Canal Defence Light

The idea is credited to a Greek citizen, Marcel Mitzakis, who devised the system for the de Thoren Syndicate in the 1930s; they were advised by J F C Fuller. The device was demonstrated to the British War Office in 1937. Although three examples were ordered for tests, the trials did not begin until 1940, when the War Office took over and ordered 300 lights for fitting to tanks.[2] A prototype was constructed using a Matilda II tank. The tank's normal turret was replaced with a cylindrical one containing both a 13 million candlepower (12.8 million candela) searchlight and a machine gun.[1]

The searchlight turret included a station for an operator, who had the task of changing the light's carbon electrodes when they burned out.[1] The light emerged from a vertical slit that was just 2 inches (5.1 cm) wide by 24 inches (61 cm) tall, a small size which reduced the chance of battle damage to the optical system. The beam diverged at 19° horizontally and 1.9° vertically, forming a pool of light of around 34 by 340 yards (31 m × 311 m) at a distance of 1,000 yards (910 m). The turret could rotate 360° and the light beam could be elevated or depressed by 10° from the horizontal.[3]

Blue and amber filters allowed the light to be coloured as well as white. A shutter could flash the beam on and off up to twice a second. It was found the blue light caused the CDL tank to appear to be at a greater distance, and blue and amber light beams from two CDL tanks could combine to illuminate a target with white. A flashing beam would further dazzle and disorient enemy troops by not giving their eyes a chance to adapt to either light or darkness.[3]

The Matilda tank was later replaced by the US M3 Grant, which was superior in several ways. It was a larger, roomier and better-armoured tank, and also faster and better able to keep up with tanks such as the Sherman. It was armed with a 75 mm gun mounted in the hull and a 37 mm gun in a turret, so could retain some fighting capacity when the searchlight turret was mounted.[1] A dummy gun-barrel fitted to the turret made it resemble a normal M3 tank. The operator was the only occupant of the turret—the vehicle commander had a seat to the left of the driver.

The project was shrouded in secrecy. It was tested during Exercise Primrose in 1943 at Tighnabruaich, Scotland; it was concluded that it was "too uncertain to be depended upon as the main feature of an invasion".

An American-built T10E1 Shop Tractor, January 1943

The CDL was shown to senior US officers (including generals Eisenhower and Clark) in 1942 and the US decided to produce their own tanks using the CDL design. The codenames "Leaflet" for the tank, and "Cassock" for the training programme for crews were used.[4] For secrecy the construction was dispersed. Conversion of the M3 to take the CDL was by the American Locomotive Company as "Shop Tractor M10", turrets were produced by Pressed Steel Car Company as "coast defence turrets", and the arc lamps were sourced through the Corps of Engineers. The final assembly of the CDL tank was at Rock Island Arsenal. By the end of 1944, American Locomotive had produced 497 tanks.[5]

American crews were trained at Fort Knox and in the California-Arizona manoeuvre area. The six battalions of tanks then moved to the UK to join the British CDL tanks in Wales.[6]

The British and American CDL units deployed to the continent did not cross over to France until August, the British as part of the 79th Armoured Division. The British 35th Tank Brigade and US 9th Armored Group were retained in the UK. The system was highly secret as surprise was considered essential to its use. This hampered its employment, as commanders were often unfamiliar with, or did not know of, it, and did not consider it when drawing up plans for attack.[3]

Rather than let trained tank crews sit idle, most of the special units were converted either to other special roles (such as mine clearance tanks) or regular tank units.

For the crossing of the Rhine, some CDL units were used. The one British squadron that had not been converted from CDLs was used in the north, 64 American CDL tanks were brought back into use with their former crews. The US tanks were spread across the First, Third and Ninth armies.[7]

The 11th Royal Tank Regiment was raised in January 1941 and designated for the CDL role in May 1941. The unit trained at Lowther Castle near Penrith, and was based at Brougham Hall, Cumberland. It spent 1942 and 1943 in the Middle East without seeing action, returning to the UK in April 1944. It landed in Normandy on 12 August 1944, seeing no action until 29 September 1944, when it was ordered to transfer all of its equipment to the 42nd and 49th Royal Tank Regiments, and was retrained to operate the American amphibious LVT-4, known by the British Army as the Buffalo Mark IV.

In their turn, the 42nd and 49th Royal Tank Regiments were largely inactive for the remainder of the war, when all three units were disbanded after the end of hostilities.

Battalions of the American 9th Tank Group trained using the Grant variant of the CDL tank at Camp Bouse in the Arizona desert. In 1944, before deployment in the European Theatre of Operations, they continued training on the Preseli Hills in Pembrokeshire, West Wales.

At 06:00, before dawn, on 18 November 1944, giant searchlights ("canal defence lights") of the 357th Searchlight Battery, Royal Artillery provided hazy indirect light for the mine-clearing flail tanks supporting the infantry in Operation Clipper.

Some British tanks were sent to India in 1945. The US Tenth Army requested deployment of CDL tanks for use during the Battle of Okinawa, but fighting there was complete by the time they arrived.

During the Korean War, there was a requirement for searchlights on the battlefield. There was brief interest in resurrecting a CDL on an M4 Sherman design (T52) that had started in 1944, but it was recognized that four battalions could be equipped with normal searchlights for the cost of a single CDL tank.[10]

CDL-equipped Matilda II on display at Bovington Tank Museum

The only surviving CDL-equipped Matilda tank is in the collection of the Royal Armoured Corps at The Tank Museum, Bovington, Dorset, in Britain. One CDL-equipped M3 Grant is displayed at Cavalry Tank Museum, Ahmednagar in India.[11][12]

• Camp Bouse US test site.
• Leigh Light
• List of U.S. military vehicles by supply catalog designation SNL G-193

• Fuller, J.F.C. (1949). The Second World War - 1939-45 - A strategical and Tactical History. Duell, Sloan and Pearce.
• Hunnicutt, R.P. (1994) [1978]. Sherman - A History of the American Medium Tank. Presidio Press.

Wikimedia Commons has media related to Canal Defence Light.

• Report on DDay preparations
• Now It Can Be Told! - Tanks That Turn Night Into Local Day, The War Illustrated, November 23, 1945.