Leigh Light
The ASV Mk. II radar set had a minimum detection range of about 1,500 yards. This distance proved fatal for night operations over the Atlantic Ocean. When an aircraft approached a surfaced U-boat, the target vanished from the radar screen before the pilot could see it with naked eyes. Early attempts to reduce this blind spot failed completely. Crews tried dropping flares to light up the water below them. A series of flares was necessary until the submarine appeared in the beam. By that time, the U-boat had already dived deep into the dark depths. The entire process gave the enemy enough time to escape and reload their torpedoes.
Wing Commander Humphrey de Verd Leigh devised a solution after chatting with returning aircrew members. He proposed mounting a searchlight under the aircraft pointed forward. This device would illuminate the submarine the moment the radar lost contact. Leigh developed the system on his own without official sanction. Even the Air Ministry remained unaware of its existence until they saw the completed prototype. At first, fitting the heavy equipment onto existing planes proved difficult. He persisted in testing the idea and eventually garnered support from Sir Frederick Bowhill. In March 1941, a Vickers Wellington DWI modified with a retractable dustbin lamp proved the concept sound. Development assistance and production were handled by Savage and Parsons Ltd. of Watford led by Jack Savage.
Two distinct types of Leigh Light entered operational use during the conflict. The Turret type fitted on Wellington aircraft featured a 30-inch searchlight mounted in a retractable under-turret. Hydraulic motors and rams controlled the movement of this massive unit. Its maximum beam intensity reached 50 million candelas without the spreading lens. With the lens attached, the output dropped to about 20 million candelas. Total weight for this configuration was approximately 600 pounds. The Nacelle type fitted on Catalinas and Liberators used a 40-inch searchlight slung from bomb lugs on the wing. Electric controls managed this larger unit which weighed around 800 pounds. It produced a maximum beam intensity of 90 million candelas without the lens. With the lens, the intensity settled at about 17 million candelas.
By June 1942, aircraft equipped with ASV radar and the Leigh Light operated over the Bay of Biscay. They intercepted U-boats moving to and from their home ports along the French coast. The first submarine successfully sighted was the Italian submarine Torelli on the night of the 3rd of June 1942. The first confirmed kill occurred on the 5th of July 1942 when a Vickers Wellington of 172 Squadron sank an enemy vessel. Pilot Wiley B. Howell led that attack. In the previous five months not one submarine had been sunk while six aircraft were lost. The Leigh Light turned the tables completely by August. U-boats preferred taking chances in daytime when they could see approaching aircraft and fight back.
Germany introduced the Metox radar warning receiver to counter the combination of ASV and Leigh Light systems. This device provided submarine crews early warning that an aircraft using radar was approaching. Because the receiver detected emissions at greater ranges than the radar detected vessels, it often gave the U-boat enough time to dive. Having expected this response, the Allies introduced the centimetric ASV Mk. III radar. This new system regained control of the battle against the German fleet. Although the German Naxos countered these radars later, the U-boat force was already damaged beyond repair by then. The technological arms race shifted decisively toward Allied superiority during this period.
The observation post and concrete indicator arrow at Putsborough were built specifically for Leigh Light training. A buoy anchored in Morte Bay served as a target for practice runs. Two other sites located at Woolacombe and midway along the bay provided triangulation to assess bombing accuracy. Wing Commander Peter Cundy received the Air Force Cross for his part in developing the Leigh Light. At least one Fairey Swordfish I biplane torpedo bomber was trialled with a Leigh Light under the lower port wing. A large battery pack slung under the fuselage powered the system where torpedoes normally went. Performance suffered significantly with such heavy loads resulting in top speeds marginally above stall speed.
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Common questions
What is the minimum detection range of the ASV Mk. II radar set used with the Leigh Light?
The ASV Mk. II radar set had a minimum detection range of about 1,500 yards. This distance proved fatal for night operations over the Atlantic Ocean because the target vanished from the radar screen before the pilot could see it with naked eyes.
Who devised the Leigh Light system and when did development begin?
Wing Commander Humphrey de Verd Leigh devised the solution after chatting with returning aircrew members. He developed the system on his own without official sanction and presented a modified Vickers Wellington DWI in March 1941 that proved the concept sound.
How many distinct types of Leigh Light entered operational use during the conflict?
Two distinct types of Leigh Light entered operational use during the conflict. The Turret type fitted on Wellington aircraft featured a 30-inch searchlight mounted in a retractable under-turret while the Nacelle type fitted on Catalinas and Liberators used a 40-inch searchlight slung from bomb lugs on the wing.
When was the first submarine successfully sighted by aircraft equipped with the Leigh Light?
The first submarine successfully sighted was the Italian submarine Torelli on the night of the 3rd of June 1942. The first confirmed kill occurred on the 5th of July 1942 when a Vickers Wellington of 172 Squadron sank an enemy vessel.
What countermeasure did Germany introduce to detect ASV radar before the Allies switched to centimetric systems?
Germany introduced the Metox radar warning receiver to counter the combination of ASV and Leigh Light systems. This device provided submarine crews early warning that an aircraft using radar was approaching because it detected emissions at greater ranges than the radar detected vessels.