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Anti-submarine tactics in the First World War

Written by Sakhal

In the spring of 1917, the offensive of the U-boat reached its peak point, threatening with forcing England to starvation. However, the development of anti-submarine weapons, such as depth charges, and the adoption of defensive tactics, particularly the convoy system, caused tremendous damage to the submarine weapon, which went from being the chaser to being the prey of surface ships.

In 1914, the only effective methods to counter the submarine threat were gunnery and ramming. However, these methods could be used only if a submarine were sighted while navigating in surface or localized in depth, which was unlikely to happen. In total, only fourteen U-boat were sunk through ramming, which caused as well damage to the attacking ship. In 1918, the old destroyer HMS Fairy was sunk because of the damage sustained when ramming the UC75. Another two - purely defensive - methods existed: the mine field and the anti-torpedo net. In order of precedence, nets were used first, fitted with an alarm system; later this one was fitted with small mines which, attached to the net, exploded when making contact with the trapped submarine. The British spent a lot of time developing an effective mine, but when the H2 finally became operative in 1916, it demonstrated to be a terrible weapon against the U-boat, sinking circa the 25 percent of the total.

Submarine weapons

Depth charges were devised in 1916 for allowing surface units to attack submerged submarines; they were 136-kilogram bombs fitted with an hydrostatic device which made them to explode at a certain depth. Then numerous systems were studied for localizing a submarine before it could attack; the result was the development of diverse types of hydrophones, rater simple contraptions which working in horizontal direction could detect the noise emitted by the electric motors of a submerged submarine. In July 1916, the motorboat HMS Salmon achieved the first success when sinking the UC7 with the help of either hydrophones or depth charges. At the end of 1918, numerous scientists began to work in a new system of submarine localization; from these studies it was born the ASDIC (Anti-Submarine Devices Investigation Committee), device which during the Second World War would be very important to save England once again.


The MK III depth charge of D type demonstrated to be very effective, but it could not be prepared until 1916 and it was available, in large quantity, only in 1917. The hydrostatic device which triggered the explosion, at a certain depth quota, had to be inserted by hand; only then the charge was rolled on the deck astern and released into the water. Known by the German as "Wasserbom" or "wabo", it was capable of destroying a U-boat from a distance of 15 meters.


The British company Thornycroft built a small mortar capable of launching depth charges at a distance of 70 meters. With two launchers, located astern, and a rail, for releasing depth charges into the water, surface ships had more possibilities of destroying a submarine.

Surface ships used a very simple method when facing the submarine threat: to follow a zig-zag course, because in that time the commander of a U-boat had to calculate the course and speed of enemy ships through simple estimation. Any error, even the slightest one, caused the torpedo to miss the target. For further deceiving the U-boat, surface ships were fitted with a floating device which stirred the water on the prow to give the impression of a cruising speed higher than the real one. Moreover, numerous mimetic colors were used, which applied to some areas of a ship could confuse even more the commander of a submarine regarding the course of the ship. Also submarines were camouflaged by using a wide range of colors which rendered difficult their localization and, in some cases, the U-boat hoisted, by means of a special device, a sail rigging in the attempt of being mistaken with fishing ships.


Zig-zag navigation was important for surface ships which, operating in such a way, had success in avoiding the torpedoes launched by the U-boat. This kind of navigation had to be studied and regulated beforehand to avoid collisions between the ships of a convoy. On the other hand, the explosive towing was a contraption towed by a destroyer; it was fitted with an indicator whichsignaled eventual obstacles and its explosive charge was detonated from the very destroyer.


These coastal ships, of rather inoffensive appearance, were actually warships. The corvettes HMS Gardenia (bottom) and HMS Polyanthus (top), initially designed and built as conventional escort ships, were transformed into bait ships, wholly camouflaged as depicted here; on the other hand, the sloop PC69 was turned into a P-boat (center). They were armed with two types of cannons, of caliber 76 and 102 millimeters, depth charges and bomb launchers; they differed from the Q-ships in that they were actual warships. Their armament was not visible from outside, for the cannons amidships and astern were covered by screens.

Depth charges demonstrated to be the best weapon against a submerged submarine, but some other anti-submarine weapons had been created earlier. One of them was an explosive charge, kept at a certain depth by a special floater and towed by a destroyer. When an object hit the charge an indicator sent a warning to the towing ship, from where the charge could be electrically triggered. Another anti-submarine weapon consisted of an explosive screen, towed at high speed by a destroyer with the hope that a submarine were hit by it. None of these methods was approved by any commander, because it was possible that the towing cable got entangled in the propellers and the explosive charge were brought to explode just beneath the ship.


The screen - variant of the explosive towing - was towed at a sustained speed by a destroyer. When in contact with a submarine it exploded or was exploded through command. Such devices had a bad reputation among the commanders of destroyers, who feared that the cable got entangled in the propellers and the charge exploded against the very destroyer.

Before arriving to proper depth charge launchers which represented the last technical development in the field of anti-submarine warfare, they were adopted a series of devices which could launch bombs at a certain distance in the area where a submarine had been seen for the last time. Firstly, it was created a 190-millimeter recoilless howitzer, a breech-loading weapon which fired spherical bombs; being easily maneuverable and having a weight of 1800 kilograms, it could be installed even onboard small trawlers. The 254-millimeter launcher was another muzzle-loading weapon capable of launching either normal projectiles or 91-kilogram spherical bombs. Another weapon was the 280-millimeter breech-loading mortar, which fired 160-kilogram projectiles at a distance of 2800 meters; due to its size it could be installed only onboard destroyers and, since it could fire only conventional projectiles, it was useful only when a submarine was visible in the surface.


The 190-millimeter bomb launcher was one of many weapons produced before the introduction of depth charge launchers. It was capable of launching its bombs at a distance of 1900 meters with an elevation of 45 degrees, but it was not particularly useful given that its projectiles, lacking an hydrostatic fuze, could not be set to explode at a certain depth. It was effective only when a submarine was hit while in surface or immediately after immersion.

The most spectacular anti-submarine weapon was the "Q-ship", an apparently inoffensive merchant ship which was fitted with a powerful armament, concealed until an enemy submarine showed up. The first victims of the Q-ship were easily deceived but then the deception became more difficult as the commanders of the U-boat knew the existence of these ships. To further improve the deception new techniques were elaborated; for example, the crew of a Q-ship, pretending to be terrified, abandoned the ship wearing life vests, leaving onboard the gunners on their cannons; if the submarine approched the ship the cannons immediately opened fire. Since the U-boat could launch a torpedo instead of using their cannon, Q-ships were filled with wooden boards and beams, which would allow the ship to remain afloat for a longer time, with the hope that the submarine emerged and so the onboard cannons could attack her.

In 1917 a special Q-ship was launched, the HMS Hyderabad; she was armed with one 102-millimeter cannon, due 76-millimeter cannons, four bomb launchers, one torpedo launcher and numerous depth charges, and her draft was only 2.06 meters, so a torpedo could pass beneath the hull without hitting it. Many units of the Flower class were remodelled as Q-ships and accordingly renamed as Flower-Qs. Also, twenty P-boat units were reconverted as PC-boat; these were actual warships to which, for deceiving enemy submarines, a wide range of modifications was added and a false name was assigned.

The trap of the trawler

A variant of the deception technique was adopted in 1915, as soon as the U-boat began to attack the British trawler fleet along the Scottish coast. Each group of trawlers was escorted by a trawler of the Royal Navy, disarmed and properly reinforced in their structure; this had the purpose of towing an old C-class submarine to which the trawler was connected via phone. In the event of aggression, the trawlers of the Royal Navy would transmit via phone the exact position of the attacker; then the allied submarine, after having released the towing cable, would have tried to reach the best position to attack the U-boat. The first time when this experiment was attempted, in June 1915, the submarine C24 did not manage to release the towing cable but, even if bridled by a 188-meter cable dangled to her prow, she managed to torpedo the U40.


Through this method two U-boat were sunk in 1915. A trawler towed a C-class submarine and, when a U-boat attacked with her onboard cannon, the trawler gave the position and course of the attacker via telephone. Once briefed about the position of the U-boat and after having released the towing cable, the C-class submarine attempted to torpedo the U-boat.

Circa one month later, the Allies benefited from another chance, with the unit C27; this time the phone contact between the towing ship and the towed submarine failed but, even so, the commander of the latter, by using his imagination and reasoning, managed to realize what was happening in the surface and, once the proper firing position was reached, to sink the U23. But the most important problem, how to localize a submerged submarine, still remained unsolved and, in April 1917, the losses registered by the Allies clearly demonstrated the failure of all the countermeasures adopted until then. The fundamental problem was the impossibility of providing a good escort in a space as large as the ocean.

A response was the convoy system, the navigation in group of merchant ships escorted by warships, the classic system used since the 14th century to avoid assaults from pirate ships. But, for a number of reasons, the naval strategists of the 20th century could not accept the fact that such a method, which had demonstrated its validity also during the Napoleonic wars, could be still regarded as valid in the era of the steam machine and the battleship. Not later than January 1917, the Naval High Staff had in fact declared that convoy navigation would not have the power to stop the submarine weapon; this declaration was made public notwithstanding the fact that the Grand Fleet had been declared, since the beginning of the war, to be immune to submarine attacks, just because it always moved in formation like a proper convoy does.

During 1917, nobody reinvented the convoy system, but it is due to acknowledge that it was insistently recommended to the British Prime Minister, Lloyd George, by Cabinet Secretary Hankey. The first change was verified in February 1917, when, following the insistence from the French, it was decided that the merchant traffic on the English Channel were organized through the convoy system. April 1917 was the worst month for the British merchant units and the best for the U-boat, which sank hundreds of thousands of tonnes of ships. The first convoy set out at the end of that terrible month and, after thirty days, the Allies were already certain that the losses had been reduced from 25 to 0.24 percent. The system worked just because the enemies of the U-boat were all concentrated in the same place, which forced the submarines to converge toward the very convoy, in an area where they in turn could be attacked by the Allies.

We have, in this regard, numerous testimonies: diverse U-boat commanders indeed stated that, before the introduction of the convoy system, their submarines were stationed in strategic points, waiting for an isolated ship to pass by; but when the convoy system was introduced, it seemed like the sea was suddenly depopulated of ships and when one eventually appeared she was surrounded by destroyers, corvettes and other ships of any size and type. The convoy system set in crisis the U-boat, which from that moment had to operate defensively. They were tested in a hard way even in the vicinity of their base, Heligoland Bight and the Belgian coast, where the Allies laid numerous mine fields. The British mine layers, on the other hand, deployed barriers of mines, either conventional or magnetic, in all the access routes, and from that moment massive sinkings of German submarines began. Although the huge Anglo-American North Barrier stretched between Norway and the Orkney Islands served little for sinking the U-boat, the Dover Barrier, in 1918, became totally inaccessible to the passage of enemy submarines. In the Mediterranean, the Allied tried to block the German and Austrian submarines in the Adriatic, by means of the Otranto Barrier, but they achieved little success due to the depth of the water.

Overall it can be stated that, without any doubt, the convoys forced the U-boat to face major risks, either by forcing them to enter mined waters or by intercepting the continuous flow of messages that they had to exchange for organizing the attack against the convoy. The ships which traveled in convoys amounted a total of 84,000 tonnes, of which only 257 (the 0.4 percent) were sunk by the U-boat; on the other hand, during the same period, not less than 2,616 ships which operated in isolation were sunk.


These charts illustrate the effects of the convoy system on the U-boat. The number of merchant ships sunk by submarines fell dramatically (top) in direct relation to the number of ships which traveled in the convoys, and despite the number of U-boat in the sea (bottom).

Another anti-submarine weapon which was developed during the First World War was the aerial one. Technically, the first submarine sunk after an aerial attack was the British unit B10, in the port of Venetia, in August 1916, but the first actual attack against a submarine on the high seas was carried out by two Austrian seaplanes which attacked the French unit Foucault, off of Cattaro, the 15th September 1916; the submarine, severely damaged, was forced to resurface. The British, during the First World War, created the "Blimp", a small plane that demonstrated to be particularly valid for anti-submarine patrol, especially when the convoys began to navigate. Another weapon particularly adapted for anti-submarine warfare was devised by the British: the Davis recoilless cannon, which installed onboard aircraft demonstrated to be extremely effective.

Which defenses could the U-boat adopt to face all of these new offensive weapons? They had to confront armed merchant ships, Q-ships (bait ships), normal warships and countless other small units created on purpose for anti-submarine warfare. The first days of war demonstrated that a submarine was always able to escape the attack from a surface ship if this one was sighted with a certain anticipation. Anti-submarine nets did not constitute a severe problem, for a submarine fitted with a powerful wire cutter on her prow was almost always capable of passing through them. Periscopes were gradually improved; in a relatively short time they came from being single devices fitted with a single visual field to being more complex devices fitted with two different visual fields, one for observation and search and another one for attack. Observation periscopes were fitted with lenses of great angulation which granted the maximum visual field, whereas attack periscopes had a very restricted visual field; and, toward the end of the conflict, periscopes for aerial observation were produced as well.


The growing danger of aerial attacks against submarines compelled the technicians to create a periscope for maritime and aerial observation, capable of providing a visual field of up to 80 degrees in elevation. Submarine operations, always very complex, also pushed technicians to create a periscope fitted with two different visual fields, one of large field for observation and another one of narrow field for attack.

Aircraft could be counterattacked with the onboard cannon, but the anti-aircraft weapons of that time were totally primitive, even more than the techniques of aerial bombardment, so a combat between a submarine and an aircraft was solved only after a lucky hit of either contender. The most severe problem for a submarine still was that of the short autonomy during the state of immersion and, for solving it, ever more powerful electric batteries were created. Although the onboard cannon, already installed in almost all the submarines which took part in the conflict, was extremely useful for sinking minor ships, allowing to save torpedoes, in 1917 the Germans, after the beginning of the convoy system, were forced to use torpedoes almost exlusively, for it was not possible for the U-boat to cannonade an entire convoy.


Due to the modest size of submarine units it was not easy to introduce a torpedo onboard a submarine. With the help of a winch, installed temporarily in the deck, the torpedo was inclined and introduced through a small hatch into a duct to be then transported to the torpedo chamber. Among the many inventions patented during the First World War there was also a torpedo mine, projected by a certain Myers. It was a torpedo which, once its course was complete, automatically released a mine.

Numerous submarines, those of larger size, were fitted with supplementary torpedoes; the UEII class had twelve assigned. At the beginning of 1915 the British unit E14 went out on patrol, in the Sea of Marmara, with supplementary torpedoes bound to the deck, but this was an exceptional measure which was not subsequently encouraged. The war demonstrated that submarines were capable of operating in precarious conditions, much worse than those which had been theoretically imagined before the war. The conning tower was modified with the purpose of granting a greater safety to the crew, but the submarines, generally, demonstrated to be much more resistent and compact than many surface ships.


The German 150-millimeter L45 cannon was installed in the U-Cruiser belonging to the classes U151 and U139.


The 58-millimeter Hotchkiss cannon was installed in 1915 as emergency weapon in many British submarines, after the success of the campaign in the Sea of Marmara. Its arc of fire was 4100 meters with an elevation of 9.5 degrees and its projectiles traveled at 554 meters per second.


Many projects were studied for developing a retractable cannon for being installed on submarines, but the units from the First World War were all fitted with conventional cannons. Many years later technicians returned to the idea, for it was felt the necessity of rendering the hull of a submarine as hydrodynamical as possible, and thus all the modern units were fitted with retractable cannons. The German 88-millimeter L30 folding cannon (center) was a characteristic of the U19 class; but, since its folding device was very cumbersome, the successive U-boat were fitted with conventional cannons, fixed to the deck structure. Another retractable design was the Fielding cannon, installed internally in the prow of a submarine (bottom).

However, their very low profile limited enormously their possibility of action, either because it allowed just a very limited vision field, either because it did not prevent waves from hitting the deck; but, as every submarinist knew, the rough sea could be faced through immersion, for the effect of waves does not reach a great depth. Other truly severe problems did exist: the habitability in the very restricted space and the poor hygienic conditions that the crew had to endure. These problems could be solved only by building larger submarines; on the other hand, the larger size would allow them to operate with increased autonomy. However, the problem of the restricted dimensions of submarines was not addressed until the postwar period.

Still, the First World War was the event which contributed to the largest extent to the development of submarines; first of all, military authorities finally realized about the threat that they posed for surface ships; after that, they came into light all the technical improvements that rendered them ever more fearsome. Moreover, during those five years the crews acquired ever growing experience and a confidence on the submarine weapon which increased on par with the technical improvements which were gradually added to their war machines. Engines were made more powerful, torpedoes were made more deadly and, in 1918, submarine projects had suffered as many rapid transformations as those added during the same period to the field of military aeronautics.

When thinking about the submarines of that time, small and fragile vessels, and the courageous men who navigated them, we should take into account that, in 1918, a submarinist would be in turn amazed by those audacious pioneers who brought to sea such small vessels as the Lake or the Holland. But this does not detract from the fact that the submarinist of that time should have steel nerves for operating such small units in such disastrous conditions. The great paradox of submarines was that they required the most courageous men for one of the most ruthless conflicts.

Categories: Naval Warfare - Engineering - World War One - 20th Century - [General]


Website: Military History

Article submitted: 2019-09-11

Article updated: 2019-09-11

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