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	CHAPTER XI
	U-BOAT STRATEGY AND TACTICS
	(Note.  This is a summary of evidence to date.  As a result of our increased and more efficient sea and air offensive and also of the enemy's improved weapons, U-boat tactics are continually changing.
	It is emphasised that this chapter is mainly a short historical survey of the U-Boat offensive up to the autumn of 1943.  Recently the enemy has seldom been able to develop the tactics described below, although they are probably still applicable.  Paragraph (xii) gives the latest trend of the U-boat offensive up to March, 1944.)
	(i)  Types of Operation.
	U-boats operating in the Atlantic may theoretically be ordered to operate in one of the following ways.  (They are rarely followed exactly in practice.)
	(a)  "Einzelstellung" (Single U-boat).  A single U-boat reporting shipping movements and weather and not attacking unless ordered to do so.
	(b)  "Freijagd" (Independently-operating boat).  A U-boat operating at the commanding officer's discretion in a given area.  This has been known in the St. Lawrence and off the South American coast.  In such cases, signaling is reduced to a minimum.
	(c)  "Vorpostenstreifen" (Patrol line.)  U-boats up to perhaps 25 in number, spread about 20 miles apart on a given line of bearing across the convoy routes, patrol at right angles to the line, not going farther then 10 miles or so from it.  The sighting of the convoy is reported to control, which passes the signal to other U-boats in the line.  An independent ship can be pursued without referring to control.
	(d)  "Aufklaerungsstreifen" (Reconnaissance Sweep).  U-boats are withdrawn from the patrol line by control, which signal the limits of the area to be swept, the time of the beginning of the sweep, and all the units to take part.  The boundaries of the area to be swept may be 150 miles apart, the U-boats being spread about 20 miles apart.
	(e)  "Angriffsaufstellung" (Attack Formation).  When U-boats are in contact with a convoy but have not yet made the initial attack, commanding officers decide what formation to take up:  usually a semi-circle round the line of approach of the convoy.
	(f)  "Lauerstellung" (Lying in Wait).  A U-boat proceeds to a given position where shipping is likely to be encountered and lies in wait for it.
	(g)  Special Missions.  U-boats are sometimes sent on special operations such as landing saboteurs and spies, particularly in America, and minelaying expeditions.
	(h)  Groups.  U-boat operating together or in neighbouring areas are usually formed into a Gruppe (Group) which is given a code name.  Such groups are not necessarily identical with any of the formations above which are sometimes formed, on orders from Control, from selected boats belonging to different groups operating in adjacent areas.
	(i)  Unattached U-boats.  U-boats not attached to one of the particular formations above are normally allotted an operational area by grid-square references in which they wait for orders.
	(ii)  Attacks on Convoys.
	(a)  Detection.  A convoy may be detected either by U-boat or aircraft, and there is evidence that information on movements of convoys is obtained by agents.  The commonest means of detection is by I-boats in "Vorpostenstreifen" (see (i) (c) above).  U-boats may detect targets visually, by H.E. or rarely by Radar.
	(b)  Procedure on Detection.  The U-boat first sighting a convoy become contact-keeper and immediately reports her position and all detail of the convoy to Control.  The average range at which a convoy is sighted is 10 to 15 miles.
	Control then orders specified boats in the area to take up their position in the "Angriffsaufstellung" (see (i) (e) above).  Most, thought not necessarily all, of these boats come from the same "Vorpostenstreifen" as the contact-keeper.
	On gaining visual or hydrophone contact with the convoy, each boat makes a short H/F signal to Control.
	(c)  Duties of Contact-keepers.  (1)  To shadow and report the convoy continuously.  This is generally done by keeping just inside visibility distance, but contact-keepers are often forced to submerge by aircraft and ships following up HF/DF bearings, and thus lose touch.
	(2)  To report her position approximately once an hour.
	(3)  In bad visibility, to transmit D/F signals on M/F according to a predetermined routine.
	(4)  She may not attack until the general order is given.
	(d)  The Attack.  (1)  Signal to Commence.  When Control considers that sufficient boats have assembled, the order to attack is given.  All U-boats aim to attack simultaneously in order to create the maximum confusion but seldom achieve this ideal.
	(2)  Independence of Action.  Commanding officers choose their own targets and positions; there is no such thing as co-ordinated action between U-boats once the battle has begun.

 

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	(ii)  Attacking Position in General.
	(a)  Since the war began, U-boat attack tactics have passed through three phases -
	(1)  Submerged attack at medium range.  In the early days the enemy under-estimated the efficiency of the Asdic.  He was soon forced to abandon this method when attacking well-escorted convoys.
	(2)  Night Attack - surfaced.  This was most successful particularly in 1941 when escorts were weak, but has been found costly in the presence of sufficient Radar fitted A/S vessels.  It is still effective against stragglers and against convoys when the defence is saturated.
	(3)  Reversion to submerged attack by day after gaining position on the surface.  The latest method favoured.  U-boats possibly prefer Asdic to Radar opposition.
	(b)  Other things being equal, the following are the factors which effect the selection of the direction of attack:
	(1)  In bad weather, attack down wind in order to close and get away quickly and to prevent spray obscuring the view.
	(2)  When submerged, attack down sun, to prevent periscope shine.
	(3)  Dawn and dusk are favorite attack periods, as they provide the sharpest silhouettes against a light horizon.
	(iv)  The abandonment in the Summer of 1944 of attacks on Convoys by packs of U-boats.
	Air cover in the Atlantic has improved considerably and U-boats have consequently found it very costly to attempt approaching as a pack on the surface whether by day or night.  The increase in available surface escorts has made it possible to hamper the U-boats' efforts further by sending hunting-groups well away from the convoy.  It must be remembered that a submerged U-boat's speed and endurance are such that a submerged pursuit of even a slow convoy is out of the question.
	(v)  Attacks on Distant Areas.
	The U-boats have pushed further and further afield as air and surface protection of shipping has increased.  They will always tend to seek out the areas which can give them the best results in merchant ships sunk against U-boat losses.
	(vi)  Intercommunication during Attacks.
	U-boats rarely communicate directly with one another by W/T, the only occasion being when communication with Control breaks down.  Otherwise, U-boats only know what others in their group are doing during an attack by listening in on W/T service in use for the area of the attack.
	(vii)  Tactics Against Surface Craft.
	Tactics adopted are governed by the experience and temperament of individual commanding officers, weather and light conditions, composition of attacking forces and, even more, by the submerged endurance of the U-boat at the time of the attack.  The following short notes therefore can necessarily only give a pointer to the forms of evasion likely to be taken.
	(a)  When attacked in daylight.  U-boats when attacked in daylight remain submerged at about 1 or 2 knots, make frequent alterations of course and eject S.B.T. pills as necessary.  The depth varies considerably with different commanding officers; the general policy used to be to lie at about 150 metres (492 feet), but it is now known that depth charges can be set to greater depths and some U-boats will go much deeper.  No U-boat will, however, remain at constant depth when it knows that it is being hunted.  Unless forced to the surface, U-boats usually do not surface until all indications of the presence of hunting craft have ceased for some time.
	(b)  When attacked at Night.  Night attacks on U-boats are in the nature of things generally made in the vicinity of convoys.  Whether the U-boat is attempting to attack on the surface or has been detected submerged the tactics may well be to surface as soon as possible and escape at high speed.
	(c)  Asdics.  Some of the limitations of the asdic are known to the enemy.  It is realised that contact must necessarily be lost when running in to attack and sometimes as soon as the attacking vessel is committed U-boats increase to full speed and work their pumps if required.  Many commanding officers endeavour to keep their boat bow or stern on to the attacking vessel in order to give the narrowest extent of target.
	The ping of transmission can be heard by hydrophones in a U-boat at greater range than that at which it is first possible to get an echo.  When the A/S vessel is at close range the transmission can sometimes be heard aurally.
	(d)  U-boats sometimes deliberately allow themselves to be sighted while other boats in the vicinity get away or attack without interference.
	(e)  Experiments have been reported (C3) of a mine field designed to be discharges by submerged U-boats against attacking vessels, also some form of decoy towed about 800 yards astern (C.O.).
	(vii)  Tactics against Aircraft.
	(a)  U-boat have for some time regarded aircraft as their worst enemy, although earlier in the ear, when there were many fewer, they were not considered very seriously.  Much increased A/A armament in the summer of 1943 did not, however, have the desired effect largely because the persistence of pilots in pressing home an attack under heavy fire was under-estimated.

 

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	(b)  Previous Tactics.  Until 1943, although some U-boats occasionally chose to fight it out on the surface, they normally submerged immediately on detecting aircraft without attempting even to identify them.  A normal depth to which U-boats would go in such circumstances was about 120 feet and the wheel was usually put hard over on diving.
	(c)  Present Tactics.  In the spring of 1943, the greatly increased A/A armament then being fitted, was considered sufficient to warrant a U-boat remaining on the surface whenever aircraft were encountered by day.  There are now signs, however, that a U-boat will only accept battle with an aircraft if surprised and if her task (e.g., keeping in touch with the convoy) necessitates surface speed.
	(d)  Use of Armament.  When surfaced in areas patrolled by enemy aircraft, whether by day or night, U-boats always have sufficient men on the bridge to man A/A guns immediately, while the full gun's crews come up from below.
	Fire is generally held until the aircraft is within about 1,000 yards.  Although a number of aircraft have been shot down, the percentage lost to attacks carried out on surfaced U-boats is very small, possibly because of the short time within which a fast target can be fired at.
	Cases have been known when a U-boat purposefully dived, leaving the gunners on deck, and the presence of survivors in the water cannot therefore always be taken as evidence of the loss of the U-boat.
	Reports have been received of P.A.C.'s and rocket projectiles being fitted, but so far only standard armament has been encountered.
	(e)  Detection.  Until the middle of 1942, U-boats relied entirely on the look-outs to detect aircraft.  From about August, 1942, onwards however, it became the settled policy to fit all boats with G.S.R. (see Chapter III, Section iv).  Although capable of detecting Radar transmissions from surface-craft as well, it was principally designed to give warning of aircraft in the vicinity.  By the summer of 1943, several U-boats had been fitted with aircraft-warning Radar as well.  (Chapter III, Section iii.)
	(ix)  Deception.
	(a)  The following forms of deception, details of which are in Chapter III, Section v (i) have been used by U-boats during the war.  (Fact).
	(i)  S.B.T.'s have been used successfully to give false Asdic echoes.
	(ii)  R.D.B.'s were introduced in the autumn of 1943 to give false Radar echoes, their purpose being to deceive surface craft and aircraft into pursuing false Radar contacts.
	(iii)  R.D.S.'s are a variant on the principle of R.D.B.'s and were introduced early in 1944 in order to produce a multiplicity of echoes in congested U-boat areas such as the Bay of Biscay.
	(b)  The following devices have also been mentioned:
	(i)  An oil pill ejected through the S.B.T. tube, which leaves an oil trace designed to make the enemy think that the U-boat has been damaged.
	(ii)  A small torpedo also ejected through the S.B.T. tube which creates a dummy H.E. (C3).
	(iii)  The device used in the last war of purposefully allowing wreckage, cap ribbons, etc., to come to the surface has never been substantiated during this war.
	(x)  U-boat and Aircraft co-operation.
	Sometimes patrol lines operate with German aircraft.  When the aircraft sights a convoy it reports to Control.  U-boats then receive instructions either to proceed to a given position or to home on a/c beacon signals.
	Experience has shown aircraft positions to be inaccurate, and U-boats usually home on the aircraft beacon signals, reporting their own position and the bearing of the aircraft to Control, which plots the exact aircraft position (i.e., the convoy) after which further instructions to U-boats follow.
	Schwann and Fühlungshalter Buoys (see Chapter III, Section 5 (xvii) ) may be used to assist homing.
	Aircraft and U-boat co-operation was frequent early in the war, thereafter was not found for a long time, and only recently have attempts again been made to enable U-boats and aircraft to carry out co-ordinated attacks on convoys proceeding close enough to the aircraft bases in France.
	(xi)  U-boats as Escorts.
	U-boats often escort inward and outward-bound blockade-runners.  The screen is generally distant.  They are, too, often used to support other U-boats which have been damaged or are short of fuel.
	(xii)  Trend of U-boat Offensive.
	The heavy losses incurred during 1943 and early 1944 have induced the enemy to withdraw a number of U-boats from operations for re-arming and to fit new devices for defence.  Concentrations in the North Atlantic have been on a reduced scale, but there has been increased activity on the Russian convoy route.  Independent shipping in the Indian Ocean has suffered casualties and the enemy is trying to reinforce his flotillas in the Mediterranean.
	In the summer of 1943, the enemy, frustrated by our improved Radar, particularly during night attacks, introduced the GNAT - a homing torpedo.  This weapon was designed for sinking escorts and thus leaving the convoy undefended.  Though normally fired by a U-boat which has recently been forced to dive they have been used in "unseen" attacks.  A U-boat which has been hunted for many hours and was forced to surface at limit of endurance might well surface and fire a GNAT at his attacker.