Anti-Submarine Warfare in the North Atlantic During World War 2

Allied shipping in World War Two suffered great losses at the hands of enemy submarines, particularly of the German Kriegsmarine which were named Unterseeboots (under sea boats or U-boats). When asked to describe German submarines Winston Churchill said “U-boats are those dastardly villains who sink our ships,” ( – Gannon 1998). From the start of hostilities against England in 1939 until the end of 1942, German U-boats sent 2,780 merchant ships totaling more than 14 million tons to the bottom of the Atlantic. During the first half of 1943 however, Allied anti-submarine tactics caused a major shift in their favor. Between May and August 1943, the Allies sunk more than 120 U-boats and shipping losses were drastically reduced (Tarrant p.14). By the end of the war, the Allies had managed to sink more than 700 U-boats costing Germany some 33,000 U-boat crewmen (Botting p.6). It is the intent of this paper to explain what brought about the sudden reversal which allowed the Allies to defeat the Germans in the North Atlantic.

The success of the Allies defeating the Germans in the North Atlantic did not come from one solid factor; it instead came as a result of a combination of three factors. The first, was the development and use of technology such as radar and anti-submarine weapon systems such as the “Hedgehog”. The second, was of convoy tactics using sonar and airplanes to repel submarine attacks. The third, was the increased intelligence gathered as a result of breaking the German naval code. It was the coming together of those three factors in early 1943 that allowed the Allies to be successful in repelling the U-boats from the North Atlantic, and left the first officer of U-230 asking, “What caused this sudden stream of messages that told us of nothing but dying?” (Botting p.145). We will first look at how technology influenced the Allied victory.

It was common practice for U-boats to remain on the surface as long as possible. This was because the main diesel engines were not able run under water. There was no source of oxygen to feed the combustion engines, and there was no effective way of venting exhaust. Moving the boat while submerged required the use of electric engines, which had a limited battery capacity. Instead of remaining in relative safety underwater, U-boat captains chose to remain on the surface to conserve battery power. It was because of this limitation that the most important and deadly piece of anti-submarine technology during World War Two was developed. This was the development of radar.

Early radar systems used large wave radio signals to locate distant objects. The system would emit a radio signal and measure the amount of time it took for the radio signal to be reflected back by the distant object. Surface escorts and airplanes had great difficulty in finding U-boats during the night or bad weather. Jurgen Oesten, the commander of U-61, said that “At night, if you were closer to your target than 3000 to 4000 meters…You offer only a small silhouette to your target, almost invisible,” ( Because the radar’s radio signals were not affected by weather or darkness it allowed pinpoint accuracy in determining the location of U-boats. Allied airplanes equipped with radar and a Leigh Light (an ultra-bright spotlight that would illuminate the surface at night) proved to be especially effective in the later years of the war. The combination forced U-boats to submerge more often and be on their guard constantly.

In 1943, Germany countered early Allied radar by equipping its U-boats with receivers that could detect the large wave radio signals (Metox receivers). But the Allies soon phased out large wave radar and equipped their escorts and patrol airplanes with short wave radar. Short wave radar was essentially no different from long wave other than the fact that it used shorter radio wave lengths to detect distant objects. The use of short wave radar rendered the U-boat radar receivers useless. It wasn’t until the end of the war that Germany had the ability to detect short wave radar, but by then it was too late. Another radio-based invention that proved to be deadly to German U-boats was the High Frequency Direction Finder.

The High Frequency Direction Finder (Huff-Duff) is a radio direction finder, which enables the user to pinpoint the direction of radio transmissions. U-boats were often placed in groups of 10 or more boats called “Wolf Packs” to increase their effectiveness against enemy convoys, and their main method of communication was by simple ship-to-ship radio. Escorts equipped with “Huff-Duff” could accurately see which direction enemy transmissions were coming from. Using “Huff-Duff” from land bases and other escorts enabled escorts to triangulate the exact location of enemy submarines. Combined with radar, the use of “Huff-Duff” by the Allies proved to be especially effective in finding and deterring the enemy away from Allied convoys. Merchant losses by the Allies dropped from over 2 million tons every four months at the end of 1942, to just under 300,000 tons every four months by the end of the war (Tarrant p.14). The reduction in merchant losses was in part due to the ability of escorts to prevent penetration of U-boats to the convoy. A larger reason why merchants weren’t falling victim to U-boats was because large numbers of U-boats were being sunk.

The detection of U-boats was an important part in repelling them, but the Allies needed weapons to remove them for good. At the same time radar and “Huff-Duff” were appearing in greater numbers, new anti-submarine weapons technology began wreaking a devastating blow to the U-boat arm. New inventions such as the “”Hedgehog”” and “”Squid”” were developed to counter the inaccuracy of traditional depth charges.

A World War Two U-boat’s main advantage was its ability to remain hidden underwater, where it would strike at its target. As such, the U-Boat was never conceived to be much more than a hit-and-run vessel to wreak havoc on enemy merchants and warships. They were fitted with weaponry to allow them to achieve their immediate goals, but were never equipped to fight in a traditional surface-to-surface manner. As such, submarines were known to be very vulnerable on the surface. Technology such as radar and “Huff-Duff” enabled the Allies to engage submarines in a more traditional manner, but a good submarine crew could dive their boat very quickly if they needed to. U-boats were much more difficult to destroy once they were underwater and the use of depth charges had long been the only method with which to fight against a submarine once it had submerged.

Depth charges were extremely inaccurate once they left the deck of the destroyer. Depth charges used a firing pin which would detonate the charge after a certain depth was reached or a set amount of time had passed. Sonar was in wide-spread use by escorts during the entire war and a single escort using sonar could reveal a U-boat’s approximate position, but it could not reveal its depth in any great detail. The escorts would be forced to make an educated guess when setting the depth charge’s firing depth, because they would lose contact with the submarine once they were directly overhead. Veteran U-boat commanders knew that they could avoid the charges by diving or changing course at high speed. Depth charges released such a violent explosion that it was often possible for the U-boat to escape while the explosion confused the escorts sonar. In 1943, the Allies solved this problem in two ways. One was by adding two additional escorts on a U-boat attack (which we will discuss later), and the other was with the development of the “Hedgehog”.

The “Hedgehog” was put in to service in 1943 and drastically changed the ability of escorts to destroy enemy U-boats. “Hedgehogs” used 25 small mortars that were fired from the front of the escort once the ship was about to pass over the U-boat. The distinct advantage that “Hedgehogs” provided over depth charges was instead of relying on time or depth firing pins, they did not explode until they made contact with the submarine’s hull. “Hedgehogs” were sometimes magnetized and would be attracted to the enemy’s hull once they passed close enough.

An action report concerning the sinking of U-515, states that, “At 0813 PILLSBURY fired “Hedgehog” and 2 explosions were observed, bringing up debris. Sound contact was immediately regained contact and attack resumed”

( The action report reveals another advantage of the “Hedgehog”, in that is the mortars were a fraction of the size of a standard depth charge. An escort’s sonar would not be confused once the mortar had exploded. This allowed escorts to continue to track the U-boat and severely limited its ability to escape. The same year the “Hedgehog” came in to service, another weapon called the “Squid” was also arriving.

The “Squid” is a three-barreled mortar system which fires three depth charges from the front of the boat. The advantage the “Squid” had over normal depth charges was that it was directly tied in to the escort’s sonar system. The “Squid” would fire when the sonar believed it was appropriate to do so. It would set the depth firing pin automatically according to the sonar information and fire one salvo of depth charges. Each salvo contained two charges fired at an angle and one which was fired straight ahead. This created a triangle of depth charges. After a few seconds, a second salvo would be launched in the same fashion as the first. Once the first salvo of charges reached its detonation depth of 25 feet below the sub, it would detonate all 6 depth charges. The resulting blast wave would crush anything inside of the triangle of charges. The “Squid” proved so effective in defeating U-boats that Allied escorts soon replaced the “Hedgehog” system for the much more destructive “Squid”.

Developing technology greatly improved the ability of the Allies to wage war on the U-boat arm. The development of both the “Hedgehog” and “Squid” systems, when combined with radar and “Huff-Duff” could take away the U-boat’s ability to wage any sort of warfare on the Allies. But the development of anti-submarine technology was not the only reason the Allies were successful against the U-boats. Changes in convoy tactics and the increased usage of airplanes also played a large role.

Merchant convoys were used by the Allies since the beginning of the war. The large merchant convoy was in widespread use during World War One and was immediately adopted by the Allies after the start of World War Two. At the start of the war, Convoys were poorly defended. Convoys usually contained no more that five total escort vessels and sometimes even fewer (Kuenne p.113). The escorts would promptly leave the convoy once their operational mileage had been met. This created many problems, because arriving escorts were usually be late. This left convoys open to attack from German “Wolf Packs”. A wolf pack of 10 or more boats could easily subvert any anti-submarine tactics presented by so few escorts. One strategy that the Allies employed to counter this problem was to simply increase the number of escorts in a convoy.

By the start of 1943, most convoys employed no less than 2 sloops, 6 support frigates, multiple armed trawlers and 4 destroyers. They also added supply tankers to each convoy to extend the operational range of the escorts. Each convoy was also joined by at least one aircraft carrier. By the end of the war, Britain had 6 aircraft carriers whose sole purpose was to aid in the defense of convoys (Middleton p.84). Each carrier had Royal Air Force high-speed airplanes which could intercept any U-boats found threatening a convoy. The operational range of the Royal Air Force fighters shortened the distance Allied convoys had to travel with no air cover.

A method known as “swamping” was quickly adopted after airplanes became readily available to Allied convoys. “Swamping” involved packing an area where a U-boat was know or suspected to be operating with aircraft and escorts. The aircraft and escorts would then search continually and systematically until the U-boat was found and destroyed (Tarrant p.118). The practice of “swamping” proved to be so successful that Royal Navy historian S.W. Roskill states that “…it was the defensive strategy of sailing ships in convoy and of providing those convoys with powerful surface and air escorts which did most to accomplish that decisive victory [over the U-boat]” (Kuenne p.111).

Herbert Werner of the U-230 describes a “swamping” attack: “The [incoming] small aircraft grew enormous fast…neither the mate nor I [were] able to fire a single bullet…Four bombs in a row erupted alongside our starboard saddle tanks.” After the first airplane left the scene, another showed up to drop its payload, and then another. During the attack Werner remembered that his submarine received radio distress signals from three other U-boats. He later went on to say “With a shudder, I pictured what would happen to us once our hull was cracked” (Botting p.153).

Including more escorts in to convoys proved to be useful in locating an enemy submarine as well. One escort using sonar was at a severe disadvantage in finding submerged U-boats. The depth finding abilities of the attacking escort was limited. Once the Allies increased the numbers of escorts, many convoys applied a 3 escort attack technique. One escort would directly engage the submarine, while the other two escorts would stay away from the battle zone keeping the submarine in sonar contact (Showell p.137). Each escort would communicate information back and forth while hunting the submarine. It was this 3 escort hunting technique that made the “Hedgehog” and “Squid” weapons so effective. Over 400 of the total submarines sunk during the war were a direct result of the advancement in anti-submarine technology and changes in convoy tactics employed in 1943. Despite those advances, perhaps nothing had a greater impact on the total defeat of the U-boat arm than the breaking of the German naval code.

In December 1942, British crypt-analysts succeeding in breaking the German “Triton” code which allowed the Allies to read encoded naval messages. A great portion of the decrypted messages provided the Allies with intelligence regarding the departure times and dates of U-boats, the number of U-boats at sea, and their movements and operational orders (Tarrant p.13). The greatest advantage given to the Allies from the deciphered information was their resultant ability to divert convoy traffic away from waiting U-boats. This reduced the number of merchants ships sunk and increased the chances of escorts to find and destroy the waiting U-boats.

After the sharp decline in merchant sinking began to take hold in 1943, the leader of the U-boat arm, Admiral Karl Donitz, wrote in his war diary that the sudden decline “cannot depend on chance…Is it possible that from some source or other the British obtain information on our concentrated formations?” (Van Der Vat p.221). Donitz and German intelligence did not believe that the “Triton” code could be broken due to the sophistication of the encoding machine (the Enigma). What he didn’t realize was that two Enigma machines had been captured from U101 and a captured German weather ship. He and his staff firmly believed in the inability to crack the encoded messages; he later went on to write that the British might be gaining the information “by decrypting our radio messages…[which is] out of the question…or by radar” (Van Der Vat p.221).

German technological advancements were also discovered from these decrypted messages. Metox radar detectors were installed on most German U-boats to counter the Allied long wave radar. The Allies knew that the Germans knew how to counter their long wave radar and were quick to develop the short wave radar which they knew the Germans believed was impossible. The Germans also developed a new kind of torpedo which would listen for a ship’s propeller and home in on the sound. After the Allies learned of the new torpedo from the intercepted messages, they developed the “Foxer”. The “Foxer” was towed by ships and emitted loud noises which confused the new acoustic torpedoes (Van Der Vat p.306). An unintended benefit from the “Foxer” was that the noise emitted from the device often scared the men in the U-boat. A report from the U.S.S Frost states that after the capture of prisoners from U-490, many of the captured stated that, “They were scared out of their wits by our [“Foxer”] gear” (

Winston Churchill stated that “All was ruled by that harsh and despotic factor, shipping,” ( – Churchill 1950). And as we have seen, the reversal brought on the by the Allies in 1943 gave them an advantage over the Germans. This was not only true in the battle for the North Atlantic but in the entire war. The decrease in merchant ships being sent to the bottom of the ocean had a great impact on the total war effort, but that reversal did not come as a result of any one change. The development of technology such as radar and the “Hedgehog” gave the Allies a greater ability to detect submarines and dispatch them. Changing tactics using sonar and airplanes decreased the access submarines had to merchant convoys. And increased intelligence gathered as a result of the breaking of the German naval code allowed the Allies to intercept vital operational information. There were invariably more factors that contributed to the success of the Allies, such as the entrance of the Unites States in to the war, but the three factors expounded here provided the technological and tactical information the Allies used for victory. It was the coming together of these factors that sealed the defeat of the U-boats in World War Two.

Reflecting on the war, Winston Churchill said “The Battle of the Atlantic was the dominating factor all through the war. Never for one moment could we forget that everything happening elsewhere, on land, at sea, in the air, depended ultimately on its outcome… Many gallant actions and incredible feats of endurance are recorded, but the deeds of those who perished will never be known. Our merchant seamen displayed their highest qualities and the brotherhood of the sea was never more strikingly shown than in their determination to defeat the U-boat,” ( – Churchill 1962).

About Matthew Schroder

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