Convoy ONS5 (North Atlantic, April-May 1943)
Posted: Tue May 25, 2021 7:36 pm
The North Atlantic, April-May 1943
Prologue
From September 1939 until April 1943, Britain had been fighting a loosing battle against the U-boat in the North Atlantic for three and a half years. In 1942 alone, over 1,000 merchant ships, or 5.5 million tons were lost, at the cost of 86 U-boats. Ships were being sunk faster than they could be replaced, and the size of the U-boat fleet was increasing.
In the first three months of 1943, North Atlantic shipping losses were 155 ships at 930,000 tons for 40 U-boats sunk. In March alone, 82 ships at 476,000 tons were lost for 15 U-boats sunk. Britain could not sustain losses on this scale for much longer.
In his memoirs Admiral Doenitz wrote “At the end of March 1943, after three and a half years of war, we had brought the British maritime power to the brink of defeat in the Battle of the Atlantic, and that with only half the number of U-boats we had demanded”.
An Admiralty review later stated “The Germans never came so near to disrupting communications between the New World and the Old as in those first 20 days of March 1943”.
But two months later, in May 1943, for the first time, more U-boats (41) were sunk than ships lost (34). In his memoirs, Admiral Doenitz wrote “On May 24 I ordered them (the U-boats) to proceed, with utmost caution, to the area south east of the Azores. We had lost the Battle of the Atlantic”.
What had happened in such a short space of time to cause this dramatic reversal of fortunes?
Well, nothing really, except for the experience gained in using the weapons and systems currently in operation. The major developments in weapons, aircraft, technology etc. were just staring to come into operation, but not in numbers at this point in time.
But probably the biggest single factor was the advanced training given to escort officers, by the Western Approaches Tactical Unit (WATU), in convoy protection and anti U-boat warfare. This course developed tactics for dealing with all kinds of attack by U-boat and situations developing from them.
The General Situation
Background
Suitable land based aircraft were not available to the Allies in quantity, and those that were had insufficient range to cover convoys the whole way across the North Atlantic. Thus there was an area, south and southeast of Greenland, which became known as the “Mid Atlantic Gap” or “Greenland Gap”, where U-boats could operate free from the danger of air attack. It was in this gap of a few hundred miles that most U-boat patrol lines were positioned ready to intercept both east and westbound convoys.
At the end of April 1943, the number of U-boats available to operate in the North Atlantic reached a high point – 193, of which only about one third would actually be on patrol at any one time. Sixty of which were in the North Atlantic, positioned mainly in the so called “Greenland Gap”. This was the highest number ever to participate in one convoy battle.
British Intelligence
Towards the end of April, Bletchley Park had one of its periodic “blackouts” when the four rotor “Triton” cipher could not be broken, so naval Ultra was not available to the Navy’s Operational Intelligence Centre (O.I.C.). This did not mean that there was no available intelligence about U-boat movements. There were other sources.
For example, every time a U-boat either left of arrived at its base, it was met by an escort which reported the U-boat’s arrival or departure to its base in a cipher that was already broken. Thus the identity and exact number of U-boats at sea was known. It also indicated which U-boats were lost by their non arrival.
A Triton signal from a U-boat, although unintelligible, gave away its position through land based HF/DF intercepts. From this, its route and speed could be estimated, and thus its probable destination.
Because U-boat signals were formalized, the length of a signal gave a clue as to its content. Thus a weather report could be distinguished from a sighting report which was different from a position report. When a convoy was sighted, the necessary sending of many signals alerted O.I.C. to the fact.
The Submarine Tracking Room used all of these sources to estimate the position and numbers U-boats and whether they posed a threat to a convoy. The main purpose of the Submarine Tracking Room at this time was to route convoys away from concentrations of U-boats.
After sighting a convoy, it could take the U-boats a couple of days to concentrate around it, generally giving the O.I.C. enough time to organize reinforcements for the convoy such as air support and escort support groups.
German Intelligence
The German radio and cryptographic service, the Beobachtungsdienst or Bdienst, had been reading the Anglo/American Naval Cipher No.3 since 1941, and had intelligence about most convoys including their sailing dates, routes and destination.
When the Bdienst informed the BdU (Befehlshaber der Unterseeboote, or U-boat Command) about a convoy’s departure date and probable route, the BdU organized available U-boats into patrol lines (sometimes called wolfpacks) across the expected routes to intercept the convoy.
When a U-boat sighted a convoy it sent a sighting report to the BdU, indicating the convoy’s speed and direction, then continued to shadow the convoy, transmitting a radio homing signal. The BdU then signaled other U-boats in the area, directing them to concentrate at a point ahead of the convoy before attacking.
U-Boat Patrol Lines
Because of the multiplicity and transient nature of patrol lines, it is difficult to keep track of the big picture of which U-boats were where at any one time.
Patrol lines were both temporary and mobile; they were constantly being formed, reformed and dispersed as convoys came and went. Each time they reformed they acquired a new name. They were not static, they moved, sweeping the ocean before them to maximize their chances of making an interception.
On May 1, in the Greenland Gap, there were 56 U-boats organized into 4 patrol lines. The following show the patrols active in the area at this time.
Patrol Line “Specht”.
Position: NE of Newfoundland, to intercept SC.127 and HX.235.
Active from April 19 to May 4.
Consisted of 24 U-boats.
On May 5 most go to “Fink” and some to “Amsel” 3 and 4.
Patrol Line “Amsel”.
Position: East of Newfoundland, southeast of Specht, to intercept HX.235 and SC.128
Active from April 22 to May 3.
Consisted of 16 U-boats.
On May 4 this split into four lines, “Amsel” 1,2,3 and 4.
Patrol Line “Star”.
Position: SE of Iceland, to intercept northern routed convoys.
Active from April 24 to May 1.
Consisted of 16 U-boats.
On May 3 all go to “Fink”.
. .
Patrol Line “Fink”.
Position: South of Greenland, to intercept SC.128.
Active from May 3 to May 6.
Consisted of 28 U-boats from “Specht” and “Star”.
Patrol Lines “Amsel 1-4”.
Position: South and southwest of Fink.
Active from May 3 to May 6.
Consisted of 4 groups of 5 or 6 U-boats.
Formed from lines “Amsel” and “Specht”
. .
Convoys at Sea
During this critical period, there were about ten convoys en route, arriving or departing, providing no shortage of potential targets for the U-boats.
Eastbound convoys from Halifax, Nova Scotia to U.K.
HX.234, HX.235, HX.236, HX.237.
Eastbound from Halifax to U.K. (Slow)
SC.127, SC.128, SC.129.
Westbound from U.K. to North America
ONS.5, ON.180, ON.181.
But, if there is one of these convoys, in its battle with the U-boats, which can be described as a turning point in the Battle of the Atlantic, it has to be convoy ONS5. Captain Stephen Roskill, the naval historian, described it thus:-
“The seven day battle fought against thirty U-boats is marked only by latitude and longitude and has no name by which it will be remembered; but it was, in its own way, as decisive as Quiberon Bay or the Nile.”
(To be continued . . .)
Prologue
From September 1939 until April 1943, Britain had been fighting a loosing battle against the U-boat in the North Atlantic for three and a half years. In 1942 alone, over 1,000 merchant ships, or 5.5 million tons were lost, at the cost of 86 U-boats. Ships were being sunk faster than they could be replaced, and the size of the U-boat fleet was increasing.
In the first three months of 1943, North Atlantic shipping losses were 155 ships at 930,000 tons for 40 U-boats sunk. In March alone, 82 ships at 476,000 tons were lost for 15 U-boats sunk. Britain could not sustain losses on this scale for much longer.
In his memoirs Admiral Doenitz wrote “At the end of March 1943, after three and a half years of war, we had brought the British maritime power to the brink of defeat in the Battle of the Atlantic, and that with only half the number of U-boats we had demanded”.
An Admiralty review later stated “The Germans never came so near to disrupting communications between the New World and the Old as in those first 20 days of March 1943”.
But two months later, in May 1943, for the first time, more U-boats (41) were sunk than ships lost (34). In his memoirs, Admiral Doenitz wrote “On May 24 I ordered them (the U-boats) to proceed, with utmost caution, to the area south east of the Azores. We had lost the Battle of the Atlantic”.
What had happened in such a short space of time to cause this dramatic reversal of fortunes?
Well, nothing really, except for the experience gained in using the weapons and systems currently in operation. The major developments in weapons, aircraft, technology etc. were just staring to come into operation, but not in numbers at this point in time.
But probably the biggest single factor was the advanced training given to escort officers, by the Western Approaches Tactical Unit (WATU), in convoy protection and anti U-boat warfare. This course developed tactics for dealing with all kinds of attack by U-boat and situations developing from them.
The General Situation
Background
Suitable land based aircraft were not available to the Allies in quantity, and those that were had insufficient range to cover convoys the whole way across the North Atlantic. Thus there was an area, south and southeast of Greenland, which became known as the “Mid Atlantic Gap” or “Greenland Gap”, where U-boats could operate free from the danger of air attack. It was in this gap of a few hundred miles that most U-boat patrol lines were positioned ready to intercept both east and westbound convoys.
At the end of April 1943, the number of U-boats available to operate in the North Atlantic reached a high point – 193, of which only about one third would actually be on patrol at any one time. Sixty of which were in the North Atlantic, positioned mainly in the so called “Greenland Gap”. This was the highest number ever to participate in one convoy battle.
British Intelligence
Towards the end of April, Bletchley Park had one of its periodic “blackouts” when the four rotor “Triton” cipher could not be broken, so naval Ultra was not available to the Navy’s Operational Intelligence Centre (O.I.C.). This did not mean that there was no available intelligence about U-boat movements. There were other sources.
For example, every time a U-boat either left of arrived at its base, it was met by an escort which reported the U-boat’s arrival or departure to its base in a cipher that was already broken. Thus the identity and exact number of U-boats at sea was known. It also indicated which U-boats were lost by their non arrival.
A Triton signal from a U-boat, although unintelligible, gave away its position through land based HF/DF intercepts. From this, its route and speed could be estimated, and thus its probable destination.
Because U-boat signals were formalized, the length of a signal gave a clue as to its content. Thus a weather report could be distinguished from a sighting report which was different from a position report. When a convoy was sighted, the necessary sending of many signals alerted O.I.C. to the fact.
The Submarine Tracking Room used all of these sources to estimate the position and numbers U-boats and whether they posed a threat to a convoy. The main purpose of the Submarine Tracking Room at this time was to route convoys away from concentrations of U-boats.
After sighting a convoy, it could take the U-boats a couple of days to concentrate around it, generally giving the O.I.C. enough time to organize reinforcements for the convoy such as air support and escort support groups.
German Intelligence
The German radio and cryptographic service, the Beobachtungsdienst or Bdienst, had been reading the Anglo/American Naval Cipher No.3 since 1941, and had intelligence about most convoys including their sailing dates, routes and destination.
When the Bdienst informed the BdU (Befehlshaber der Unterseeboote, or U-boat Command) about a convoy’s departure date and probable route, the BdU organized available U-boats into patrol lines (sometimes called wolfpacks) across the expected routes to intercept the convoy.
When a U-boat sighted a convoy it sent a sighting report to the BdU, indicating the convoy’s speed and direction, then continued to shadow the convoy, transmitting a radio homing signal. The BdU then signaled other U-boats in the area, directing them to concentrate at a point ahead of the convoy before attacking.
U-Boat Patrol Lines
Because of the multiplicity and transient nature of patrol lines, it is difficult to keep track of the big picture of which U-boats were where at any one time.
Patrol lines were both temporary and mobile; they were constantly being formed, reformed and dispersed as convoys came and went. Each time they reformed they acquired a new name. They were not static, they moved, sweeping the ocean before them to maximize their chances of making an interception.
On May 1, in the Greenland Gap, there were 56 U-boats organized into 4 patrol lines. The following show the patrols active in the area at this time.
Patrol Line “Specht”.
Position: NE of Newfoundland, to intercept SC.127 and HX.235.
Active from April 19 to May 4.
Consisted of 24 U-boats.
On May 5 most go to “Fink” and some to “Amsel” 3 and 4.
Patrol Line “Amsel”.
Position: East of Newfoundland, southeast of Specht, to intercept HX.235 and SC.128
Active from April 22 to May 3.
Consisted of 16 U-boats.
On May 4 this split into four lines, “Amsel” 1,2,3 and 4.
Patrol Line “Star”.
Position: SE of Iceland, to intercept northern routed convoys.
Active from April 24 to May 1.
Consisted of 16 U-boats.
On May 3 all go to “Fink”.
. .
Patrol Line “Fink”.
Position: South of Greenland, to intercept SC.128.
Active from May 3 to May 6.
Consisted of 28 U-boats from “Specht” and “Star”.
Patrol Lines “Amsel 1-4”.
Position: South and southwest of Fink.
Active from May 3 to May 6.
Consisted of 4 groups of 5 or 6 U-boats.
Formed from lines “Amsel” and “Specht”
. .
Convoys at Sea
During this critical period, there were about ten convoys en route, arriving or departing, providing no shortage of potential targets for the U-boats.
Eastbound convoys from Halifax, Nova Scotia to U.K.
HX.234, HX.235, HX.236, HX.237.
Eastbound from Halifax to U.K. (Slow)
SC.127, SC.128, SC.129.
Westbound from U.K. to North America
ONS.5, ON.180, ON.181.
But, if there is one of these convoys, in its battle with the U-boats, which can be described as a turning point in the Battle of the Atlantic, it has to be convoy ONS5. Captain Stephen Roskill, the naval historian, described it thus:-
“The seven day battle fought against thirty U-boats is marked only by latitude and longitude and has no name by which it will be remembered; but it was, in its own way, as decisive as Quiberon Bay or the Nile.”
(To be continued . . .)