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.
by William H. Pugsley. National
Archives of Canada, PA-139273.
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.
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.
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
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.
War Museum, 19470003-008.
Throughout the war, U-boats used radio communications
to report their positions and the position
of their targets to their HQ. This is how
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
Before the war British authorities already
knew about Enigma 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)
and Kriegsmarine (war navy), that
the Allies intercepted.
Enigma encryption device.
War Museum, 19470003-008.
On May 8th, 1941, a German submarine, U-110,
was rammed by HMS 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
This remarkable advantage was lost in February
1942, when the Germans upgraded Enigma
by 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.