Type VII U-boat

When Germany began openly rebuilding its navy in the 1930s, one officer above all understood where its best hope of victory at sea lay. Karl Dönitz, a submariner from the First World War, argued that Britain — an island wholly dependent on seaborne imports — could be brought to her knees not by battleships but by a fleet of submarines severing her Atlantic supply lines. The weapon he chose to do it was the Type VII.

Developed from First World War designs and the clandestine submarine work Germany had pursued abroad during the years of disarmament, the Type VII was a medium, ocean-going boat — not the largest or longest-ranged submarine afloat, but cheap, tough, nimble and quick to build. It struck a balance between seakeeping, armament and economy that made it ideal for mass production, and it became the backbone of the U-boat arm and the most numerous submarine class ever built.

The Type VII was a classic diesel-electric submarine of its era: diesels for running and charging batteries on the surface, electric motors for the slow, silent, short-ranged world beneath. On the surface it could make nearly 18 knots; submerged it crawled at little more than walking pace and could stay down only so long as its batteries and its crew's air lasted. In truth it was less a true submarine than a surface torpedo boat that could submerge to hide and to attack.

Its punch came from five 533mm torpedo tubes — four in the bow, one in the stern — and a stock of around fourteen torpedoes, backed early in the war by an 88mm deck gun for finishing off lone merchantmen. The definitive Type VIIC, built in huge numbers, was a strong, deep-diving boat that could take its crew down to depths that helped them survive depth-charge attacks which would have crushed a weaker hull.

But the design's virtues were also its limits. It was cramped and spartan, with appalling living conditions for its fifty-odd men, and its modest range meant the long Atlantic patrols depended on resupply. Most fatally, the Type VII's whole way of fighting — surfacing to charge and to attack at night — left it terribly exposed once the Allies developed the radar, aircraft and tactics to find boats on the surface.

Dönitz's great tactical innovation was the Rudeltaktik — the "wolfpack." Rather than send boats to hunt alone, he spread them in patrol lines across the convoy routes; the first to sight a convoy would shadow it and radio its position, drawing other boats to converge for a mass night attack on the surface, where the low-slung U-boats were nearly invisible and could outrun the escorts.

Co-ordinated from Dönitz's headquarters ashore by a constant stream of radio messages, the wolfpacks turned the Type VII from a lone raider into a strategic weapon. Through 1940–42 they sank merchant ships faster than the Allies could build them, and for a time it seemed they might genuinely starve Britain into defeat. This reliance on radio, however, would also become their fatal vulnerability.

From the opening days of the war the Type VIIs waged the Battle of the Atlantic, the longest continuous campaign of the conflict. In the period U-boat crews called the "Happy Time," aces such as Otto Kretschmer and Günther Prien ran up huge tonnage scores, and at the campaign's height in early 1943 the wolfpacks came close to overwhelming the convoy system altogether. It was, Churchill admitted, the one thing that truly frightened him during the war.

At the campaign’s peak the arithmetic was terrifying: a handful of boats could find a convoy, gather into a wolfpack and tear it apart over successive nights, and for a time the Allies were losing merchant ships faster than they could build them. Britain’s survival hung on the thin line of the escorts — and, more than she knew, on the codebreakers of Bletchley Park.

Then the tide turned with brutal speed. A combination of long-range aircraft, escort carriers, centimetric radar, better depth charges, code-breaking at Bletchley Park and overwhelming Allied shipbuilding broke the U-boat offensive in the spring of 1943 — "Black May," when losses became unbearable and Dönitz pulled his boats back from the North Atlantic. The hunters had become the hunted.

For the rest of the war the Type VIIs fought on in worsening conditions, sent out on patrols from which ever fewer returned. Of the roughly 40,000 men who served in the U-boat arm, some three-quarters were killed — the highest casualty rate of any branch of any nation's armed forces in the war, a grim measure of how completely the Allies had mastered the threat.

With around 700 built, the Type VII was the most-produced submarine class in history and the instrument of the gravest threat Britain faced in the entire war. For three years its wolfpacks waged a campaign that came genuinely close to severing the Atlantic lifeline on which Britain — and the build-up for the invasion of Europe — depended.

That it ultimately failed, and at so terrible a cost to its own crews, does not diminish its place in history. The Type VII embodied both the deadly promise and the final futility of Germany's war at sea: a cheap, tough, brilliantly handled weapon that for a time held the outcome of the war in its hands, before Allied science, industry and code-breaking sent its grey wolves to the bottom in their hundreds.

• Roger Chesneau (ed.), Conway’s All the World’s Fighting Ships 1922–1946 (Conway Maritime Press).
• V. E. Tarrant, The U-Boat Offensive 1914–1945 (Arms & Armour Press).
• Gordon Williamson, Kriegsmarine U-boats 1939–45 (Osprey, New Vanguard).