Reinforcements

From 30 May on: Destroyer Oribi

From 1 May on:
The 3rd SG (Capt MacCoy) consisting of the destroyers Offa, Panther, Penn and Impulsive.

From 5 May on:
The 1st SG (Capt Brewer) consisting of: sloops Pelican and Sennen frigates Jed, Wear and Spey

Between 4-6 May :

The wolfpack Amsel I of 6 boats U-107 (Kptlt. Gelhaus), U-402 (Korvkpt. Freiherr von Forstner), U-504 (Kptlt. Luis), U-575 (Kptlt. Heydemann), U-621 (Oblt. Kruschka), U-638 (Kptlt. Staudinger) ++

The wolfpack Amsel II of 7 boats
U-223 (Kptlt. Wächter), U-266 (Kptlt. von Jessen), U-377 (Kptlt. Köhler), U-383 (Kptlt. Kremser), U-634 (Oblt. Dalhaus), plus 2 homebound boats

The wolfpack Fink of 28 boats:
U-125 (Kptlt. Folkers) ++, U-168 (Kptlt. Pich), U-192 (Oblt. Happe ) ++, U-209 (Kptlt. Brodda), ** sunk already on May 6 U-226 (Kptlt. Borchers), U-231 (Kptlt. Wenzel), U-260 (Kptlt. Purkhold), U-264 (Kptlt. Looks), U-270 (Kptlt. Otto), U-358 (Kptlt. Manke), U-378 (Kptlt. Mäder), U-381 (Kptlt. Graf von Pückler und Limburg), U-413 (Kptlt. Poel), U-438 (Kptlt. Heinsohn) ++, U-514 (Kptlt. Auffermann), U-531 (Kptlt. Neckel) ++, U-533 (Kptlt. Henning), U-552 (Kptlt. Popp), U-584 (Kptlt. Deecke), U-614 (Kptlt. Sträter), U-628 (Kptlt. Hasenschar), U-630 (Oblt. Winkler) ++, U-648 (Kptlt. Stahl), U-650 (Oblt. Witzendorff), U-662 (Kptlt. Müller), U-707 (Oblt Gretschel), U-732 (Oblt. Carlsen), U-954 (Kptlt. Loewe)

* U-boats that fired torpedo or used the deck gun

After the successful convoy battles of March, most u-boats were on the end of their patrol endurance and had to return to base for supplies and repairs. Hence, with lesser u-boats at sea and reinforced convoy escorts, results in April dropped by half. At the end of the month, most boats got out to sea again and together with the new boats arriving directly from Germany, u-boat strength on the North Atlantic convoy routes reaches a new peak.

Also at its peak is the then ongoing intelligence war. Both sides have succeeded in breaking into each other secret codes : the Allied 'ULTRA' code breakers are in general more quicker to decrypt Germans messages and are able to route convoys around u-boat packs, much to the frustration of the Germans, who at that time are provided with much information about convoy routing by the B-service.

On 25 April however, there is a temporarily 'black-out' in the British code breaking, when the Germans unexpectedly change their codes. The next day the patrol line Star convoy is formed to catch an expected westbound and this movement is not detected in time in order to reroute ONS-5.

ONS-5 was routed on a northerly course in order to get for a longer period air escort from aircraft based at Iceland. The more recent convoy battles took place more to the south so it was also hoped that this route is clear of wolf packs. On the 24th the air escort sinks U-710 who happened to come close to the convoy on its way out of Germany to the Atlantic.

The convoy runs into the patrol line on the 28th and is reported by U-650. The contact reports from U-650 and other boats trying to close are detected by the HF/DF of the escorts so they are warned that a wolf pack attack is mounted. Trying to anticipate, the convoy takes evasive action and the escorts make sweeps towards the HF/DF bearings.

Only 4 other boats manage to come up in the evening and during the night their attacks are unsuccessful: only U-532 is able to launch but misses and in return, U-386 and U-532 are heavily damaged by the escorts and have to return to port.

On the 29th U-258 sinks one ship in a daylight submerged attack, but U-528 is damaged by the air escort and is already the third boat that returns damaged to port. As a reaction on the attacks, the 3rd SG and also the destroyer Oribi (from SC-127) are directed towards ONS-5 in order to reinforce the escort.

During the next days the weather deteriorates and only U-192 manages to attack, but is unsuccessful. In the heavy and difficult weather contact is lost. The gale gives the Allied also their share of trouble: it is impossible for the convoy escorts to refuel at sea and the convoy is scattering. Eventually 6 ships became stragglers and another 6 are rerouted apart from the main body, escorted by Pink.

Meanwhile the B-service has located the eastbound convoy SC-128 and focus is shifted towards this more valuable target. Nearly all available boats are concentrated to intercept this convoy: the gruppen Specht and Start together with other boats establish the new patrol line Fink on the expected route of the convoy. Fresh boats coming from France form another patrol line Amsel who is subdivided in 4 smaller groups Amsel 1-4.

Although SC-128 is sighted by U-628 on May 1st, the convoy manages to avoid all U-boats by the aid of Enigma decrypts. Aircraft are sent out to sweep the u-boat concentrations and a Canso damages U-209 who is on its way to its station in the patrol line and goes missing several days later. U-438 is also damaged by aircraft.

In the evening U-628 does not report the eastbound and expected SC-129 but the westbound ONS-5. In the heavy gale, this convoy has advanced only 20 miles a day and now runs into the patrol line from the other side. The BdU reacts at once and directs the gruppen Fink, Amsel I and Amsel II towards ONS-5.

Meanwhile at ONS-5, the Oribi and the 3rd SG have arrived and most of the scattered ships are back in the convoy. Only 4 ships have fallen far behind and are following a separate route, escorted by the Pink. On 3 May the destroyers Duncan, Impulsive, Panther and Penn and the trawler Northern Gem leave the convoy because of fuel shortage. From the increasing number of HF/DF receptions in the vicinity of the convoy it is understood that the u-boats are tightening again their grip on the convoy and the 1st SG is sent as reinforcement.

The attacks start already before night, when U-125 sinks a straggler. At night the escort at first manages to keep the u-boats at bey. U-270 and U-732 are damaged by depth charges from Oribi and Snowflake, and Vidette respectively. Both boats have to make for port. U-514 is damaged by Vidette and has to retire from the battle but resumes patrol after some days.

But finally the defence collapses and the boats get through: U-707 attacks from the front, dives under the convoy, emerges on the rear and there sinks a straggler. Then U-628 gets to the convoy through a gap in the escort screen, launches 5 torpedoes at 5 targets, but damages only 1 ship. (This ship is finished off by the same u-boat the next day.) Only minutes later U-264 launches also 5 torpedoes and is more accurate: she sinks 2 ships with 2 hits each. Finally U-258 moves in and sinks 2 ships with 3 torpedoes.

At the U-boat headquarters, Dönitz is leading the battle in person. Smelling a possible big victory after 6 weeks of disappointing results, he continuously presses his u-boat commanders to press home an attack, whenever an occasion occurs. Indeed the escorts are now operating so effective during night battle that submerged daylight attacks seem more promising.

So during daytime the attacks continue and the escorts are very busy, to the extent that most are short on depth charges or/and fuel. 1 ship is sunk probably by U-638, but this boat is sunk in the counterattack of Sunflower. Towards evening U-266 sinks 3 ships with 4 torpedoes but is damaged by Offa.

Some boats get in contact with the group of the Pink. While this corvette is giving the first attacker U-358 a plastering that forces the u-boat to limp back to port, U-584 gets to the unprotected group and sinks 1 ship.

Towards evening a B-24 appears and provides cover for a little time but cannot avoid the massive build-up of u-boats around the convoy. Before dusk Tay alone has already 7 boats in sight, and pressed on by Donitz at least 15 boats are in contact.

Then the convoy reaches a thick fog bank and at once the tactical situation is reversed completely. While the U-boats expected to overrun the escorts by their large number, the escorts can now detect the U-boats easily and in time with their radar. All ships are equipped with the new Type 271M radar, working on yet smaller waves for which the German 'METOX' detector gives no warning.

From the evening until next morning, the boats make around 25 attacks, but they are all repulsed in time by the escorts : Shortly before midnight, Vidette gets U-531 on the radar and destroys the u-boat with 2 depth charge patterns when it belatedly makes a crash-dive. Vidette goes on to chase U-707 and other radar contacts but with no result. Then towards morning a hedgehog attack on an ASDIC contact results in the destruction of U-630. Loosestrife makes first an unsuccessful attack with D/C on the U-575 and then gets U-192 on the radar. Just as the corvette appears out of the fog at 500 meters, U-192 launches 2 torpedoes at it but they miss. The u-boat is destroyed on the surface with a shallow set D/C pattern. Snowflake repels U-107 with some D/C and gets another 4 contacts on its radar during this action. While engaging some of these with gunfire, it forces U-125 to scuttle while it has been rammed and immobilized by Oribi. Offa makes 5 attacks before midnight, but without inflicting any damage. Only towards morning it slightly damages U-223 with gunfire and D/C. Sunflower makes numerous attacks on radar contacts around her, is fired at two times by the u-boats, but has to wait until morning to see some result: U-533 is rammed but the boat escapes damaged and is able to resume patrol after repairs.

When the 1st SG arrives in the morning, they disperse around the convoy as a distant screen before joining the close escort and drive off all remaining boats lurking around the spot. Pelican follows a radar contact and due to fog is able to close in to 300 meters before being spotted by its target. The surprised U-438 crash-dives too late and is destroyed by 2 shallow set D/C patterns. Sennen goes to the Pink-party and on its way attacks U-650 and U-575 with D/C and hedgehog, but does not inflict any damage. Spey takes up station behind the convoy and drives of U-634. The U-boat escapes but is hit twice by gunfire.

When towards morning the full scale of the disaster becomes clear to the BdU, the operation is immediately broken off. The boats are regrouped but only half of the remaining boats are in a condition to battle. The Germans lay the blame of the failure on the fog and the general fact that most results are usually obtained in the first night of the battle when the u-boats have the advantage of surprise. More worrisomee however is that a very large wolf pack has been defeated by surface escorts only. It is also remarkable that only half of the boats did make contact with the convoy and sometimes seemed reluctant to press home an attack. As for the fog, it should have been anticipated as this these were the usual weather conditions in this region of the Atlantic where the cold and hot gulf stream mix.

The convoy lost 12 ships, as it turned out, the last time that a convoy would lose a majorr number of ships. For these 12 ships, the Germans lost 6 u-boats in the battle, to which could be added U-710 and U-209. Another 7 boats had to abort due to severe battle damage. So this convoy battle is often seen as the turning point in the battle for the Atlantic. In fact, in the 3 weeks following this disaster, the Germans were unable to mount an attack on any other convoy but continued to suffer severe losses. This would prompt Donitzz to recognize his defeat and on 23 May he halted all convoy operations and recalled his boats from the North Atlantic convoy routes.