BIR scientists moved from Hawkcraig, Commander Ryan continued his hydrophone development unencumbered by the civilian scientists. His inventive mind, however, was also able to tackle the underway pursuit problem. Ryan had designed two systems that could be towed aft of a ship; one referred to as the “porpoise”, the other his “rubber eel.”
Porpoise towed listening devices developed by Captain Ryan at the Hawkcraig Experimental Station. (Wilson, 1920)
We now reach the era of the rubber ‘eel’ and ‘porpoise’ hydrophones…. Both these instruments spelt a new departure. They could be towed from the stern of a vessel under way—a marked advance—and furthermore the latter instrument was a direction finder.22
The rubber eel was available by 1918, with 463 being issued to a variety of vessels, including trawlers and P-class ships. The device was a free-flooding, three-inch diameter rubber cylinder, eighteen inches long—the sensor consisting of a two-inch diameter button microphone with a phosphor-bronze diaphragm. When towed at speeds up to eight knots, the listener could detect a submarine at ranges up to four miles. Because the microphone was non-directional, two eels were often towed and the operator relied on binaural listening to obtain a target bearing.23
Ryan’s most effective device, the porpoise, was not available until September, 1918. The porpoise was similar to the Nash fish, but only carried a single, improved uni-directional hydrophone, also rotated by an electric motor. In comparison trials with the Nash fish, the porpoise had a longer range to detection, performing well at speeds up to six knots. By Armistice, only thirty-one of the one hundred units ordered by Admiral Jellicoe’s Anti-Submarine Division were in operation.24 Nonetheless, the British hydrophone service was glad to have them.
A new school of porpoise instruction was started at Elie for training personnel in its use; and hunting flotillas, using the porpoise, commenced operations in the summer, and carried on until Armistice …”25
Whether hunting at sea or from shore, it was hoped that with all the new technology available, including American listening devices being installed on destroyers and other British submarine hunting vessels, neither a U-boat nor a lovesick mermaid could avoid detection.
CHAPTER 4 HUNTING SUBMARINES FROM SHORE
For the detection of submarines at certain congested lanes near the coast, and at entrances of harbors and bays, the British rather early in the war developed a tripod, with a microphone mounted upon it, which was lowered to the bottom and a listening station established on shore with connection by cable to the tripod.
—Admiral Robert S. Griffin, History of the Bureau of Engineering, 19221
Admiral Griffin, Chief of the U.S. Navy’s Bureau of Engineering, recognized the work of the British early in the war to use shore-based listening stations. During the spring of 1915, Commander Ryan, initially at his station on Inchkeith Island and then at Edinburgh’s Granton Pier, had demonstrated to an anxious Admiralty the application of hydrophones for submarine detection and, in particular, for creating a shore station listening post. Between March and August, 1915, hydrophone stations were established within the Firth of Forth at Oxcars, Inchcolm, and Elieness.
Ryan’s success led to the rapid expansion of his shore station operations with installations to the north at Cromarty and to the south at Lowestoft near the entrance to the English Channel. Because the hydrophones used for these listening stations were non-directional, several units were installed in a line, where the listener could switch to each individual hydrophone in sequence. Where the sound was loudest indicated the position of an approaching submarine. The shore station listener was in contact with antisubmarine patrols assigned to the area, the submarine’s position transmitted via wireless telegraphy. The vessels would then speed to the location, dropping their depth charges.
Installing these stations required running cables along the bottom, connecting the listeners to their offshore hydrophones. Three cable-laying drifters, Vanguard, Couronne, and Eros, were added to Ryan’s small fleet.2 Now with listening stations fully operational and a staff that would increase to twenty officers and eighty enlisted, he was able to deal with station logistics, maintenance, and listener training. Ryan was no longer confined to the Firth of Forth; he also continued to expand his hydrophone development beyond shore stations.
Hawkcraig Admiralty Experimental Station, along the north shore of the Firth of Forth. (Wilson, 1920)
Once accepted by the Admiralty as an effective U-boat counter-measure and with the creation of the Anti-Submarine Division (ASD) in December 1916, multiple shore stations were rapidly established, covering strategic locations along the English Channel; across choke points at the northern and southern entrances to the Irish Sea; and as far north as the Orkney Islands. Stations were also established in the Mediterranean at the Malta hydrophone school and at locations in Italy associated with the Otranto Straits, through which U-boats stationed in the Adriatic had to pass when entering the Mediterranean. Another shore station was installed at Cap Griz Nez south of Calais as part of the mine fields that stretched between England and France at the Dover Straits.3
Mine fields across areas transited by U-boats were referred to as “mine barrages.” The Dover Mine Barrage was a major deterrent that discouraged U-boats stationed at Zeebrugge and Ostend in Belgium from passing down the Channel, forcing them to transit past the north coast of Scotland in order to access their hunting grounds in the Atlantic.4 This route, and passage beyond the Shetland Islands, however, was also defended by aggressive antisubmarine patrols and the extensive mine fields known as the North Sea Mine Barrage, much of which had been created by mine laying ships from the U.S. working with the British between the Orkneys and Norway.5 As the reliability of the mines, as well as their numbers, increased, their effect on U-boat operations became a major concern of submarine commanders, including Ernst Hashagen, captain of U-62:
The danger of the mines mounted from year to year throughout the war. There was no effective defense against them. We could only try to avoid the mines whenever possible, and, if there was no other way, we had to pass over them or dive through them. But they were dangerous foes, because they were always invisible and took us unawares by stealth.6
At the beginning of 1918 the British succeeded too, at last, in blocking effectively the Straits of Dover. Between Folkstone and Griz Nez they had laid a wall of mines, ten deep, hermetically sealing the passage under water. Should a submarine be reported in the neighborhood, a row of buoys would light up, at a signal, with bright magnesium flares, turning night into dazzling brilliance. The submarine, forced to dive, was almost certainly destroyed upon the walls of mines. In the net lurked many a spider, ready for its victims. This barrage had indeed become a ‘barrier of death.’ Its completion spelt the end of the U-boat base at Zeebrugge, on the Belgian coast. When one U-boat after another had left it, never to return, it had to be abandoned.”7
Shore stations installed in the vicinity of coast and harbor defense mine fields were sometimes equipped with the ability to remotely detonate the mines. When a listener was sufficiently certain that a U-boat was in close proximity to the mines, he could trigger them electronically. On May 4, 1918, U-59 was heard on hydrophones at Cap Griz Nez, only two weeks after the shore station was installed. The listener triggered the mines, severely damaging the sub, which barely managed to return to her base at Ostend. It is thought that UC-78 may have been destroyed at this same location. While the majority of the U-boats lost passing through the Dover Straits were from direct contact with a mine, U-boat commanders were very aware of the listeners on shore.8 Ernst Hashagen:
[By 1918] the British concentrated their mines in areas constantly traversed by German submarines, such as the English Channel. Between the mines were ‘listening-buoys,’ electrically connected with the nearest lookout post on shore. From far away,