Atlantic U - boat Campaign ( World War I ) The Atlantic U - boat Campaign of World War I was the naval campaign fought by German U - boat s in Atlantic waters - the seas around the British Isles , the North Sea , and the coast of France . Initially directed against the British Grand Fleet , later it was extended to include action against the trade routes of the Entente Powers . The campaign was highly destructive , and resulted in the loss of nearly half of Britain ' s merchant marine fleet during the course of the war . The campaign and the Allied response is also known as the Battle of the Atlantic . To counter the German submarines , the allies moved shipping into convoys guarded by destroyers , blockades such as the Dover Barrage and minefields were laid and aircraft patrols monitored the U - boat bases . The U - boat campaign was not able to cut off supplies before the US entered the war in 1917 and in later 1918 , the U - boat bases were abandoned in the face of the Allied advance . 1914 : initial campaign . First patrols . On 6 August 1914 , two days after Britain had declared war on Germany , the German U - boats , , , , , , , , , and sailed from their base in Heligoland to attack Royal Navy warships in the North Sea in the first submarine war patrols in history . The U - boats sailed north , hoping to encounter Royal Navy squadrons between Shetland and Bergen . On 8 August , one of " U - 9 ' s " engines broke down and she was forced to return to base . On the same day , off Fair Isle , " U - 15 " sighted the British battleships , , and on manoeuvres and fired a torpedo at " Monarch " . This failed to hit , and succeeded only in putting the battleships on their guard . At dawn the next morning , the 1st Light Cruiser Squadron , which was screening the battleships , came into contact with the U - boats , sighting " U - 15 " , which was lying on the surface . There was no sign of any lookouts on the U - boat and sounds of hammering could be heard , as though her crew were performing repairs . " Birmingham " immediately altered course and rammed " U - 15 " just behind her conning tower . The submarine was cut in two and sank with all hands . On 12 August , seven U - boats returned to Heligoland ; " U - 13 " was also missing , and it was thought she had been mined . While the operation was a failure , it caused the Royal Navy some uneasiness , disproving earlier estimates as to U - boats ' radius of action and leaving the security of the Grand Fleet ' s unprotected anchorage at Scapa Flow open to question . On the other hand , the ease with which " U - 15 " had been destroyed by " Birmingham " encouraged the false belief that submarines were no great danger to surface warships . First successes . On 5 September 1914 , commanded by Lieutenant Otto Hersing made history when he torpedoed the Royal Navy light cruiser . The cruiser ' s magazine exploded , and the ship sank in four minutes , taking 259 of her crew with her . It was the first combat victory of the modern submarine . The German U - boats were to get even luckier on 22 September . Early in the morning of that day , a lookout on the bridge of , commanded by Lieutenant Otto Weddigen , spotted a vessel on the horizon . Weddigen ordered the U - boat to submerge immediately , and the submarine went forward to investigate . At closer range , Weddigen discovered three old Royal Navy armoured cruiser s , , , and . These three vessels were not merely antiquated , but were staffed mostly by reservists , and were so clearly vulnerable that a decision to withdraw them was already filtering up through the bureaucracy of the Admiralty . The order did not come soon enough for the ships . Weddigen sent one torpedo into " Aboukir " . The captains of " Hogue " and " Cressy " assumed " Aboukir " had struck a mine and came up to assist . " U 9 " put two torpedoes into " Hogue " , and then hit " Cressy " with two more torpedoes as the cruiser tried to flee . The three cruisers sank in less than an hour , killing 1 , 460 British sailors . Three weeks later , on 15 October , Weddigen also sank the old cruiser , and the crew of " U 9 " became national heroes . Each was awarded the Iron Cross Second Class , except for Weddigen , who received the Iron Cross First Class . The sinkings caused alarm within the British Admiralty , which was increasingly nervous about the security of the Scapa Flow anchorage , and the fleet was sent to ports in Ireland and the west coast of Scotland until adequate defenses were installed at Scapa Flow . This , in a sense , was a more significant victory than sinking a few old cruisers ; the world ' s most powerful fleet had been forced to abandon its home base . End of the first campaign . These concerns were well - founded . On 23 November penetrated Scapa Flow via Hoxa Sound , following a steamer through the boom and entering the anchorage with little difficulty . However , the fleet was absent , being dispersed in anchorages on the west coast of Scotland and Ireland . As " U - 18 " was making her way back out to the open sea , her periscope was spotted by a guard boat . The trawler " Dorothy Gray " altered course and rammed the periscope , rendering it unserviceable . " U - 18 " then suffered a failure of her diving plane motor and the boat became unable to maintain her depth , at one point even impacting the seabed . Eventually , her captain was forced to surface and scuttle his command , and all but one crew - member were picked up by British boats . The last success of the year came on 31 December . sighted the British battleship on manoeuvres in the Channel and torpedoed her . " Formidable " sank with the loss of 547 of her crew . The C - in - C Channel Fleet , Adm . Sir Lewis Bayly , was criticized for not taking proper precautions during the exercises , but was cleared of the charge of negligence . Bayly later served with distinction as commander of the anti - submarine warfare forces at Queenstown . 1915 : War on commerce . First attacks on merchant ships . The first attacks on merchant ships had started in October 1914 . On 20 October became the first British merchant vessel to be sunk by a German submarine in World War I . " Glitra " , bound from Grangemouth to Stavanger , Norway , was stopped and searched by , under the command of Kapitänleutnant Johannes Feldkirchener . The operation was performed broadly in accordance with the " cruiser rules " , the crew being ordered into the lifeboats before " Glitra " was sunk by having her seacock s opened . It was the first time in history a submarine sank a merchant ship . Less than a week later , on 26 October , " U - 24 " became the first submarine to attack an unarmed merchant ship without warning , when she torpedoed the steamship " Admiral Ganteaume " , with 2 , 500 Belgian refugees aboard . Although the ship did not sink , and was towed into Boulogne , 40 lives were lost , mainly due to panic . The U - boat ' s commander , Rudolf Schneider , claimed he had mistaken her for a troop transport . On 30 January 1915 , , commanded by " Kapitänleutnant " Otto Dröscher , torpedoed and sank the steamers , , and without warning , and on 1 February fired a torpedo at , but missed , the hospital ship " Asturias " , despite her being clearly identifiable as a hospital ship by her white paintwork with green bands and red crosses . Unrestricted submarine warfare . In February 1915 the first unrestricted campaign against Allied trade was started . The U - boat had several deficiencies for a commerce raider ; its low speed , even on the surface , made it scarcely faster than many merchant ships , while its light gun armament was inadequate against larger vessels . To use the U - boat ' s chief weapon , the attack without warning , using torpedoes , meant abandoning the stop - and - search required to avoid harming neutrals . In the first month 29 ships totalling were sunk , a pace of destruction which was maintained throughout the summer . As the sinkings increased , so too did the number of politically damaging incidents . On 19 February " U - 8 " torpedoed " Belridge " , a neutral tanker travelling between two neutral ports ; in March U - boats sank " Hanna " and " Medea " , a Swedish and a Dutch freighter ; in April two Greek vessels . In March also , " Falaba " was sunk , with the loss of one American life , and in April " Harpalyce " , a Belgian Relief ship , was sunk . On 7 May , sank with the loss of 1 , 198 lives , 128 of them American citizens . British countermeasures were largely ineffective . The most effective defensive measures proved to be advising merchantmen to turn towards the U - boat and attempt to ram , forcing it to submerge . Over half of all attacks on merchant ships by U - boats were defeated in this way . This response freed the U - boat to attack without warning , however . In March 1915 , this tactic was used by the packet to escape an attack by . For this her captain , Charles Fryatt , was executed after being captured by the Germans in June 1916 provoking international condemnation . Another option was arming ships for self - defence , which , according to the Germans , put them outside the protection of the cruiser rules . Another option was to arm and man decoy ships with hidden guns , the so - called Q - ship . A variant on the idea was to equip small vessels with a submarine escort . In 1915 , three U - boats were sunk by Q - ships , and two more by submarines accompanying trawlers . In June also was sunk by while attacking " Taranaki " , and in July " U - 23 " was sunk by " C - 27 " attacking " Princess Louise " . Also in July " U - 36 " was sunk by the Q - ship " Prince Charles " , and in August and September and were sunk by " Baralong " , the former in the notorious Baralong Incident . There were , however , no means to detect submerged U - boats , and attacks on them were limited to efforts to damage their periscopes with hammers dropping hand - thrown guncotton bombs . Use of nets to ensnare U - boats was also examined , as was a destroyer , " Starfish " , fitted with a spar torpedo . In all , 16 U - boats were destroyed during this phase of the campaign , while they themselves sank 370 ships totalling 750 , 000 grt . 1916 : in support of the High Seas fleet . In 1916 the German Navy returned to a strategy of using the U - boats to erode the Grand Fleet ' s numerical superiority by staging a series of operations designed to lure the Grand Fleet into a U - boat trap . Due to the U - boats ' poor speed compared to the main battle fleet these operations required U - boat patrol lines to be set up , while the High Seas fleet manoeuvred to draw the Grand Fleet to them . Several of these operations were staged , in March and April 1916 , but with no success . Ironically , the major fleet action which did take place , the Battle of Jutland , in May 1916 , saw no U - boat involvement at all ; the fleets met and engaged largely by chance , and there were no U - boat patrols anywhere near the battle area . A further series of operations , in August and October 1916 , were similarly unfruitful , and the strategy was abandoned in favour of resuming commerce warfare . 1917 : renewed " unrestricted " campaign . In 1917 Germany decided to resume full unrestricted submarine warfare . It was expected to bring America into the war , but the Germans gambled that they could defeat Britain by this means before the US could mobilize . German planners estimated that if the sunk tonnage exceeds 600 , 000 tons per month , Britain would be forced to sue for peace after 5 to 6 months . In February 1917 U - boats sank over 414 , 000 GRT in the war zone around Britain , 80 % of the total for the month ; in March they sank over 500 , 000 ( 90 % ) , in April over 600 , 000 of 860 , 000 GRT , the highest total sinkings of the war . This , however was the high point . In May , the first convoys were introduced , and were immediately successful . Overall losses started to fall ; losses to ships in convoy fell dramatically . In the three months following their introduction , on the Atlantic , North Sea , and Scandinavian routes , of 8 , 894 ships convoyed just 27 were lost to U - boats . By comparison 356 were lost sailing independently . As shipping losses fell , U - boat losses rose ; during the period May to July 1917 , 15 U - boats were destroyed in the waters around Britain , compared to 9 the previous quarter , and 4 for the quarter before the campaign was renewed . As the campaign became more intense , it also became more brutal . 1917 saw a series of attacks on hospital ship s , which generally sailed fully lit , to show their non - combatant status . In January , " HMHS Rewa " was sunk by ; in March , " HMHS Gloucester Castle " by ; in June , " HMHS Llandovery Castle " by . As U - boats became more wary , encounters with Q - ship s also became more intense . In February 1917 was sunk by , but only after Gordon Campbell , " Farnborough ’ s " captain , allowed her to be torpedoed in order to get close enough to engage . In March " Privet " sank " U - 85 " in a 40 - minute gun battle , but herself sank before reaching harbour . In April " Heather " was attacked by " U - 52 " and was badly damaged ; the U - boat escaped unscathed . And a few days later " Tulip " was sunk by whose captain was suspicious of her appearance . 1918 : final year . The convoy system was effective in reducing allied shipping losses , while better weapons and tactics made the escorts more successful at intercepting and attacking U - boats . Shipping losses in Atlantic waters were 98 ships ( just over 170 , 000 GRT ) in January ; after a rise in February they fell again , and did not rise above that level for the rest of the war . In January , 6 U - boats were destroyed in the theatre ; this also became the average loss for the year . The Allies continued to try to block access through the Straits of Dover , with the Dover Barrage . Until November 1917 it was ineffective ; up to then just 2 U - boats had been destroyed by the Barrage force , and the Barrage itself had been a magnet for surface raids . After major improvement in the winter of 1917 it became more effective ; in the four - month period after mid - December seven U - boats were destroyed trying to transit the area , and by February the High Seas Flotilla boats had abandoned the route in favour of sailing north - about round Scotland , with a consequent loss of effectiveness . The Flanders boats still tried to use the route , but continued to suffer losses , and after March switched their operations to Britain ’ s east coast . Other measures , particularly against the Flanders flotilla , were the raids on Zeebrugge and Ostend , an attempt to blockade access to the sea . These were largely unsuccessful ; the Flanders boats were able to maintain access throughout this period . May 1918 saw the only attempt by the Germans to muster a group attack , the forerunner of the wolf - pack , to counter the Allied convoys . In May , 6 U - boats sailed , under the command of K / L Rucker in . On 11 May " U - 86 " sank one of a pair of ships detached from a convoy in the Channel , but the next day an attack on the troopship led to the destruction of " U - 103 " , while was sunk by British submarine . Two more ships were sunk in convoys in the next week , and three independents , but over 100 ships had passed through the groups patrol area in safety . During the summer , the extension of the convoy system and effectiveness of the escorts made the east coast of Britain as dangerous for the U - boats as the Channel had become . In this period , the Flanders flotilla lost a third of its boats , and in the autumn , losses were at 40 % . In October , with the German army in full retreat , the Flanders flotilla was forced to abandon its base at Bruges before it was overrun . A number of boats were scuttled there , while the remainder , just 10 boats , returned to bases in Germany . In the summer , too , steps were taken to reduce the effectiveness of the High Seas Flotillas . In 1918 the Allies , particularly the Americans , undertook to create a barrage across the Norwegian Sea , to block U - boat access to the Western Approaches by the north - about route . This huge undertaking involved laying and maintaining minefields and patrols in deep waters over a distance of 300 nautical miles ( 556 kilometers ) . The North Sea Mine Barrage saw the laying of over 70 , 000 mines , mostly by the United States Navy , during the summer of 1918 . From September to November 1918 6 U - boats were sunk by this measure . On 11 November 1918 , World War I ended . The last task of the U - Boat Arm was in helping to quell the Wilhelmshaven mutiny , which had broken out when the High Seas Fleet was ordered to sea for a final , doomed sortie . After the Armistice , the remaining U - boats joined the High Seas Fleet in surrender , and were interned at Harwich . Of the 12½ million tons of Allied shipping destroyed in World War I , over 8 million tons , two thirds of the total , had been sunk in the waters of the Atlantic war zone . Of the 178 U - boats destroyed during the war , 153 had been from the Atlantic forces , 77 from the much larger High Seas Flotillas and 76 from the much smaller Flanders force .