The first problem posed by the U-boat was its location; the second was its destruction. In 1917 the Allied Research Commission for the Detection of Submarines had worked seriously and for a long time to solve the first point, and towards the end of the war they created a device capable of detecting a submerged vessel by means of sound waves. Although not very safe, this weapon was known as ASDIC, after the initials of the Commission. The principle of the device was to emit a sound pulse - a "knocking" that propagated through the water and bounced off the submarine; his return was detected by the transmitting ship. The time taken by the sound wave, between its emission and its reception, made it possible to know the distance of the submarine, thanks to the calculation of the speed of propagation of sound in water.
Over the years, it was realized that the speed of sound in water was fanciful, that it depended on temperature, salinity, density, etc. Nevertheless the ASDIC was useful and from 1930 almost all the ships of the Royal Navy were equipped with it. Through intensive training, the sailors became able to detect a good number of submarines.
The Americans had worked for the same purpose; their device was called the SONAR (Sound Navigation and Ranging*); in 1939, with the ASDIC, they were the only existing devices for the detection of submerged targets. Unfortunately, experience soon showed that neither worked as well as hoped. The scientists persevered and managed to improve their qualities and their performance. Despite this, an experienced submarine commander, knowing well the difficulties of an ASDIC operator, could make the latter's task difficult, by taking advantage of the differences in density of the layers of water, and other natural phenomena. that cause sound waves to be distorted or absorbed.
Asdic
The Asdic consists of a transmitter-receiver which sends sound pulses and receives an echo if these waves hit an obstacle. The time between the transmissions and the return gives an indication of the importance of the objective.