Between 1939 and 1943, German U-boats have a clear advantage over Allied escort ships. They can close in on convoys without being detected, fire torpedoes and escape without too much difficulty. To counter their devastating attacks, Allied scientists will develop underwater (ASDIC) or surface (radar) detection systems. Other systems can follow U-boats by plotting their radio signals on a map (HF/DF). Finally, the breaking of the Enigma code used for communications between U-boats and their HQ will allow Allied naval authorities to foresee and counter their attacks. It is only in 1943, though, that the Allies, with more ships, better armament and better detection systems will be able to regain control of the seas.
The ASDIC is a sonar system for submarine detection developed by British, French and American scientists during WWI; the name is derived from that of the Anti-Submarine Detection Investigation Committee.
The ASDIC emits a sound signal at regular time intervals. The sound waves travel through water and, when they hit a solid body, bounce back as an echo, which is intercepted, amplified and then heard by the operator. The return sound wave also sets in motion a stylus that records the echo on a chart. Position is estimated based on the direction of the echo, and distance based on the delay between emission and interception. The operator will immediately notify the bridge of any suspicious reading.
ASDIC sound impulses use a frequency between 14 and 22 kilocycles; the operator of each ship in a convoy must select a frequency different from that used by the neighbouring vessels; otherwise he could intercept the original signal from another ASDIC, resulting in a very loud “ping”. The transmitter is located in a dome under the hull and the signal is sent forward. In 1940, ASDIC detection could locate a submarine, a whale or a school of fish at a distance of 2,000 metres.
The ASDIC system has its limitations: it is affected by the turbulence created by propellers or by the motion of ships; it becomes inefficient, therefore, once the submarine has succeeded in slipping inside the convoy. In addition, when there are layers of water of contrasting temperatures, the signal is deflected and readings are unreliable, as Canadian operators realized when tracking German submarines in the Gulf of St. Lawrence.
Radar (RAdio Detection and Ranging) emits radio waves that are reflected by solid objects and intercepted on their way back to their source. An amplified image of the echo is displayed on a cathode-ray screen, and the operator can estimate direction and distance. The radar had such potential that Allies and enemies each developed their own top-secret radar projects.
When the war started, the Royal Air Force (RAF) already had a radar system for coastal defence, which was used successfully during the Battle of Britain in 1940. It is only with the development of radars using shorter wavelengths and smaller antennas that those devices could be installed on ships. In 1940, the Royal Navy started using ASW (Air/Surface Warning) systems, also known as Model 286, radars with a 1,5 m-wavelength. Canada’s National Research Council (NRC) soon developed a similar model, called SW1C (Surface Warning 1st Canadian), using the same wavelength, and gradually installed on board of Royal Canadian Navy (RCN) ships, starting in late 1941. Unfortunately, their efficiency in anti-submarine warfare was limited as the wavelength used by both 286 and SW1C radars is still too long to detect an object as small as a submarine’s conning tower.
British scientists countered this limitation by developing the cavity magnetron that can reduce a radar wavelength to 10 cm. As early as 1941, Royal Navy vessels are equipped with this improved radar, Model 271. This development was such a technical breakthrough at the time that Canada’s NRC was unable to come up rapidly with a Canadian version. The RCN will benefit from this upgrade only in 1943 and 1944, long after its British counterpart.
Starting in 1942, British escort ships were equipped with compact, high-frequency radio-goniometry systems known as “HF/DF” (High Frequency Direction Finder), or “Huff-Duffs”. Just like land-based radio-goniometry stations, HF/DFs allow an operator to determine the direction of a radio signal. An experienced operator can also estimate the distance. When a radio message from a U-boat is intercepted, a ship can be detached from the escort group and sent in the direction of the submarine. Two ships using Huff-Duffs and working together can pinpoint the position of a U-boat through triangulation of their results.
Radio-goniometry is extremely effective in locating German submarines, since when preparing an attack, they must report their position and that of their target to their HQ. Even if the message itself is enciphered it can reveal the position of the attacker.
Throughout the war, U-boats used radio communications to report their positions and the position of their targets to their HQ. This is how Admiral Karl Dönitz could command the submarine fleet as a whole, forming “wolf packs” to attack convoys. Those communications were encrypted through a system code-named Enigma, using a typewriter-like machine that produced enciphered messages that could only be read by someone using another such machine.
Before the war British authorities already knew aboutEnigma and how Polish scientists had started working on the problem. British intelligence services launched the Ultra Project, a team of cryptography experts located in Bletchley Park, north of London, and dedicated to breaking the coded messages of the German Luftwaffe (air force) andKriegsmarine (war navy), that the Allies intercepted.
On May 8th, 1941, a German submarine, U-110, was rammed byHMS Bulldog. The crew evacuated the submarine, but she did not sink as fast as her commander thought she would and the Allies were able to seize her Enigma machine with its operating instructions. This allowed Ultra scientists to break the code used by the Kriegsmarine. The British could now be informed within 48 hours on the position, condition and strategy of all German submarines and warships; convoys could be routed to avoid U-boat concentrations.
This remarkable advantage was lost in February 1942, when the Germans upgraded Enigmaby adding a fourth rotor. For ten months, while cryptologists worked on breaking the new code, the Allies were deprived of the valuable information that allowed them to escape U-boats. To make things worse, the Germans succeeded in breaking the No. 3 Code used by the Allies for coordinating convoys and their escort ships. Allied losses in 1942 and early 1943 were staggering.
In the spring of 1943, with the decipherment of the new Enigma system by Bletchley Park and improved detection systems, the Allies gained a decisive advantage.