[Subnuts]

Quite a few people have asked about late-war US submarine countermeasures, but not much information seems to be available out there. In Thunder Below Eugene Fluckey mentions using a large number of decoys while being hunted by a DD in shallow waters, but not much is actually said about them.

I recently bought a copy of Friedman's US Submarines Through 1945, which includes a few paragraphs on the subject, which I'll post here for anyone curious.

A submarine decoy program began in April 1943. Evolved from ASW training aids (echo repeaters) that the University of California Division of War Research (UCDWR) began to develop in 1942, decoys could be released from a submarine's 3-inch signal ejector tube and from her torpedo tubes. NAC was a barrage jammer with only limited output at any one frequency band. After tests on board an S-boat in February 1944, 450 NACs were ordered in June 1944; that increased to 5,450 in August. Another 5,000 NAC-1s were ordered in June 1945. A total of 4,308 were supplied to the submarine force during the war (contracts were cut back drastically at the end of the war). NAC proved only partially effective against experienced sonar operators. Its successor, NAH, used a high-velocity tape or disk recorder to pick up the sonar ping, then retransmitted it for 15 sec, beginning 0.10 sec after it arrived. It worked over the 10-30 kHz band. Development began in May 1945.

NAC did not move through the water like a real submarine. Work on a self-propelled NAD began in May 1943. The 3-inch NAD-3 simulated a submarine running at periscope depth at 120 turns. It could not accommodate a sonar repeater. A 6-in (diameter) decoy ejector was proposed; beginning in December 1943, an enlarged NAD-6 (6 in x 48 in) incorporating a repeater was designed to fit it. The new ejector was then canceled, and NAD would have to be fired from a torpedo tube. UCDWR enlarged NAD again (the 10-inch NAD-10 was made from an M30 mine body). The first NAD-6, carrying a sonar repeater, was completed in August 1944. BuShips demanded a simulator; a redesigned NAD-6 was tested in October 1944. Contracts for 500 NAD-6s and 500 NAD-10s were let late in 1944 and doubled in 1945; at the end of the war they were cut back to 500 each. These decoys held a straight course within two degrees so that the launching submarine where they were and evade away from them; gyro angle was preset to within plus or minus 90 degrees. NAD-3 ran at 5 kt at a depth of 50 ft and slowed to 3 ft to begin noisemaking at a preset distance. NAD-6 ran out of a torpedo tube at 4 kt; noisemaking began after 35 sec, and lasted 30-35 min. NAD-10 ran out at 7 kt, then slowed to 3.75 kit after 1 min; it lasted an hour. At lease some NADs were in fleet service in the summer of 1945.

David Taylor Model Basin began work in 1944 on NAE, a mechanical noisemaker derived from the towed FXR and effective above 6 kHz. Mk 1, a wideband masker, was field-tested in October 1944 and operational in the spring of 1945. Mk 2, introduced in the summer of 1945, differed from Mk 1 in that it could be launched down to 400 ft and its supporting balloon hovered below the surface of the water so that it was not visible from the air. Mks 3 and 4 were high-frequency maskers and jammers. NAE was considered effective at high sonic and supersonic frequencies and could even jam some sonars, but it was less effective at lower sonic frequencies. Postwar, it was developed mainly as a rocket-fired noisemaker (acoustic torpedo countermeasure) for use by surface ASW ships.

UCDWR and David Taylor conceived the NAG in 1943 as a lower-frequency companion to NAC to counter hydrophones in the 0.1-10 kHz range. A second sound head was later added, so that it could also operate at supersonic frequencies. NAG was under test at the end of the war. There were also false target shells (FTSs) and false target cans (FTCs). MIT designed pepper signals (Mk 14 and Mk 20) to mask submarine self-noise at sonic frequencies in shallow water. Reverberation would add its own masking; in deeper water, sonar operators could maintain contact between explosions.

A combined-use doctrine was quickly worked out. If the submarine was not sure that the enemy had contact, she launched an NAD or ejected FTSs and FTCs. If the enemy had contact, quantities of NACs and NAEs could jam receivers, confuse an attack, and break contact; then FTSs and FTCs would be ejected to add to the confusion. Once contact was clearly broken, an NAD emerging from the masked area would attract attention. The submarine had to choose her evasive course so that the NAD emerged before she did. Decoys always had to be ejected in rapid succession to avoid forming a trail that pointed to the submarine. These ideas and decoy categories (static noise beacon, mobile submarine simulator) have persisted ever since. Analogous radar beacons were developed to decoy radar-directed ASW aircraft. Hopes of an antisonar coating were not realized.