Aviation History

The day U-boat U-134 shot down K-74: the story of the only US Navy blimp lost to enemy action in World War II

While US Navy blimps were useful for finding and pinning down U-boats until other help arrived, they were not supposed to attack surfaced U-boats.

As 1942 opened, both Nazi Germany and the Allies were ready for the climactic battle of the Atlantic to begin. Germany had 91 operational U-boats, and over 150 in training or trials. Production for 1942-44 was planned to exceed 200 boats annually. Karl Dönitz, running the Kriegsmarine’s U-boat arm, would finally have the numbers needed to run the tonnage war he wanted against the Allies.

Meanwhile, the British had, at last, assembled the solution to the U-boat peril. Its weapons and detection systems had improved to the stage that maritime patrol aircraft could launch deadly attacks on U-boats day and night. Airborne radar, Leigh lights, Magnetic Anomaly Detection (MAD) and the Fido homing torpedo all turned the anti-submarine warfare (ASW) aircraft into a submarine-killer, while shore and ship-based technologies such as high-frequency direction finding and signals intelligence could now help aircraft find enemy U-boats. Following its entry into the war in 1941, the United States had also thrown its industrial muscle behind the campaign, supplying very long range (VLR) Liberator bombers to the RAF and escort carriers to the Royal Navy.

As told by Mark Lardas in his book Battle of the Atlantic 1942–45 The climax of World War II’s greatest naval campaign, the US Navy also used blimps extensively in the Battle of the Atlantic, typically in regions where winter was not a problem: the Eastern Seaboard south of New York, the Gulf of Mexico, the Caribbean, the central Atlantic and the Mediterranean. The main type used in the Atlantic was the K class, of which 134 were built before and during World War II. While blimps were useful for finding and pinning down U-boats until other help arrived, they were not supposed to attack surfaced U-boats. Instead, doctrine called for them to attack only as or after the U-boat dived.

What happened to blimp K-74 of Blimp Squadron ZP-21 on a patrol east of the Florida Straits in the Bahamas shows why this rule existed. Having spotted a U-boat on radar, K-74 made visual contact with U-134 running surfaced, heading in the direction of a nearby tanker and freighter. To protect these ships, K-74’s commander, Lieutenant Nelson G. Grills, chose to attack U-134. The encounter ended badly for K-74.

Attacking just before midnight on Jul. 18, 1943, K-74 opened fire with its .50-calibre machine gun at 250 yards. Passing directly overhead, the bombardier dropped two depth charges – which failed to sink the U-boat. A second pass would be necessary.

Meanwhile, U-134’s anti-aircraft battery returned K-74’s fire, and with a target larger than U-134, successfully hit it. The blimp’s starboard engine was knocked out and the cells holding the blimp’s helium were repeatedly punctured. Once past U-134, the blimp’s bag began deflating, especially aft. The uncontrollable blimp soon settled in the water.

The crew abandoned K-74 after it reached the water. The balloon took a long time to sink. Lieutenant Grills re-entered the gondola to check that all classified material had been tossed overboard. (It had been.) When he exited, he became separated from the others, and decided to swim to shore. Grills was picked up by a SC-657 the next morning, 6 miles from K-74.

Meanwhile, the other eight crew members had stayed together and were spotted the next morning by a Grumman J4F amphibian. While the water was too rough for the amphibian to land, it found the destroyer Dahlgren, which rescued seven of the eight remaining crewmen. The eighth was attacked and killed by a shark minutes before Dahlgren arrived.

U-134 submerged after the attack, which slightly damaged its ballast tanks. It resurfaced before dawn, its men boarded the still-floating gondola, and photographed artefacts aboard the blimp. It transferred the photographs to another U-boat later in the patrol.

Continuing its patrol, U-134 was badly damaged by a second attack in the Florida Straits, this time by carrier aircraft from USS Croatan, and was finally sunk in the Bay of Biscay by HMS Rother while running the Bay barrier.

Because Grills sidestepped doctrine to attack a U-boat, some called for his court martial. Others claimed he should receive a commendation for his aggressiveness. He received neither a court martial nor a commendation, instead being transferred to a position where he had responsibility for improving airship tactics.

The two ships Grills was trying to protect from U-134 escaped attack. K-74 was the only blimp lost to enemy action in World War II.

Battle of the Atlantic 1942–45 The climax of World War II’s greatest naval campaign is published by Osprey Publishing and is available to order here.

Photo credit: Edouard A. Groult via Osprey

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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