Leigh Light

• 1. Introduction

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  The Leigh Light put out 22 million candelas of white glare, enough to turn a black sea into a lit stage. It was a carbon arc searchlight, built in Britain during the Second World War. Patrol bombers of the Royal Air Force's Coastal Command carried it so crews could spot surfaced German U-boats at night during the Battle of the Atlantic. Radar could already find a submarine in the dark, yet something about radar left aircrews blind at the decisive moment. Who noticed that gap? Why did the answer begin as a private project that officials knew nothing about? And how long could one lamp keep the hunters ahead of the hunted?

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• 2. The Radar Blind Spot

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  A minimum range of about 1 km was the flaw in the early ASV radar. ASV stood for air-to-surface-vessel. The Mk. II set lost its target from the display inside that distance, which was still too far for a pilot to see a darkened submarine by eye. Efforts to shrink the minimum range did not work.

  Flares were the first stopgap. A single flare lit only the patch of sea directly beneath the aircraft, so crews had to drop them one after another until a submarine showed. Then the plane had to circle back for its attack. The whole routine gave the submarine time to dive.

  A buoy dropped earlier by the plane led to the next improvement, the time-delayed flare. It was fired into the air from the buoy, which bought the attacking aircraft time to circle. The surfaced submarine then appeared in silhouette as the plane approached. Even so, the better answer would come from an RAF personnel officer who spent his days chatting with returning aircrew.

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• 3. A Secret Prototype

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  Wing Commander Humphrey de Verd Leigh found his idea by talking to the crews who came back from night patrols. He proposed mounting a searchlight under the aircraft, pointed forward. The crew would switch it on just as the target was about to vanish from the radar display. The U-boat then had too little time to dive, and the bombardier had a clear view of it.

  Leigh developed the lamp on his own, in secret and without official sanction. Even the Air Ministry knew nothing about it until it was shown the finished prototype. The size of the lamp made it hard to fit on aircraft at first. Leigh kept pushing his tests and won the support of Sir Frederick Bowhill, Commander-in-Chief of Coastal Command.

  In March 1941 a Vickers Wellington DWI proved that the concept was sound. That aircraft already carried the generator the lamp needed, because it had been used for anti-magnetic mining with a large electromagnet. It was modified with a retractable "dustbin" to hold the lamp.

  The Turbinlite was the Air Ministry's preferred system once it accepted the idea was worthwhile. It had been developed as an aid for night-time bomber interception, and it was less effective. After trials the Ministry also turned to Leigh's system, but aircraft were not modified to carry it until mid-1942. Savage and Parsons Ltd. of Watford, led by Jack Savage, gave development help and handled production. Wing Commander Peter Cundy also received the Air Force Cross for his part in the work.

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• 4. Turret and Nacelle

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  Wellington bombers carried the Turret type of Leigh Light. It was a 24 in searchlight in a retractable under-turret, worked by a hydraulic motor and ram. Without its spreading lens the beam reached a maximum of 50 million candelas, and with the lens about 20 million. The whole installation weighed 1,100 lb.

  Catalinas and Liberators carried the Nacelle type instead. This was a 20 in searchlight inside a nacelle 32 in in diameter, slung from the bomb lugs on the wing. Its controls were electric. The maximum beam was 90 million candelas without the spreading lens and about 17 million with it. The total weight was 870 lb.

  A Fairey Swordfish I biplane torpedo bomber was also trialled with the light under its lower port wing. A large battery pack hung under the fuselage where the torpedo would normally go, and a rack of anti-submarine bombs sat under the other wing. With such a heavy load the top speed was only marginally above the stall speed. A Swordfish III carried the light under its starboard wing, though it is unclear where the battery pack went, because the ASV radar scanner apparently filled the torpedo area.

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• 5. Practice at Morte Bay

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  Putsborough has an observation post and a concrete indicator arrow that were built for Leigh Light training. A buoy anchored in Morte Bay served as the target. Two other sites provided triangulation, one at Woolacombe and the other midway along the bay. Together the three points let observers assess how accurate each bombing attempt had been. Crews who practised there went on to a far larger stretch of water, the Bay of Biscay.

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• 6. Hunting Over Biscay

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  By June 1942, aircraft with ASV radar and the Leigh Light were flying over the Bay of Biscay. They intercepted U-boats moving to and from their home ports on the coast of France. The first submarine successfully sighted was the Italian submarine Torelli, on the night of the 3rd of June 1942.

  The 5th of July 1942 brought the first confirmed kill. A Vickers Wellington of 172 Squadron sank the submarine, piloted by an American, Wiley B. Howell. The five months before had produced no sunk submarines at all and six lost aircraft. The Leigh Light turned the tables.

  By August the U-boat crews preferred to take their chances in daylight. For a time they were forced to charge their batteries by day, when they could at least see aircraft coming. They could also warn each other and fire their anti-aircraft weapons back. Germany was already working on a device to restore the balance, a receiver called Metox.

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• 7. The Radar Race

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  Metox was the German answer to the pairing of ASV radar and the Leigh Light. It was a radar warning receiver, and it told a submarine crew that an aircraft using radar was approaching. Because it could detect radar emissions at a greater range than the radar could detect vessels, it often gave the U-boat enough time to dive.

  The Allies had expected such a countermeasure. They introduced the centimetric ASV Mk. III radar and regained control of the battle. Germany then fielded Naxos, which countered these newer radars. Naxos came too late to matter, because the U-boat force was already damaged beyond repair.

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