbers of escort vessels down to R-boats as escorts at each convoy.  Close escort at a distance of 2000-6000 meters, remote escort deployed mostly in full screen focusing on U-boat concentration side set off up to 25 nm from the convoy.  Number of the escort vessels according to the value of the convoy cargo and U-boat danger varies strongly from about 5 to 25.  An auxiliary aircraft carrier is in or at almost every convoy.
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  With regard to defense, the speed of their appearance and their employment, the number and quality of the existing aircraft and naval forces, the equipment and quality of the devices of the "U-boat finders", there is for the enemy, as opposed to our "Find-problem", no "expanse of the Atlantic", for him it is an inland sea
English Problem?
    a "mare nostrum".
A. The boat as lone warrior in the open Atlantic.
          Employment as a lone warrior is not the general rule, since agglomeration of own strength against combined defense (close to the convoy) is advantageous, however the boat appears as a lone warrior:
    on reaching the operations area,
    as long as there are not enough Type XXI boats for agglomeration,
    directed approach is available,
    between any group dispositions.
          The efficiency of the individual Type XXI boats is considerably higher than previous combat boats because once reaching the enemy firing multiple shots is allowed (two to three salvos equal to 12 to 18 torpedoes), so significant combat power can be put to use.
  If the type XXI boat keeps its promises, it means every boat is a special risk for the enemy, that is to say, each boat will be fought intensively by him to and/or be rendered harmless by continual detection and holding submerged.  The appearance a few Type XXIs already brings a far greater fragmentation of the defense than significantly greater numbers of existing combat boats. Therefore in the fight for pure existence in the operations area, as opposed to the current condition, relief may even take place. The superior characteristics of the Type XXI to resist enemy defenses are not yet considered.
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  1.) Disposition:
    Attack area ordered by leadership. Usually a larger surveillance area (strip, attack area with depth etc), in which the boat moves back and forth according to traffic indication and enemy defenses.
    Traffic indication by:
            own observation,
            observation of other boats,
            cuing from B-Dienst [Beobachtingsdienst = Navy Intelligence Service] and convoy plotting (passed from leadership by Radio Message),
            own air reconnaissance (anticipated to be slight, cannot be expected).
  2.) General conduct of the boat.
    In strict compliance with the requirement to remain unnoticed, the following possible worsening degrees of own behavior are sorted by degree of identified enemy defenses (locating enemy air activity is viewed as an enemy defense):
            a)  day and night surfaced,
            b) days dived, nights surfaced,
            c)  basically dived, surfaced only to charge,
            d)  basically dived, charging with snorkel.
    In general - particularly in calm weather with favorable enemy detection conditions - the behavior of the boat is as set forth in c) and d).  In winter and with especially adverse weather conditions for enemy air operations easements are possible up to level a).  Conduct according to levels a) and b) is also possible if Fu.M.B. is present, warning of all locating frequencies used by the enemy in time to allow a dive before detection by the enemy locating is possible.
            Use of own Fu.M.O. - to safeguard against the enemy air attack during surface transit - before being noticed is incorrect, because by this the presence of a U-boat is betrayed. Use has been released,
    as soon as the boat is noticed anyway and surface charging transit (for example, snorkel failure, etc.) is forced, or when no tactical disadvantage arises because a large submerged position change is intended anyway.
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  3.) Action on detection by enemy air.
    Increased enemy air activity - determined by sightings or by Fu.M.B. - typically foreshadows the imminent appearance of enemy traffic (convoys, warships antisubmarine formations). Therefore by day the boat must dive to get avoid a more precise detection and escape after combat.  In view of an imminent attack opportunity the boat must begin charging in snorkel mode if necessary.  For this use the periscope and frequent all around listening.  Snorkeling will probably be difficult to implement in a calm sea due to risk of locating.
      How far "snorkeling" can be heard by the enemy in comparison to "periscope range", can be stated only after practical testing.
            At night, after temporarily diving before locating or an approach, in many cases it will be possible to remain on the surface and charge.  Depending on visibility and weather conditions, it will now be necessary to monitor the area by frequent all around listening on the surface or submerged.
    Remaining on the surface is better since enemy sea locating [radar from surface ships] (and thus the presence of enemy vessels) can be detected at greater ranges than the enemy propeller sounds by own listening device.  The direction of detections as well as monitoring and bearings of the enemy's convoy voice frequencies give indications of the enemy and the own intercept course to be steered.
    It should be noted that bearings can be taken on sea locating and convoy voice communication over very long distances in the Fu.M.B., or MF direction finder, so it can take hours until the boat comes in sight by the locating enemy.
  4.) Action on enemy contact.
    a)  By day, boat is submerged:
    The enemy will be detected depending on the listening or visibility conditions only by listening or periscope observation, in many cases listening observation may be further than periscope observation (especially with the balkon listening device).
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    Action of the boat after detecting a sound bearing:
      Periscope all around sweep, when clear surface and set off on the last bearing.
      Also by day try - but only as long as one proceeds unnoticed - to cover as much distance as possible on the surface to save battery capacity.  Recent experience has clearly shown that convoys and independents were often heard at distances of more than 30 nm, i.e. at distances that would cause high speed submerged cruise to prematurely exhaust battery capacity.
      (If sound bearings show that the boat is beaing approached by the enemy do not surface).
      Dive on encountering aircraft, otherwise continue on. Once surface locating appears and convoy voice traffic is observed the approximate advance course of the enemy is determined from the individual bearings (depending on the sea area, certain general courses of the enemy already may be assumed at the first sound bearing). Start at top speed on course to maneuver ahead. After first enemy sighting, or when some clarity is gained about the location, nature of the enemy and its submerged accessibility, dive and approach submerged at the highest endurance speed appropriate to the situation.  (Further see under V. A).  From the number and type of the sound bearings and from periscope observation  clarity will soon result concerning the type and disposition of the enemy, if independent, U-boat hunting group, convoy with outer and inner screen.
      Action of the boat after detecting a sound bearing:
              Periscope all around sweep, enemy aircraft sighted, do not surface, instead operate submerged on the suspected general course of the enemy.  Form a picture of the situation from sound bearings .  As soon as within visual range, at periscope depth, all around sweep, continue as before.
    b) By day, boat is surfaced:  (exceptional case).
              First indication of the enemy not through locating or MF bearing, but instead by visual observation. With a good lookout sighting an enemy generally takes place from a position forward of 90°.  Independents generally cannot be kept up with given the low speed advantage of the type XXI on the surface, on sighting a convoy the surface screen or enemy aircraft
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      will soon detect the boat while maneuvering ahead and force it to submerge. So normally after finding an enemy the boat immediately dives and begins to attack submerged. (continue as under V. A.)
    c) By night, boat is surfaced:
      First enemy indication is normally from detecting localization.  If air locating, dive momentarily - otherwise remain on the surface. Turn on own Fu.M.O., ready for immediate use.  Take bearings on detections, stop for all around listening. Depending on the situation from all around listening proceed on the surface or submerged, work by briefly (if necessary) with Fu.M.O. form picture of the situation and approach on the surface until sighting the enemy. At night, always remain on the surface as far as possible..
              The boat must exploit the given monitoring devices (Fu.M.B., balkon listening device, MF direction finder) as well as the Fu.M.O. for the approach. Skillful tactical behavior will allow in many cases the conduct of a surface attack. (Experience Schröteler and Poel). (further see V. B. 1)
    d) By night, boat is submerged:
      First indication by listening gear.  According to the situation (Volume of the bearing, weather, visibility conditions), surface and get ahead in accordance with c) or remain submerged and carry out a blind approach by sound and S-gear [active sonar].  (See V. B. 2 )
        Request for training:
        Analysis of the advance course of convoys from sound and other bearings to achieve the optimum own approach course at maximum economical submerged speed has already lead to the formation of a particularly erudite plotting technique. Cooperation between the Commander and the Chief Engineer must be particularly close when determining the highest sustainable submerged speed and this must be trained in theory prior to the shooting and tactical training exercises.
                The Commander must decide after assessing the weather and listening conditions if he will approach on the surface or submerged, if he can perform a "creeping"-approach or a strepitous ("racket") approach.
                The young Commanders must also be taught this in training.
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B. Type XXI as lone warrior close to the coast.
          Under his own coast - particularly in "traffic funnels" (North Channel, Minch) - with relatively little effort the enemy can apply a continuous sea and air response - thorough U-boat hunts and keep each submarine movement under control.
  Therefore the principle applies here even more than in the Atlantic:  Be sure to remain undetected until the first attack.
          Staying in a "traffic funnel" offers more targets than the open Atlantic.  While outside the boat out must attack everything that comes before the tubes, it can and must allow some selection of targets here and may also dispense with the execution of a seemingly unfavorable attack with low hit prospects.
          The employment of the boat is costly, therefore the first attack must lead to a worthwhile success.
  For longer stays of boats under the enemy coast the following properties are required:
          Long submerged endurance,
          Ruggedness in the face of depth charge pursuit.
  The XXI satisfies both requirements in great measure with its large battery capacity and large diving depth .
  1.) Outbound transit:
    Unnoticed, basically dived, charging only with snorkel.  In heavy seas or unfavorable flying weather, it can under certain circumstances charge at night on the surface.  Establish boat routine, eating so that as little waste as possible accumulates for the intended use of coastal and surfacing to empty 'bailers and containers " is avoided.
  2.) Conduct in operational area:
    Only attack with certain-shot opportunities. Perform traffic monitoring, improve position afterwards so that the next attack can be a completely worthwhile success.  Change area depending on the onset of defenses, or remain undetected for multiple days, lying on the bottom and only at periscope depth for ventilation.
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    If possible charge with snorkel under locating protection from cliffs.  Attacking as soon as a degree of calming has occurred.
            One difficulty with permanent submerged cruise is navigation.  Any opportunity for terrestrial fixing by periscope observation day or night must be exploited. It is not likely that there will be opportunities to surface.
            The enemy has its traffic narrows under surveillance by land radar so that on surfacing the boat will be detected.  Therefore, do not even make the attempt.
III.  Type XXI as combat boat in convoy warfare.
        The massing of defense (protection of convoys) must be countered by a massing of attackers. When boats are looking for the convoy themselves, they must solve the "problem of finding" themselves, massing of the attackers is difficult.
        With the Type XXI it will be possible to arrive in new dispositions by snorkeling and relative quickness, especially near the convoy starting points unnoticed. Because in these areas large evasive movements are hardly possible and - as long as U-boats are unnoticed - and are not to be expected, it is possible to carry our dispositions at some depth, and thus come closer to the always demanded "ideal" formation.
        Each and every small indication, even the slightest sign of the presence of U-boats will be gathered by the enemy in painstaking work and adds up for him to the final form of a group, but a group formation is useless if the enemy knows its extent; he handles it with ease, for him a known aggregation of submarines represents a diminution of U-boat danger in other areas. Therefore, remaining unnoticed is the duty of every boat.
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A. Group disposition for detection of convoys.
  1.) Disposition in places of traffic concentration.
    a) Conceivably:  A formation of many boats in a checkerboard pattern in the traffic funnel or close off it (North Channel - Minch), i.e. the operation of many boats in a small area. This is not to ignore whether a convoy sighting may be (belatedly) relayed from the sighting boat to other boats. Report with rod antenna (Kurier method [burst transmission], see also under VII.). In general, only boats that are directly approached by the convoy come to attack.
    b) Conceivably:  A formation of several boats as "scouts" in the funnel, one or more groups just off the funnel. The task of the scouts is delivery of the sighting report with the rod antenna, if necessary deploy one or more radio repeaters outside of the funnel, but within rod antenna range.  The radio repeater transmits the report to leadership and thus to other boats.
              In principle, the relocation of the groups off the funnel after receipt of the report of the scout takes place entirely submerged.
      Disadvantage of this disposition: Convoy course in the funnel is geographically limited, convoy course on departure will be chosen arbitrarily depending on the intended route by the convoy leader, it is questionable whether the group's disposition will still be correct.
  2.) Disposition (Patrol lines) in open sea areas
      (without being brought up by aircraft bearing signals).
            Presuming that own aerial reconnaissance, by which the sea areas in question are often seen, carries out a certain amount of area reconnaissance, convoys can be detected but contact cannot be held. Contact holding, sending bearing signals are impossible due to strong fighter defense by remote fighter or carrier-based aircraft.
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            Procedure from 1942 and 43: Patrol line disposition on convoy routes based on information from convoy plotting, results of the B-Dienst and own aerial reconnaissance. Additionally, new disposition from surfaced radio repeater parallel to the patrol line.
            1943 procedure was unsuccessful in most cases, because the U-boat disposition was located [with radar] by the enemy on the surface and was avoided by the convoy.  The Type XXI offers the advantage of proceeding to the patrol line unnoticed and unnoticed shifting of the patrol line submerged.  Detection of convoys is possible in certain cases because smaller evasive movements are to be expected.
    Disadvantage of this disposition:  Because information is imprecise, relatively broad dispositions are required.  Just a few boats gain contact by sound bearings or periscope observation or receive convoy reports via rod antenna and can get to attack position with a submerged approach.  Other boats must attempt to get ahead on the surfaced due to the great range (as in 1943), which seems very difficult due to heavy air surveillance and is expected to succeed only at night.
    Type XXI advantage:  Lower advance delay from frequent diving before aircraft because the boat may continue to run on its old course at high submerged speed (13-14 knots) for the brief period of diving.
    Type XXI disadvantage: Lower surface speed than VII C boats, in good weather in the Atlantic at best 12-13 knots.
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  3.) Disposition (Patrol line) in open sea areas with aircraft bearing signals.
      (brought up by aircraft bearing signals).
            1943 process was carried out in cooperation with  Fliegerführer Atlantik. It is not expected that appropriate German reconnaissance aircraft such as these will exist for warfare in 1945 because fighter defenses are too strong in the Atlantic. However, if this method is possible, the XXI has the following advantages:
            Fast, unnoticed formation of patrol lines submerged, so multi-day advanced reconnaissance by air is not required. Relocation of patrol lines submerged after receiving the last reconnaissance report, narrowing of patrol lines to small boat intervals is possible quickly.
  4.) Patchwork disposition [individual action stations]
    Conceivably:  a patchwork disposition of boats at a certain depth and width on an assumed convoy route. After receiving an aircraft reconnaissance or U-boat contact report, boats shift individually ahead of the advance course of the convoy or the formation of individual small patrol lines (2-5 boats to increase the chances of convoy detection) at the level of the previous submarine disposition ahead of the advance course of the enemy. Everything basically submerged, contact on the convoy only by sound bearings.
B. Conduct of the boats in the disposition:
          How the cooperation of the boats in a convoy operation takes place and is advantageously directed may become apparent only after the presence of practical experience from training exercises and eventually from the front. Therefore some issues can only be touched on.
  1.) Advance and occupation in patrol lines:
            Advance into the disposition as shortly as possible before the time of the expected convoy passage in order not to give the enemy evidence for the presence of boats by a longer wait in the disposition. Advance to get there submerged in principle, charging by snorkel operation.
            Occupation submerged in principle. Surfacing only on command, or if the enemy is reported (where necessary).  Systematic control of VLF Program times is required for this.
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  2.) Conduct on enemy contact:  (First boat which gets in contact).
    a) By day, boat is dived.
      If the procedure for Radio Message delivery with the rod antenna (Kurier method or passing by radio repeater) proves successful, as for all dispositions, [attack is] to be achieved through a massed approach of the boats (except for dispositions in accordance with III. A. 1 a).  The report [comes] before the attack.  However, if despite repeated sending of the report with the rod antenna it is not received, then the Commander will have to decide whether to attack without further delivery of reports. His boat alone represents a considerable combat force, it is also uncertain whether other boats will get in contact.
              Surfacing is wrong, because generally it can not be expected that this is possible within sight of a convoy protected by air for delivery of the sighting report.
              If the boat does not perform an attack due to unfavorable position and low battery capacity, then its main task is the delivery of the report. For this purpose it must surface if necessary.  If it is repeatedly forced to submerge, always surface again and report.
              In funnel disposition in accordance with III. A. 1 a) attack first, then attempt to send a report.
    b) By night, boat is dived.
      In each case (except for dispositions in accordance with III. A. 1 a), after clarifying the situation at the surface through listening and S-gear surface and report. Kurier method allows continual Radio Message transmission without the possibility of the enemy taking a bearing. Surfaced attack see point V. B.
  3.) Conduct of remaining boats on receiving enemy report over VLF.
    a) By day.
      If the enemy can be reached (with good navigation) with a submerged approach remain submerged and with required highest sustained economical speed (allowing sufficient capacity remaining for attack) approach and gain contact with the listening device.
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              If the enemy cannot be reached in a submerged approach, surface, operate on the surface. Attempt to get ahead despite protecting air, only to give up when air becomes too strong and if getting ahead by night is no longer possible.
              In dispositions in funnels and in heavily air monitored areas do not surface, the danger is too great, let the enemy go.
    b) By night.
      In dispositions under land and in funnels do not surface.  If the convoy cannot be reached submerged let it go.
              In all the other dispositions surface, operate on the surface as long as possible. (See also V. B.)
              When conducting convoy operation with approach guidance by bearing signals (See III. A. 3) operate as in 1943.
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IV.  Type XXI as remote combat boat.
(Coastal areas Central-, South Atlantic, Indian Ocean).
Situation in operations area:  The Allies are - particularly with regard to their war effort in Europe - largely dependent on the supply of raw materials from South America, South Africa and India. The distances protected by the enemy against U-boat danger are very large. Nevertheless, he has been able to largely avoid the impact of the German U-boats by taking appropriate measures.  Generally, despite the diversity of the traffic and defense situation in the various regions, the following should be noted:
Traffic:  Convoys and independents run - as far as possible - in coastal areas, sometimes in shallow water but always under the protection of air surveillance.  In individual limited areas independents only run on day treks where they can be continuously protected by enemy air. The same procedure off major ports and transport hubs, where traffic arrives and departs by day and passes through narrows by day.  In the open sea fast independents running on strongly varied routes, are warned of U-boats, or are diverted around U-boat endangered areas.
Defences:  Essentially by use of enemy air, whose effectiveness has been increased many times over by equipping of almost all aircraft with new tracking devices. An extensive, well planned airbase system developed in the course of the war, with the additional deployment of aircraft carriers, makes air supremacy possible even the broadest Atlantic and coastal routes.
        Off traffic hot spots during the day there is protection by continuous air, at night by strong air patrol activity. Operational strength of the enemy air alone is sufficient to deter boats from the previous type of approach, or to force boats already positioned off focal points to turn away, or hold them harmlessly in one position and starve them. (Experience of one boat: After being noticed enemy air about 60 nm off Colon was so strong that it was impossible
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to recharge.  The boat had to force the required charging time by defense with its antiaircraft weapons).
        In contrast, defense by sea forces is relatively weak.  Off southern bases mostly older destroyers, gun boats and PC-boats.  These, as well as the protection of convoys running there compared to the see protection of the North Atlantic in the fight against boats, are untrained and have little endurance, their effectiveness is correspondingly low.
Suitability of the Type XXI as remote combat boat.
        Best use of the Type-characteristics occurs when used as a remote combat boat.  The Type XXI is better suited for this purpose than for convoy warfare in the North Atlantic.  The following characteristics:
1.) Large action radius:
  a) The following radius of action are from theoretical forecasts.  (It is possible that the real values are slightly lower, however, fundamental changes will not occur).
    Action radius:
On the surface
6 knots 19000 nm   11 knots 11700 nm
7    " 18000 nm   12   "   9500 nm
8    " 17000 nm   13   "   7700 nm
9    " 15500 nm   14   "   5000 nm
10  " 14000 nm      
    Action radius with silent E-motors:
5.5 knots 320 nm   5 knots 360 nm
    With lower speed settings increased action radius.
    Submerged cruise with main E-motors:
6 knots 280 nm   12 knots 60 nm
7    " 210 nm   13    " 50 nm
8    " 160 nm   14   " 40 nm
9    " 135 nm   15   " 30 nm
10  " 110 nm   16   " 25 nm
11  "   80 nm      
    Surface action radius lies between the Type IX-C and IX-D-2
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  b) Which operations can be reached?
    Norwegian Sea:
    Because of the short outbound transit from Norwegian bases action radius for cruising and operations also for highest speed on the surface are completely sufficient.
    North Atlantic:
    It is assumed: transit from western base to the most distant possible disposition on the Newfoundland Banks and transit back from this area.  Disposition area square BC center, for outbound and return transit 4000 nm.
    At 10 knots fuel consumption for outbound and return transit 29% of supply.
      "  12    "                  "             "         "          "            "         42%       "
    North American coast:
    It is assumed Cape Hatteras (outbound and return transit 7000 nm).
    At 10 knots fuel consumption for outbound and return transit 50% of supply.
      "  12    "                  "             "         "          "            "         80%       "
    It is assumed Panama Canal (outbound and return transit 10000 nm).
    At   8 knots fuel consumption for outbound and return transit 60% of supply.
      "  10    "                  "             "         "          "            "         70%       "
    It is assumed Natal (outbound and return transit 8000 nm).
    At   8 knots fuel consumption for outbound and return transit 47% of supply.
      "  10    "                  "             "         "          "            "         57%       "
    New Guinea:
    It is assumed Duala (outbound and return transit 10000 nm).
    At   8 knots fuel consumption for outbound and return transit 60% of supply.
      "  10    "                  "             "         "          "            "         70%       "
    South Africa:
    It is assumed southern tip of Madagascar (outbound and return transit 18000 nm).
    At 7 knots fuel consumption for outbound and return transit 100% of supply, meaning operation in this area without intermediate supply is not possible
    With supply in Japan (outbound transit to Penang via operations area 13000 nm)
    At 7 knots fuel consumption for outbound and return transit 72% of supply.
      "  8    "                  "             "         "          "            "         76%       "
      "  9    "                  "             "         "          "            "         84%       "
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    Indian Ocean:
    It is assumed Gulf of Oman, further transit to Penang = 15000 nm).
    At   7 knots fuel consumption for outbound and return transit 84% of supply.
      "    8    "                  "             "         "          "            "         88%       "
    Operational limit without supply at sea or bases is about the level of Cape Town (outbound and return transit 14000 nm).
    At   7 knots fuel consumption for outbound and return transit 78% of supply.
      "    8    "                  "             "         "          "            "         82%       "
      "    9    "                  "             "         "          "            "         90%       "
      "  10    "                  "             "         "          "            "       100%       "
2.) Brief advance and retreat periods:
          The air supremacy exerted by the enemy in all parts of the sea has forced boats of previous types to transit largely submerged even in open areas of the Atlantic, for which their basic design was not suitable and became increasingly unsuitable because of the ever-increasing overload of the coning tower with anti-aircraft weapons and instruments. Therefore outbound and return transit times were particularly long because of low submerged speed. In May 1944 a IX-C-boat was plotted during the transit to the Caribbean as follows:
Lorient to 20°W
720 nm
        with day's run
60 nm
12 days
20°W to Panama
3800 nm
        with day's run
38 days
50 days.
Outbound and return transit approximately 100 days.
    With Type XXI the following values are calculated:
Lorient to 20°W
720 nm
(see I.1) with day's run
140 nm
5 days
20°W to Panama
3800 nm
        with day's run
23 days
(see I.2) with day's run Total
28 days.
Outbound and return transit approximately 56 days.
    Meaning:  Type XXI transits nearly twice as fast as Type IX-C.
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  Here it should be noted:
          In stormy weather and strong headwinds substantial shift of the transit submerged, that is to say less day's run loss than with a surfaced boat.
  Sufficient fuel inventory allows higher transit speed on the surface at night in the open sea, not possible up to now with Type IX-C due to low fuel inventory.
  In the open sea areas with little air surveillance boat can draw down its battery capacity below 40%, therefore run at higher submerged speed levels than silent speed.
  An average of about 160 to 170 nm day's run is always likely to be possible in the open sea in transit (to operation areas within 6000 nm).
3.) Long occupation duration in operations area:
    (full utilization of torpedo combat power).
          Shortening of the transit times means extending the length of stay in the actual operations area, only in which the boat has attack opportunities.  Unlike IX-C there is practically no limiting of the operational duration due to provisions and fuel inventory, full utilization of the existing torpedo combat power (20 torpedoes) is to be expected with some certainty. Increased attack opportunities allow some choice of targets, firing only when there are certain hit possibilities.
4.) Improved living conditions for crew:
          Stress on the crew despite long submerged transit times is lower than in previous combat boats for the following reasons:
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  Accommodation conditions are better.
          Large rooms, more berths, moderate temperatures through air conditioning.  For each patrol duration sufficient provisions outfitting, larger cooling devices.
  Better ventilation conditions during long submerged cruises.
          Larger living spaces with approximately the same number of crew as before, so more air supply.  Air quality improvement through snorkeling ventilation - (according to the latest experiences by pumping air into the boat at 200 mb and pressure equalization through snorkel mast in 1-1 / 2 minutes.) - practically anytime except for U-boat hunts. According to latest snorkel experience long underwater transits are quite possible.
  Lower nervous stress on the crew.
          The stamina of the boat against U-boat hunts is substantially larger than previous types. Starvation is very difficult for the enemy, great depth and higher silent running speed offers significant protection from depth charges. Improved equipment (Fu.M.O., Fu.M.B., Balkon listening device and S-gear) largely protect against surprise. The most nerve-wracking battle for charging times against enemy aircraft has ceased.
5.) Operation of the boat in heavy air surveillance areas is possible:
          Since May 1943 when approaching heavily monitored ports, boats of previous types must spend every minute of charging time at greater distances surreptitiously, in some cases no longer succeed in gaining getting close to the ports. However, once a boat arrived off the harbor, after being noticed one time, (for example after attack), a prolonged persecution took place. Continuous detection prevented carrying out a second attack, the boat was forced to move off or be destroyed. In contrast, it should be noted:
          The large battery capacity of the Type XXI (even without snorkel) allows underwater forays against ports from greater range. With snorkel - (result of the experience from the Channel during the invasion time) - the boat can stay for extended periods of time despite the strongest air surveillance.  If detected by air, it can by vigorously change location submerged to evade detection by anti-submarine forces that are summoned. Large location changes at silent running pose considerable difficulties for a further U-boat hunt by aircraft.
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          Principle finding: The Type XXI can arrive off traffic focal points with its large submerged cruising range, stay a longer time with snorkel and escape prosecution by large location changes submerged.
6.) High success opportunities by high submerged speed:
          The principle increased chance for success of a boat with a high underwater speed against all targets comes into play especially in the southern operations areas. Observations by boats in the spring of 1944 indicate that much traffic (usually by day) was spotted or heard on individual routes and off individual ports, however, due to low submerged speed, the boats did not reach attack position. Surfacing and tracking at night due to heavy air monitoring was not possible; boats were forced to submerge and were sometimes overrun by ships without being able to attack.
        The Type XXI will result in a fundamental reversal.  Targets heard or sighted can be reached in most cases by day or by night in submerged attack.  Especially with favorable listening conditions such as in the Caribbean - where fast running tankers were heard perfectly by individual boats at distances up to 30 nm - approaching with the Type XXI is no problem. Breaking through the screen of convoys is easier than in the North Atlantic because of the unpracticed U-boat hunting forces, so here too there are particularly good attack prospects.
  In summary:
    Short deployment times, good living conditions, adequate fuel and provisions inventory allow full utilization of the strong torpedo combat power.  Large underwater cruising range with snorkel operation allows operations off strongly air monitored traffic areas, high submerged
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    speed, periscope-less shots with S-gear allow full utilization of the offered attack opportunities.
The Type XXI is the ideal combat boat.
V.  The boat in attack.
  a) The Type XXI is not a diving boat but rather a submerged boat.  Its strength lies in its relatively high submerged speed and submerged endurance. Once it is on the surface, it resembles the previous combat boats with their weak surface combat capabilities. It is therefore questionable whether the boat can reach attack position operating on the surface after sighting or locating the enemy.  Therefore:
    If a detected enemy can be reached, even with ruthless exploitation of the battery capacity, in one submerged approach, reach it, carry out attack in one approach and force if necessary. Do not rely on the fact that the boat can achieve a new attack position operating on the surface. If the shot is taken with a battery capacity level of just 20% of total capacity, it is still sufficient to withstand a long depth charge pursuit.
  b) Tactical goal of the attack approach by day or night:  achieving a technically favorable shooting position.  Since you do not know if you can achieve it again, you must hit on the first attack.
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    Despite long range torpedoes, despite Lut get as close as possible. The shorter the shooting range the more Lut-loops, the fewer the avoidance maneuver possibilities of the enemy.  Anticipate that he is equipped with torpedo warning devices.
  c) Balkon listening gear, S-gear are technical aides to detect the enemy from greater depth and reach attack position.  However the best overview is provided by your eye.
    If you can carry out the attack at night as a surface attack, or during the day as a periscope attack, try this first. You can always still proceed with the attack with the "technical" eye . The capabilities of the type XXI enable you.
A.  Day attack:
    Attack on independents is not discussed.  It is only noted that prospect of success of the Type XXI when attacking independents is increased many times over.
1) Unnoticed attack from beyond the close escort. "External attack".
    Shooting position 3-5 nm from the heart of the convoy.  Despite large running distance shoot only from forward position, up to at most 90°.  Strive to shoot from target angle 0°, because based on range this is the best utilization of the Lut torpedo.
    Conduct of the attack:
    Sneak through the outer screen unnoticed, favorable listening speed, silent running.  Attack as periscope shot or shot with S-gear.  (The later is not always possible, because probably in many cases range is beyond S-gear range).
    Disadvantage of the far shot:
    Possibility that escort vessels will hear, avoidance maneuver, zigzag of the convoy.
    Conduct after the attack:
    Reload immediately, afterwards shoot a second fan.  If this is not possible from an external position, proceed as follows:
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  If noticed:
    a) If battery capacity allows, close the convoy at high speed, run under the escort vessels, "forced" attack by S-gear from the flank or beneath the convoy, run along with [the convoy], reload, shoot again.  Further see under Paragraph 3.).
    b) If battery capacity does not allow running along at high speed, go to great depth, hold at silent running under the convoy.  Only possible if the first attack is from a sharp target angle and low convoy speed up to about 9 knots. (However, will be possible in many cases).
    c) If battery capacity is low and underruning the convoy is not possible, run at silent running at great depth parallel to and in the direction of the convoy to lose as little ground as possible.  Conduct with depth charge pursuit see under VI.
  Assessment of the attack shot outside the screen:
  Relatively low hit prospects because
    a) Shooting data is not exact.  Due to too great range use of S-gear is not always possible, therefore target angle and especially range is by estimation.  As a result the position of Lut loops is inexact.
    b) Due to great range too few Lut loops.
      (torpedo running distance at most 9000 meters).
    c) Energetic avoidance maneuvers are especially effective. (because Lut loops do not come into effect).
  Hit prospects will be greater
      as soon as torpedo running distance is 12000 meters there will be a substantial increase in hit probability, because of increased Lut loop effectiveness, avoidance maneuvers of the enemy are also less effective.
      as soon as there is higher torpedo running speed (40 knots) decreased avoidance maneuvers of the enemy because of short running period.
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2) Unnoticed attack from within the screen.
    Only attack from forward position is possible, because breaking through the outer screen at great depth or at periscope depth must be completed at silent running.
      Remaining unnoticed is the most important requirement.
    a) Conduct of the attack at periscope depth.
              "Unnoticed periscope attack".
      The same conditions as before with submerged attack on the convoy.  The boat can be detected by S-gear because it is larger than previous combat boats, but probably not as well by listening gear, (going faster with motors above silent running 5 knots is not required).  Chances appear to be only reduced insignificantly compared with previous combat boats, if it is possible to drive the boat well at periscope depth, (this is expected as soon as the 9 meter periscope is fitted with cross member)  Prospects of breaking through the inner screen, depend on
              Sea state,
              listening and locating [Asdic] conditions,
              disposition of escorting warships,
              alertness of the escorting warships.
      Carrying out an attack at great or medium depth, thus going below the close screen and then going to periscope depth is difficult since the boat is blind. Its own S-gear offers little information at this stage.
    b) Conduct of the attack with S-gear from medium depth.
              "Unnoticed periscope-less attack".
      Procedure:  First periscope observation to get an overview of the situation. Starting at 50 meters or greater depths (if possible note favorable locating depth ahead of time), first by listening device, limited use of own S-gear.  Shot from depths between 30-50 meters when technically good shooting position is reached, therefore also from directly ahead.
      Consideration with S-gear:  Use the lowest possible number and volume of the transmitted pulses in order not to betray boat. Within the screen restriction is not
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      necessary because there is already strong noise.
    c) Conduct after attack:
      Reload immediately, second attack.  If noticed, behavior as described under Paragraph 1) a-c).
    d) Comparison of S-gear attack from medium depth with periscope attack.
      Disadvantages:  Poor overview during approach, uncertainty, if detected by the enemy.
      Advantages:  The boat is difficult to detect without periscope exposure, no ramming danger, therefore conduct of the attack with greater calm.
    e) Comparison of Type XXI with previous combat boats
      Advantages: Fast "listening safe" position changes at silent running 5 knots, relatively high turning speed. With detection by the enemy quickly reaching safe depth charge depth.  S-gear allows deep shot, so blind attack, no ramming danger, etc.
        In heavy, poor visibility weather, in which, in the recent U-boat campaign many attacks by day and from favorable positions had to be foregone, attack is quite possible. Energetic defense is severely limited, therefore there are good attack prospects in periscope-less attack.
3) Submerged attack as noticed attack.
    "Forced attack".
  Only carry out, if battery capacity allows forcing enemy escorts with high speed settings (12-15 knots).
  Strive to pass unnoticed under or around the outer screen, (so not noticed too early in the area between the outer and inner screen - therefore on a very long trek - after being noticed by the enemy combat takes place), which leads to the following disadvantages:
      depth charge pursuit.
      forced high speed settings (consumption of battery capacity),
      avoidance maneuver of the convoy on U-boat detection, possibly no further approach possibilities, because the convoy
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      can get beyond own "submerged range" by zigzagging away.
  With "as little high" speed as necessary - enough to get in shooting position - run as close as possible to the close escort.  Choice of speed settings is of great importance, according to the assessment of the listening conditions by the Commander.
      Determining own sound levels at various ranges.
      Required speed settings.
      Establishment of a noise table knot by knot, noise strength in decibels, indicating how in low, medium, good and excellent listening conditions ranges are magnified.
      Establishment of noise table for each individual boat is necessary.
  As soon as noticed, break through the screen at high speed, without regard to pursuit and depth charge attack during the run in.  Conduct, if destroyer is found to be steaming in the wake, see under VI.
  If possible, shoot while running in, if this is not possible, under the convoy, attack from below.  Further behavior as in Paragraph 4).
  Attack from beneath the convoy.
    "Shark attack"
  Attack the convoy under the protection of merchant vessels. Attack position from ahead or behind, shoot from a depth of 30-50 meters.
  Own high submerged speed allow running along at 10 knots convoy speed for a long time, therefore several attacks are possible (reloading after first attack takes 5 minutes, after second attack 15-20 minutes).
  Danger to the boat from sinking ships is seen as slight, if boat proceeds at about depth A [80 meters].  This depth is passed by a sinking ship in a few seconds.
    Conduct after attack:
    After the last successful attack still run along for a few miles, go to great depth, silent running.  Then turn off to the rear to the side.  Steer away from the stern sweeper by own S-gear and sound bearings.  Detection possibility by the stern sweeper is slight, because of the large passing velocity,
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    after own successful attack the sweeper will be engaged with rescue of the shipwrecked.  Surface as soon as possible, send the report.
B.  Night attack.
1) Surface attack:
  Assuming that the boat is nearby and ahead of the convoy stop intermittently, around listening with balkon group, possibly use S-gear.  Exploit the listening capability of the S-gear. Take bearings on received detections, if necessary use Hohentwiel [radar].  Exploit bearings of the convoy voice frequency - the use of which the enemy can not do without.
          If multiple detections occur, enter on chart.  Drawing conclusions as to the positioning of the screen, possibly even on the position of the convoy are possible.  Use Aphrodite.
  Benefits of the Type XXI compared with previous combat boats.
    All around listening is possible on the surface.
    Determining the direction of locating with the new "Fliege".
    Determining the type of the locating by subsequent Hohentwiel use.
    More certainty of the overall situation, therefore freer behavior, no time-consuming diving, no defensive attitude.
  If attack on the surface in the old manor is possible, perform.  If the boat is approached by a destroyer, gain shooting data with Hohentwiel for T 5 torpedo.  Shoot T 5, dive if necessary.  (Improved blocking path of the T 5 allows longer stay on the surface).  If the boat can move off on the surface after the attack, try this.  If detected and in any case if prosecution is expected, dive, run under convoy and attack again.
2) Submerged night attack:
  If the boat is detected while running in on the surface, or will be certainly be hindered by the screen in the implementation of the surface attack and will be forced to submerge, stay up as long as you possibly can to get a clear picture
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  of the situation. This is of great importance for the implementation of the underwater night attack. Otherwise implementation of the attack as described in A. 3).
  It should be noted:
  The forced night attack is the ideal night attack. It is practically impossible for the close escort to follow a submerged boat, attack it with depth charges and make its course changes at night due to low visibility and mutual ramming danger.  Under the convoy the boat is as safe as in a U-boat bunker.
VI.  Type XXI during depth charge pursuit.
        While assessing the enemy's defenses do not give enemy too much credit, cool deliberation seems necessary. Remember that the U-boat war with the previous combat boats - which were not submarines, but rather diving boats which generally operated on the surface, only occasionally attacking submerged - has failed.
Their characteristics:     [previous types]
    On the surface against enemy air too weak, submerged due to their low speed comparable to a mine.
Dangerous as long as unnoticed, yet harmless, easy to handle and eliminate if discovered.
    Submerged the type XXI is not a stationary mine, instead a deep going submerged boat.
1.) How have losses by depth charge pursuit occurred thus far?
          The greater part of the losses of previous types of boats took place during depth charge pursuit for following reasons: (fully confirmed from statements of U-boat officers who had returned from captivity).
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  a) Boat at great depth, silent running.  Long duration pursuit, recurring depth charge series.  From pressure hull penetration incoming water accumulates and reaches such levels that pumping is no longer possible with the unsuitable for great depths bilge pump and air. Boat gradually gets heavier and is lightly blown.  The boat floats on an air bubble.  It must be blown from time to time, consuming compressed air.  The free moving water mass in the boat causes severe trim changes which can no longer be controlled at silent running and and higher speed levels.  Due to large trim angles free flooding water leads to malfunctions in the electrical systems, in severe cases, to E-motor failure. Chlorine gas arises from penetration of water into the battery . Further compressed air and battery consumption. The Commander must decide to surface because in the future there will no longer be sufficient compressed air reserve to surface.
    A small part of the losses occur as follows:
  b) Boat at great depth, silent running.  Very long pursuit until battery and air are exhausted, boat forced to the surface.  (Starvation)
  Only in a few cases does the loss occur as follows:
  c) Through depth charge effects at great depth pressure hull shattered, or such strong leaks and failures of technical systems that the boat is immediately destroyed or forced to surface immediately.
  d) After prolonged depth charge pursuit there is calm at the surface. Commander believes that the U-boat hunter has departed. When going to periscope depth suddenly overrun by the previously lying stopped U-boat hunter, severe depth charges at shallow depth with most serious failures, boat must surface.
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2.) Comparison of the resistance of the Type XXI to previous types.
  Advantages: seeking greater depths is possible. (Type XXI was built for T = 135 meters with a 2-1/2 fold safety factor. Existing types for T = 100 meters with a 2-1/2 fold safety factor.
  High silent running speed 5 knots compared with 2 knots with VII-C.
  High submerged speed complicates targeted depth charge throwing considerably, great depth and higher creep speed alone are excellent protection.
  High underwater speed allows strong trim control and weight changes (For Type XXI also relatively - in proportion to tonnage - higher values than in type VII-C), thus blowing is required later, saving compressed air.
  Large battery capacity makes exhaustion practically impossible, 20% of battery capacity still enables long silent running with speed settings lower than 5 knots.
  The boat has two deep drain pumps, enabling pumping greater amounts of water at greater depth.  Installation of these pumps saves much tedious trimming of the water from the boat ends to the center (by makeshift means, bailers, fire hose). 
  Very good listening gear and S-gear allows better monitoring of the behavior of U-boat hunters, surprise (see paragraph 1 d) is no longer possible.
  Disadvantage:  Boat is larger than previous combat boats, therefore easier to detect.
  Boat is more complicated, has more and larger pressure hull openings, therefore larger leaks with depth charge pursuit at greater depth.
  Comparison shows:  Type XXI is, despite the given disadvantages with respect to depth charge pursuit, considerably more reliable than all previous types.
3.) Conduct with depth charge pursuit.
  a) Passive defense:
    Seek great depth, about 3 A meters, therefore a reserve still remains below.  Secondary objective: after determination of this depth the enemy will certainly carry out months-long new setting and re-equipping of his depth charges.
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    As soon as he appears with deeper set depth charges, increase own depth and put him back before a new problem.
            Exploit the biggest advantage of the boat, high creep speed, and proceed in a general course direction with easy zigzags, gain space. (Asdic range at most 5000 meters)
            If higher speed must be run temporarily, - i.e. offering good listening opportunities to the opponent - make strong evasive maneuvers.  New change of course at the moment when switching over from high speed again to silent running.
            If pursuers can not be shaken at silent running, make extensive use of all means to hinder the enemies precise localization and targeted depth charge throwing, therefore carry out speed, course, depth changes.  Eject Bolde.
            (While still training in the homeland Commanders must employ preliminary considerations for appropriate behavior with depth charge pursuit, and be taught by training establishments about it. Performing extensive tests with active U-boat hunting formations and the U-boat Defense School is essential).
  b) Active defense:
            Improved listening and locating devices provide a good overview of the situation on the surface, based on results from these devices an experienced commander can succeed by taking clever advantage of the excellent opportunities for fast speed, depth and course changes and by deception with Bolde to make enemies "malleable" or to confuse, to bring themselves to a favorable attack position for the Zaunkönig [T V torpedo] and thereby break the rule of action itself.  The times of impotent "sitting in the basement" are over for these commanders.
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VII,  Communications outfitting and operational possibilities for Type XXI U-boats.
A. Equipment outfitting:
  1  Short wave [SW] transmitter 13 - 100 meters  T 200 FK 39
  1  Stand by transmitter 40 Watt 18 - 100 meters  Lo 40 k 39c
  1  Very high frequency [VHF] transmit/receiving gear  Lo 10 UK 39
  1  "Kurier" - transmitter KZ I/44
  1  Short wave receiver E 52 b "Köln"
  1  Short wave receiver E 52 b "Köln" with medium frequency [MF] bearing device "Preßkohle" [Preßkohle = Briquette - an add on to provide DF capability]
  1  All wave receiver  E 381 S
  1  Radione  R 2
  1  Radione R 3
  1  Bearing receiver  T 3 PLLÄ 38 with 500 millimeter directional loop 1000 N XV
  1  Hydraulic extensible rod antenna (Ölasta) for operation at periscope depth (IFS)
  1  Hand driven extensible rod antenna (Handasta) for operation "surfaced" (IFS)
  1  Emergency antenna (shipyard install)
  1  Emergency transmitter
  1  Transmitter converter
  1  Under Water system hull mounted sonar Elac
B. Communication links of surfaced boats:
  1.) Short wave
    Receiving:  In the surfaced condition the boat has 3 rod antennas, all 3 of which can be used as receive and transmit antennas
    The receiving capabilities of the boat are, considering the use of strong stations on land, the same as in previous boat types. No limitations!
    Direction of the surfaced boat from land via shortwave is possible at any time in all sea areas!
    The rod antenna is probably a step back in terms of their performance from the net deflector antennas. (Accurate measurements for comparison with previous antennas are underway)
    Reports from submerged boats may not be possible at all times of day and in all sea areas. Use of remote radio repeaters is necessary.
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    The links of the boats of a group with each other will remain on the operational circuit without significant limitation the same as before. Maximum distance of the boats from one another not greater than 100 nm.
    Transmitting with Kurier.
    Kurier signals can only be received in the homeland!  Because the rod antenna is probably not as effective as the previous net deflector antenna the same limitations apply for receiving Kurier signals as for hand telegraphy.
    Kurier signals are not receivable at all time and from all sea areas in the homeland.  Therefore silent reports of the boats cannot be ensured in all cases!
    Other boats only receive Kurier signals via the land station by re-encrypting.  With the appropriate organization on land repeat can be expected on very low frequency and short wave with a time delay of 10 minutes.
    Bearings:  The boat cannot take bearings on short wave.
  2.) Low frequency [LF], very low frequency [VLF].
    Receiving:  With bearing direction finder without limitations certain in all sea areas.
    Transmitting:  No low and very long wave transmitting capability.
    Bearings:  Low and very low frequency without limitations, however not if taking bearings on medium frequency.
  3.) Medium frequency.
    Receiving:  Without limitations as before.
    Transmitting:  No transmitting capability.
    Bearings:  Without limitations as before.  However not when taking bearings on very low frequency, then only receiving.
  4.) Very high frequency [VHF].
    Receiving and transmitting:
    VHF links only for boats between themselves (only up to 15 nm!) primarily for U-boat-aircraft traffic.  No limitation compared with previous ranges.  Range 200 km.
    Bearings:  Bearings by the aircraft on the boat are not possible!
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C. Communications links of "snorkeling" boats.
  1.) Short wave.
    For transmitting and receiving there is only one antenna (Ölasta) available.  Additional reception possibilities via one wire of the snorkel runddipol.  (reception quality over the runddipol is still in testing).
    The boat can send and receive (limitedly) at the same time while snorkeling!
    The snorkeling boat can be directed from land via short wave without limitations from the area and in daytime!
    Transmitting with Kurier.
    Transmitting Kurier signals is the same as under B. 1.
    The "snorkeling boat" can  report silently with Kurier, however under area and time limits due to (probably) lower power capability of the Asta!
    Short wave reception was only interrupted for a few seconds by Kurier transmission.
  2.) Low and very low frequency.
    Low frequency:  Limited receiving via the Ölasta.  No bearing capability.
    Very low frequency:  Receiving only with the extended direction finding loop thereby speed limitation!
    The boat is capable of receiving and transmitting on short wave while snorkeling.  Therefore utilizing very low frequency only in cases where SW reception is disrupted.  Overall therefore no limitations!  Bearing possibility on very low frequency with sequencing.
    Currently the possibility of mounting a very low frequency antenna on the snorkel head is in testing, then VLF reception would be possible at any time without speed restriction.
  3.) Medium frequency.
    Only reception for monitoring, no bearing possibilities!  If it is possible to mount a direction finding antenna on the snorkel head, then also bearings!
  4.) Very high frequency.
    Receiving and transmitting as when surfaced.  However not at the same time as transmitting on short wave.
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D. Communications links of dived boats.
  1.) Only very low frequency reception and bearings without sequencing, however are possibilities are not yet fully understood.  Experiments are ongoing.  Maximum diving depth for reception 20-22 meters with strongest very low frequency transmitters and good receiving conditions.
    Diving depths are reduced with increasing distance!
    In the North Atlantic diving depths of about 18 to 20 meters are anticipated!
    Speed reduction during very low frequency reception!
  2.) Under water telegraphy.
    Operational possibilities are slight because the enemy locates immediately.  Range under favorable conditions 30 nm.



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