GORDON WILLIAMSON | ILLUSTRATED BY IAN PALMER |
Consultant editor Martin Windrow |
INTRODUCTION
Basic chronology of the U-boat war in the Atlantic
CONVOY NIGHT SURFACE ATTACKS
The Kretschmer method, 1940–41 The official doctrine, 1943
DECK-GUN ATTACKS
The theory: rules for interception of merchant ships The practice
The official doctrine
SOLO MISSIONS
U-47 at Scapa Flow The Mediterranean: U-81 and U-331
North American waters, 1942
Far Eastern waters, 1943–44
‘WOLF-PACK’ ATTACKS
‘Free hunting’ Patrol/reporting lines
Fast patrol lines
The turn of the tide
KRIEGSMARINE-LUFTWAFFE CO-OPERATION
The Fw 200C Condor Combined air-sea strikes
ASSOCIATED EQUIPMENT
Bachstelze (‘Wagtail’) Midget submarines
OFFENSIVE WEAPONS
Torpedoes Torpedo guidance systems
Mines
DEFENSIVE EQUIPMENT
Radar decoys Sonar decoys
Protective coatings
DEFENSIVE TACTICS
Against warships Against aircraft
SELECT BIBLIOGRAPHY
From the small, early Type II coastal submarines, through to the handful of advanced Type XXI and Type XXIII boats that got to sea in the final months, German U-boat design and production was forced into continual development and improvement to keep pace with wartime needs. Paramount among these was the challenge of having to face increasingly effective Allied anti-submarine warfare (ASW) efforts, and – like the submarines themselves – tactics for their effective employment needed continuous analysis and adjustment.
Submarines were employed in a variety of roles, from coastal patrols, through individual opportunist actions by single unsupported U-boats – both in the Atlantic and in more distant waters – to co-ordinated ‘wolf-pack’ ambushes far out in the North Atlantic. Numerous tactics were developed, some more successful than others, in attempts to help submarine commanders achieve combat success, and the Oberkommando der Kriegsmarine (OKM – Navy High Command) went as far as producing a U-Boat Commander’s Handbook to disseminate the practical lessons learned by the most successful captains.
Grossadmiral Karl Dönitz, Commander Submarines and later C-in-C Navy, whose own younger son was lost in U-954 in May 1943. The care shown by Dönitz for his crews’ welfare and the interest he took in the development of tactics stemmed from his extensive combat experience as a U-boat officer in World War I. (Deutsches U-Boot Museum)
We now know, of course – as they did not – that from around August 1941 the U-Boat Arm was hugely handicapped by the British cracking of the Kriegsmarine’s ‘Enigma’-encrypted radio traffic between boats at sea and the Befehlshaber der Unterseeboote (Commander, Submarines – the headquarters staff of Adm Karl Dönitz). The Bletchley Park centre needed continual radio intercepts and up-dated decryptions to supply the consequent ‘Ultra’ intelligence, but this often allowed the Allies to route convoys to frustrate German interception, and even to deploy assets to lie in wait at designated rendezvous between far-ranging U-boats and their resupply ships. Nevertheless, on the occasions when the Allies failed to intercept or decrypt message traffic, and the Germans applied the requisite tactics effectively, the U-boats were capable of inflicting devastating damage on Allied shipping, particularly in the vital North Atlantic sealanes upon which the war effort in the European theatre depended.
Kapitänleutnant Kurt Diggins, commanding the Type VIIC boat U-458, is shown at the navigation or ‘sky’ periscope in the relatively spacious control room. This larger of the two periscopes was used predominantly for scanning the horizon and sky for enemy ships and aircraft, and also to take bearings. KL Diggins survived the sinking of his boat in August 1943, and outlived the war. (Deutsches U-Boot Museum)
Such attacks were pioneered by the most successful of the ‘ace’ commanders, Korvettenkapitän Otto Kretschmer, who discovered early in the war that attacks on convoys were easier when made on the surface under the cover of darkness.
With the boat trimmed low in the water, the small conning tower was very difficult for any lookout on a merchantman or warship to spot. A further benefit of attacking on the surface was that the U-boat, powered by its diesel engines, could move much faster and further than it could underwater when reliant on electrical power. (For instance, the Type VII boat had a top surface speed and range of c.17 knots and 8,100 nautical miles, but the submerged figures were 7.3 knots, and only 69 miles before recharging the batteries.) Its surface speed would certainly be faster than that of the merchant convoys, and of some of the smaller escorts protecting them. Additionally, the escorts’ ‘Asdic’ (sonar) equipment – for acoustic underwater location and ranging – could not detect a submarine running on the surface.
The U-boat would approach the convoy submerged, and, ideally, gain a position on its beam and slightly ahead. Having identified a gap in the escort screen, it would surface and ‘sprint’ through the perimeter screen and into the heart of the convoy formation. An ideal attack position was at right angles to the overlapping parallel columns of merchant ships, which would thus present the widest possible target area. The real prizes for the U-boats were the large oil tankers and munitions ships, which would be placed at the centre of the convoy to give them the greatest protection. This precaution was to some extent effective against attacks by submerged submarines – which would struggle to penetrate deeply into the convoy without being detected by the escorts’ Asdic – but not against surface attack.
At an ideal distance of somewhere between 400m and 1,000m, torpedoes would be launched (the torpedo would not ‘arm’ until it had run about 300 metres). It was normal practice to launch a full salvo of torpedoes at several targets rather than selecting a single ship for attack, for the simple reason that once a torpedo had detonated and the convoy had become aware of the U-boat’s presence further attacks became more difficult and dangerous. The U-boat would generally launch its first torpedo, with the longest running time, against the furthest target: in theory this allowed the time to aim and fire at subsequent targets in a sequence that produced almost simultaneous strikes by all the torpedoes. (For instance, on 23/24 August 1940 KL Erich Topp’s little Type IIC boat U-57 sank three freighters with a single fast salvo from its three bow tubes.)
Taking advantage of the confusion resulting from a successful attack, the U-boat would slip out of the convoy, still on the surface, while the escorts sped off to hunt what they probably thought was a submerged boat at a much greater distance. If the boat successfully evaded detection during its withdrawal, it would reload its torpedo tubes and prepare for further action. If it was detected by escorts, then – depending on the type of warship – it might try to put some distance between it and the pursuer while still motoring on the surface, building up forward momentum in order to shorten the time taken to crash-dive (normally at least 25–30 seconds). At the forefront of the commander’s mind, however, would be the knowledge that if the warship scored a hit on the submarine with its main armament then damage to the pressure hull might render it incapable of diving, in which case it was certainly doomed.