It was hard to detect or attack submerged submarines in June 1915. The only way of finding them was visual, mainly by spotting a raised periscope and/or the wake that it produced. It was also possible to see a submerged submarine that was close to the surface in the clear waters of the Mediterranean or the Dardanelles, but not in the Atlantic or North Sea.
Hydrophones, which detected submarines by sound, were introduced later in the war. Some were based onshore. Those on ships had the problem that the ship had to stop in order to prevent the sound of her engines interfering with that from the submarine: not ideal when a submarine was around.
There was also a lack of anti-submarine weapons. One not very efficient one was for ships to towing explosive sweeps. Ships fired on periscopes with their guns or tried to ram the submarine. Most submarines sunk early in the war, struck mines, suffered accident or were caught on the surface. The limited number of torpedoes carried meant that submarines preferred to surface and use guns to sink smaller ships.
One method of attacking submarines was to trick them into surfacing in order to attack with gunfire an apparently innocuous merchant ship. It would then open fire with its concealed weapons. These ships were called Q Ships and will be the subject of several later posts in this series.
A variation on this tactic was suggested by Acting Paymaster F. T. Spickernell, Secretary to Vice Admiral Sir David Beatty. It was that a trawler should tow a submerged submarine in areas were German U-boats were expected to be operating. The trawler and the British submarine would be in contact with each other via a telephone cable. A U-boat would surface to attack a trawler by gunfire rather than wasting a torpedo on a small craft. The trawler would then inform the British submarine by telephone. It would slip the tow and manoeuvre to torpedo the U-boat whilst herself remaining submerged.
After a period of trials in the Forth in May, C class submarines from the Forth Local Defence were transferred to Aberdeen and Peterhead to work with the trawler Taranaki. The first patrol took began on 24 May, but no U-boats were encountered until 8 June, when Taranaki, towing C27, spotted U19 a mile and a half away, 30 miles east by north off Peterhead. She informed C27, which confirmed the situation by periscope before slipping her tow. She approached the German boat, raising her periscope again when she should have been in firing range only to see that U19 was heading towards her at 15 knots. C27 had to dive, and U19 was out of sight when she returned to periscope depth..
Nothing had happened to make the Germans suspicious, but Taranaki’s appearance was changed as a precaution. She sailed from Aberdeen early on 23 June, commanded by Lieutenant-Commander Harrington D. Edwards, accompanied by Lieutenant Frederick H. Taylor’s C24. At 9:30 am the next day they were at sea to the south east of Aberdeen when U40 surfaced 2,500 yards away and fired a shot across Taranaki’s bows. A problem with the telephone meant that it was three more minutes before C24 was informed. She was unable to release the cable, so Taranaki had to release it from her end. C24 was not free until 9:45 am, during when Edwards had to keep his trawler under way. 
A trawler not stopping when approached by a U-boat might have made the Germans suspicious, but U40‘s captain, Kapitänleutnant Gerhardt Fürbringer, saw nothing to fear, although one of his junior officers was suspicious.  U40 was on her first war patrol. The British hoped that a U-boat would assume that the tow line was drawing the trawler’s net.
U40 stopped 1,000 yards from Taranaki, whose crew pretended to panic and abandon ship. C24 was finding it difficult to maintain her trim because she was still attached to 100 fathoms of towing rope and telephone cable. Once this problem had been overcome, she raised her periscope, found U40 1,000 yards away, closed to 500 yards, positioned herself for a beam shot and fired a torpedo at U40 at 9:55 am. It struck the U-boat, which sank immediately. Fürbringer, another officer and a petty officer were picked up by the British. The other 29 crewmen went down with U40.
The success of this method, which was kept secret, meant that the scheme was considerably expanded: C26 and C27 were to work with trawlers from Scapa Flow; C14 and C16 from the Tyne; C21 and C29 from the Humber and C3 and C34 from Harwich.
On 18 July Lieutenant-Commander Claude C. Dobson’s C27 and the trawler Princess Marie José, temporarily renamed Princess Louise, set out on patrol from Scapa. The trawler was captained by Lieutenant L. Morton, but Lieutenant C. Cantlie and Lieutenant A. M. Tarver were also on board in order to train the crew. Cantile, who was the only regular officer of the three, the others being peacetime merchant marine officers who were members of the Royal Navy Reserve, took command during the subsequent operation.
At 7:55 am on 20 July Cantlie telephoned Dobson to tell him that a U-boat had been spotted 2,000 yards away. The phone then broke down; Dobson waited five minutes before slipping the cable; contact had not been restored, and he could hear gunfire.
The U-boat, which was U23, had fired one warning shot before firing at the trawler. She stopped, raised the Red Ensign and dipped it as a sign of surrender, whilst her crew prepared to abandon ship in an apparent panic. This was in accordance with the plan, which was to trick the Germans and hopefully persuade them to come closer. It worked so well that U40 stopped near the trawler.
The trawler’s crew did not know where C27 was, but she was only 500 yards away on U40’s starboard beam when Dobson raised her periscope. He fired a torpedo, but U40 then started her engines, and it passed under her stern. He fired another that hit and sank U40. The British rescued 10 survivors, including her captain, Oberleutnant Hans Schulthess, and two other officers. The British Naval Staff Monograph, written after the war for internal Royal Navy use only, stated that the prisoners ‘gave a good deal of information, not only of a technical character…but also on the general work of German submarines’, which it suggests may have been a result of their good treatment.
The reason why the captains and a high proportion of officers of both U-boats sunk in this manner survived was that they would have been on the bridge whilst their boats were on the surface.
In both instances the more senior of the commanders of the two British vessels involved, Edwards and Dobson, was awarded the Distinguished Service Order, and the other one, Taylor and Cantlie, the Distinguished Service Cross. The coxswains of Taranaki and C24 were also awarded the Distinguished Service Medal after the earlier action. Dobson was later awarded the Victoria Cross.
U23 was the last U-boat to be caught in this way. Her survivors managed to inform some German civilian internees who were being repatriated from the United Kingdom to Germany about her fate. Consequently no more U-boats fell into the trawler/submarine trap.
 Naval Staff Monograph (Historical) 1925 vol. xiii, Home Waters part iv, February 1915 to July 1915. p. 249.
 Ibid., p. 250.
 Ibid; R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918. (London: Constable, 1931), p. 46.
 Naval Staff vol. Xiii. p. 250.
 Ibid., pp. 250-51.
 Naval Staff Monograph (Historical) 1926 vol. xiv, Home Waters part v, July 1915 to October 1915. p. 34.
 Ibid., p. 35.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. iii, p. 48, note 1; A. S. Hurd, The Merchant Navy, 3 vols. (London: HMSO, 1921). vol. ii, pp. 55-56.
 V. E. Tarrant, The U-Boat Offensive, 1914-1945 (London: Arms and Armour, 1989), pp. 22-23.