The Grant CDL (Canal Defence Light) was a special vehicle mounting a turret in which was located a powerful searchlight that was supposed to dazzle an enemy during night operations or illuminate targets at night.
The device known under the cover name Canal Defence Light was one weapons of World War II that was destined hardly ever to be used. In essence it was a simple idea, in which the normal gun turret of a tank was replaced with another housing an intense light to illuminate battlefields at night. All manner of tactical ploys were advocated for its use, ranging from simply blinding an enemy to providing general target illumination.
The idea of mounting powerful searchlights on tanks was first mooted during the mid-1930s by a group of interested civilians who ’sold’ the idea to the War Office in 1937. The War Office carried out a series of development trials under conditions of great secrecy, and by late 1939 a turret was ready for production. The secrecy continued with the project being known as the Canal Defence Light, or CDL. The first turrets produced were for the Matilda II infantry tank, and all that the fitting of a CDL involved was the removal of the normal turret and its replacement by a new one, though changes had to be made to the Matilda’s electrical systems as well, In the turret the searchlight was positioned behind a vertical slit in which was a shutter. In use the searchlight was switched on and the shutter was opened and closed very rapidly to provide a flickering impression to an observer in front. This flickering made the range of the CDL light difficult to determine, and anyway the light was so powerful that it was difficult to look into the beam even at quite long ranges.
Some 300 CDL turrets were ordered to convert Matildas to the CDL role, and one brigade of Matilda CDL vehicles was based in the UK and another in North Africa. The military planners were determined to use the impact of the CDL units to the full and constantly awaited the chance to use them to maximum effect. That chance somehow never came and the North African campaign was over before the CDLs could prove their worth. However the Normandy landings lay ahead, and it was planned to use the CDLs there. But at the same time it was felt that the CDL turrets should be placed on something rather more up-to-date than the slow and stately Matildas, so Grant tanks became the chosen carriers.
Thus the CDL was carried throughout the war but sparingly used. However, the idea certainly attracted attention. The US Army was most impressed by what it saw of the CDL at various demonstrations and decided to adopt the CDL for itself, and thus produced 355 CDL turrets for mounting on otherwise obsolete M3 Lee tanks. These were used to equip six tank battalions for special operations in Europe. The cover name T10 Shop Tractor was used for US CDL vehicles. The U.S. Army already had two battalions equipped with the CDL in June 1944 but never considered using them on D-Day. Instead the CDLs were used for the relatively unexciting task of providing ‘artificial moonlight’ to illuminate the crossings of the Rhine and Elbe in early 1945.
The US 738th Tank Battalion (spec) manned the CDL (Canal Defence Light). This was an M3 Lee/Grant chassis, mounting a modified turret containing a 13,000,000 candle power arc lamp reflected through a mechanical shutter. This weapon induced blindness and disorientation, and the flickering prevented the enemy from identifying its source and location.
The first use of the CDL was against German frogmen, who were attempting to destroy the Ludendorff Bridge at Remagen. The device worked as intended, and the frogmen were quickly rounded up.
The Churchill Canal Defence Light is much more difficult to write about since nobody, as far as we know, has yet seen a photograph of one. There is a stowage diagram in the Tank Museum archive plus an illustrated parts list and a sketch of one in the book Sutherland's War but none are really good enough to work up a description from. However, these illustrations seem to indicate that the tanks were fitted with the earlier pattern air intake which suggests they were made quite early on. We know that the turret ring diameter of the Churchill was only 52in (1321mm) as against 54in (1372mm) of the Matilda and Grant, so the turrets weren't directly interchangeable. And we also know that the thickness of frontal armour on the Churchill CDL turret was 85mm as against 65mm on the others. On the other hand the Churchill CDL turret was of very similar layout, even down to the Besa machine gun despite the fact that on the Churchill there would have been another Besa in the hull front. But there is no evidence, from the parts list illustration at any rate, of a dummy main gun being fitted.
The evidence suggests that there was only ever one regiment equipped with Churchill CDL tanks, and that, we think, was 152 Regiment Royal Armoured Corps (which had been converted from 11th Battalion, The King's Regiment from Liverpool) in the 35th Army Tank Brigade, which became an element of the 79th Armoured Division. If it was configured along the same lines as other CDL regiments that would mean 54 CDL tanks split evenly between three squadrons along with 13 gun tanks and the usual reconnaissance, intercommunication and admin troops and their vehicles. According to a book on the Churchill tank, published back in 1971 by Chris Ellis and the late Peter Chamberlain, only prototypes were built and hulls earmarked for the remainder were later completed as armoured recovery vehicles. They also suggest that the reason for the abandonment of the Churchill CDL was on account of its slow speed, but in that respect it was the same as the Matilda, which was also adapted to the CDL role but a lot less well armoured. In any case the CDL was normally used in a static role at night, when speed wouldn't matter.
Another peculiarity of the Churchill CDL was that it required a separate engine to drive the electrical generator, a Meadows four-cylinder unit. It was not driven off the main engine as on other types. The engine and generator were located within the Churchill's fighting compartment, probably just behind the hull gunner's seat, with the exhaust funnelled through the hull roof and the air intake for the radiator incorporated in the hull gunner's hatch in the hull front. Quite how he got out in the event of an emergency is not clear.
A point has been raised, and quite legitimately, over the potential effectiveness of the CDL tank in anything but flat, level country. Wherever there are hills and irregular ground, and that must be pretty well everywhere, there will always be a problem finding a flat area for the tank to stand and project its beam. Even if that hurdle is overcome, the CDL's effectiveness is limited if there are hills in the way to obstruct the light beam or deep declivities in the ground which it will miss altogether. Indeed, this may explain the reluctance to employ it in action.