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What weapon did Germany use to sink British supply ships without any warning?

During World War II, Germany deployed a highly effective naval weapon against Allied merchant and supply ships – the Unterseeboot, better known as the U-boat. The U-boat was essentially a submarine that could operate largely unseen under the water, surfacing briefly to attack merchant ships with torpedoes before submerging again. This made them an incredibly dangerous threat to Allied shipping and supply lines.

The Rise of the U-Boat

Germany had experimented with submarines prior to World War I and deployed them to limited effect during that conflict. However, it was during the interwar period that submarine technology dramatically improved, allowing the construction of larger, faster, and more powerful U-boats. By 1939, Germany had some 57 U-boats either in service or under construction.

At the outbreak of World War II, Grand Admiral Karl Dönitz devised a new U-boat doctrine called the Rudeltaktik or “wolfpack” strategy. Instead of spreading out lone submarines, he would mass groups of U-boats along the main Atlantic sea lanes used by Allied convoys. Operating in coordinated groups or “wolfpacks”, the U-boats could more easily track and overwhelm convoys at night while surfaced.

The “Happy Time” Attacks

During the first years of the war from 1939 to 1941, the U-boats experienced great success against Allied shipping, in a period that German submariners called the “Happy Time.” There were several reasons for this success:

  • The British lacked adequate escort vessels to protect convoys and mitigate the U-boat threat.
  • Merchant ships lacked available armaments and defenses against submarine attack.
  • The British had limited capabilities to detect U-boats even when surfaced at night.
  • Allied convoys often lacked adequate organization and communications.

With these advantages, lone U-boats and wolfpacks were able to attack merchant ships on the surface using cannon fire and torpedoes. In many cases, the U-boats could simply approach the helpless ships at night and sink them with torpedoes without warning. Even when detected, Allied ships had little recourse against a U-boat attack.

U-Boat Wolf Pack Tactics

The wolf pack tactics devised by Dönitz proved devastatingly effective. A wolf pack would typically consist of around 6-8 U-boats operating in a coordinated search pattern. Using radio communication, a senior commander would direct the U-boats to converge on a detected Allied convoy. The U-boats would surface and attack at night, sinking merchant ships with impunity before slipping away. Convoys lacked the escorts and armaments to deal with multiple simultaneous U-boat attacks.

Allied Countermeasures Lacking

During the Happy Time, the British lacked adequate defenses and countermeasures against the U-boat. Most merchant ships travelled alone or in poorly guarded convoys, relying on speed rather than armament for defense. The British ships did not yet have advanced radar to detect surfaced U-boats at night. Depth charges were in short supply, and British anti-submarine aircraft could only cover limited areas. Without escorts, convoys were rarely able to mount an organized response to mass U-boat attacks. This allowed U-boats to pick off ships effortlessly.

The Toll on Allied Shipping

Between 1939 to 1941, U-boats sank over 2,600 Allied merchant ships totaling over 14 million tons. At the height of the Happy Time in early 1941, U-boats were sinking Allied shipping at twice the rate it could be replaced. With Britain’s very survival dependent on supplies shipped across the Atlantic, these losses were dire.

Year Allied Ships Sunk by U-Boats
1939 224 ships sunk
1940 528 ships sunk
1941 1,080 ships sunk

This massive destruction was wrought by no more than 10-20 operational U-boats at any given time during the period. Dönitz’s wolf pack tactics allowed the U-boats to inflict damage far in excess of their numbers. The Happy Time did not end until significant Allied countermeasures were belatedly introduced in mid-1941.

Allied Countermeasures Turn the Tide

Faced with crippling shipping losses, the British were forced to introduce extensive countermeasures that finally mitigated the U-boat threat. These included:

  • More escort ships to protect convoys: By 1943 the Allies had over 200 escort ships guiding convoys across the Atlantic.
  • Long-range patrol aircraft to hunt U-boats: Aircraft like the B-24 Liberator closed the “mid-Atlantic gap” where U-boats operated freely outside aircraft range.
  • Technologies like radar and depth charges improved tracking and attacking submerged U-boats.
  • Merchant ships were armed for self-defense and travelled in protected convoys.
  • The Allies developed code-breaking capabilities to intercept and decrypt German communications.

With these measures in place, the Happy Time ended and Allied shipping losses declined dramatically. From 1943 onwards, losses dropped while U-boat losses mounted. From 1942-45, over 800 U-boats were lost in combat compared to just over 100 in 1939-41.

Escort Groups Protect Convoys

The introduction of formal escort groups revolutionized the defense of convoys. Rather than assembling ad hoc local escorts, the British began assigning trained and equipped escort groups of destroyers, corvettes, and support ships to guard each convoy across the entire Atlantic journey. These groups were able to aggressively hunt U-boats, preventing the wolf pack attacks that previously overwhelmed convoys.

Allied Airpower Takes Its Toll

Long-range aircraft like the B-24 Liberator enabled the Allies to close the mid-Atlantic gap where U-boats operated freely outside air cover. Flying from Newfoundland, Iceland, and Northern Ireland, aircraft patrolled huge swaths of the Atlantic hunting U-boats, forcing them to submerge where they were blind and immobile. U-boats sunk at sea by aircraft increased from just 1 in 1939 to 61 in 1943. Aircraft support enabled escorts to finish off and destroy submerged U-boats more effectively.

Radar and Code-breaking Aid Defense

Radar allowed Allied warships and aircraft to detect surfaced U-boats at much greater range. Stripped of their nighttime invisibility, U-boats were forced to submerge quickly. The British also made critical breakthroughs in deciphering German naval codebooks and wireless signals. This allowed them to route convoys away from known U-boat concentrations while directing escorts to intercept wolf packs.

U-Boat Attacks After the Happy Time

After the Happy Time ended, U-boats were still able to attack and sink Allied ships, but victories came at much steeper cost. Some major examples include:

  • Operation Drumbeat – Dönitz dispatched U-boats against coastal shipping along America’s poorly defended eastern seaboard in 1942. Over 200 ships were sunk, showing the remaining threat posed by U-boats.
  • Convoy ONS-5 – In a clash over an Allied convoy in April 1943, 13 U-boats were lost in exchange for sinking 13 merchant ships. Costly battles like this became more common as escorts improved.
  • Black May 1943 – In May 1943, coordinated groups of U-boats suffered severe losses attacking convoys, losing 41 U-boats at a cost of 58 merchant ships sunk.

While still dangerous, U-boats were blunted as an existential threat to Britain’s survival. In May 1943 alone, Allied countermeasures resulted in the loss of 43 U-boats, over 20% of operational strength.

Technical Advantages of the U-Boat

The U-boat relied on several key technical features to achieve success against Allied shipping:

Submerged Operations

U-boats primary advantage was the ability to submerge below the surface, making them invisible to surface ships. This allowed them to approach targets stealthily and escape attack by diving. Early U-boats had limited underwater speed and endurance, forcing them to operate on the surface much of the time.


U-boats primary armament were torpedoes fired from forward tubes while submerged. Torpedoes allowed U-boats to sink large warships and merchant vessels quickly with single hits below the waterline. Early magnetic detonators were faulty, but contact exploders were reliable and devastating.

Deck Gun

Most U-boats carried a 105mm or 88mm cannon on deck for surface engagements. These were used to finish off damaged ships and conserve torpedoes early in the war. Latter rifles and anti-aircraft guns were added for self-defense.


Extensive radio communications allowed U-boats to operate in wolf packs directed by senior commanders tracking Allied convoys. This coordination multiplied U-boat effectiveness in massed attacks.

Diesel Engines

U-boats used powerful diesel engines to charge batteries and provide propulsion while surfaced. This gave them extended range and surface speed necessary to find targets and keep up with convoys.

Major U-Boat Types

Germany fielded several classes of U-boat during World War II, improving capabilities as technology advanced:

Type VII U-Boat

  • Formed backbone of U-boat force from 1940 onward.
  • Carried 14 torpedoes and a deck gun for surface attacks.
  • Had an operational range of 6,500 nautical miles.
  • Could dive to depths of 230 meters.
  • Had a surfaced speed of 17 knots, underwater speed of 7 knots.

Type IX U-Boat

  • Larger, longer range model used for distant patrols.
  • Carried 22 torpedoes and a deck gun.
  • Had operational range exceeding 10,000 nautical miles.
  • Dive depth and speed similar to Type VII.

Type XXI U-Boat

  • Advanced model designed in 1943, entering late war service.
  • Streamlined design with improved batteries for underwater performance.
  • Capable of 18 knots submerged speed verses 7 knots for older models.
  • Remained submerged for extended periods for stealth.
  • Too few completed before war’s end to impact outcome.

Major Engagements Involving U-Boats

U-boats participated in campaigns across the Atlantic, Mediterranean, Arctic, and Indian Ocean theatres. Major engagements include:

Battle of the Atlantic

  • Lasting the entire duration of World War II from 1939 to 1945.
  • Primarily German U-boats against Allied convoys across the North Atlantic shipping lanes.
  • Over 70% of all U-boats lost in combat were sunk in the Atlantic campaign.
  • Decisive victory by Allied escort groups, aircraft, and code-breaking efforts.

Operation Drumbeat

  • Attacks against coastal shipping along the sparsely defended American east coast starting in January 1942.
  • Also called the “Second Happy Time” by U-boat crews.
  • Over 500 ships sunk in 6 months, showing U-boats remained dangerous.
  • Forced introduction of organized coastal escort groups by Allies.

Arctic Convoys

  • Allied convoys carrying war material along dangerous Arctic route to the Soviet Union from 1941-1945.
  • Weather and long periods of darkness hampered Allied air cover and escort.
  • U-boats and Luftwaffe aircraft enjoyed successes against the convoys in harsh conditions.
  • 85 merchant ships and 16 warships lost defending Arctic supply lines.


The German U-boat campaign nearly severed Britain’s vital shipping lines, bringing the nation to the brink of starvation and defeat. Unrestricted submarine warfare came at immense moral cost, sinking millions of tons of civilian merchant shipping in contravention of international law. However, the campaign ultimately proved unsuccessful due to belated Allied countermeasures. By 1943, the Allies closed the Atlantic gap where U-boats once prowled freely and put the submarines on the defensive. While never fully eliminated as a threat, the U-boats no longer posed an existential danger after their Happy Time came to an end.