A trawling device had been developed which indicated whether contact with a submarine had been made. After the oil came up, we got out our trawling device and ran over the area for about an hour and finally got an indication.
We threw over a buoy to indicate the spot and anchored for the night, as it was getting dark. Next morning we trawled again and got another contact within a hundred yards of the buoy. We had destroyed a submarine in our first test, and the "sub" was given out by the Admiralty as a "probable." [That is, probably sunk.]
Many detection devices had been tried out and proved failures, but the American apparatus was so successful that the British ordered them for their own vessels. Thousands were manufactured, and our sub-chasers sent abroad were equipped with them. In December, 1917, it was estimated that at times two to five U-boats had passed through the English Channel in a day. After July 1, 1918, when patrol ships were equipped with the improved listening devices, only one enemy submarine is known to have passed through the Channel. Blocking the entrances to Zeebrugge and Ostend, the Dover patrol and the better mine defenses are to be credited with the larger part of this. But considerable credit is due to these "listeners," whose ability to locate under-water craft greatly increased the hazards of U-boats, especially in narrow waters.
The listeners also proved decidedly effective in high waters, off the French coast, in the Adriatic, the Mediterranean, and wherever they were used. They compelled the U-boats to change their tactics, and remain motionless for hours, fearing that the slightest movement of their propellers would disclose their presence.
Our submarine force began listening tests off Pensacola, Fla., in January, 1917, using privately-invented apparatus which gave such promise that an experimental station was established at Nahant, Mass., the General Electric, Submarine Signal, and Western Electric companies coöperating with the Navy Department and Naval Consulting Board.
The Consulting Board had created a special Experimental Committee headed by Mr. Lawrence Addicks, and on March 3 held a "Submarine Defense Conference" at New York, which was addressed by Admiral Sims, then president of the Naval War College; Captain J. K. Robison, of the Newport Torpedo Station, and Commander Yates Stirling, Jr., in charge of our submarine base at New London, Conn.
Scientists and naval officers engaged in this work held a conference in my office in the Navy Department on May 9, and two days later I created a Special Board on Anti-submarine Devices, with Rear Admiral A. W. Grant as chairman, and representatives of the electrical and signal companies, and the National Research Council as advisory members. Extensive experiments were carried on at our submarine station at New London, as well as at Nahant.
Magnetic, electrical and other apparatus having proved impracticable, attention was concentrated on listening devices. The British had been experimenting with various inventions of this nature, but none had proved very effective. The first successful listening device produced in America was the "C" tube, an application of the binaural principle—that is, hearing through both ears—which was developed by Dr. William D. Coolidge. Next was the "K" tube, developed at Nahant, an adaptation of the rotary compensator devised by Prof. Max Mason at New London, with microphones, enabling the device to be towed several hundred feet astern of the listening vessel. Subsequently the combined work at Nahant and New London resulted in production of the "Y" tube, "Delta," "O S," and "O K" tubes, all modified forms of the "K" tube, for installation on vessels of different types.
Submarine chasers were equipped with these tubes, the first of which was developed by August, 1917, and a thorough test was made with American submarines, which were easily located. But much depended on the acuteness of the operator, and a school to train "listeners" was established at New London. Phonograph records of the sound made by various craft were prepared, and used in the school for listeners, who soon became experts in determining direction, distance, type of vessel and speed at which it was moving.
"Find the submarine," was the problem when we entered the war, and this was the purpose of the listening devices. Once located, the "sub" could be destroyed or damaged by the depth-bomb. Before its advent there was no way of reaching the U-boat, once it submerged. The story is told that a British vessel chased down a "sub," which dived and remained stationary right under its pursuer. Down below them in the clear water, the Britishers could see the enemy plainly. "If we only had some sort of bomb that we could shoot down into the water, we could blow that Fritzie to Kingdom-come," an officer remarked. The general idea of the depth-bomb had long been known, and was then given its practical application.