Page 30 - Summer 2018

Canadian Navel Acoustics
number of propellers (Longard, 1993). The same report in- cluded what was likely the first direct observation of the line spectrum of the vibration of a ship’s hull (Longard, 1993), a phenomenon still used to identify individual ships.
The F0 system was developed by Ken Newbound to measure the decrease in pressure underneath a passing ship, which was thought to be a potential trigger for German “oyster” mines. However, the measured maximum negative pressure was comparable to that of background sea swell, suggesting that a pressure mine would require a second influence de- vice to avoid being falsely triggered (Longard, 1993).
In 1943, the NRE scientists received a message that the Germans were using torpedoes capable of homing on a ship’s propellers. That very evening, Lewis, John Longard, and Commander A. F. Peers designed a decoy that became known as the Canadian Anti-Acoustic Torpedo (CAT) gear. It was thought that a suitable noisemaker towed well astern of a ship could attract the torpedo and run it to exhaustion so they redesigned the minesweeping parallel-pipe noise- makers to operate around 20 kHz. The long parallel pipes were replaced with 30-inch (75-cm) steel rods, and the gear was assembled from other scraps and bolts found around the laboratory (Figure 2b). The next day, Olga Mounsey drafted proper drawings and the CAT gear went into production, and within 17 days, it was being fitted on Allied ships (Veter- ans Affairs Canada, 2017). Eventually, the CAT gear was cop- ied by the United States, where their new noisemakers with rods of the same length (30 inch) also included the “much oversize bolts” from the NRE drawing (Longard, 1993).
Oceanography and
Antisubmarine Warfare
Mine and torpedo acoustics occur at short enough ranges that sound essentially travels in a straight line. However, vertical variations in the speed of sound in water (which depends on temperature, salinity, and pressure) result in a lensing effect that redirects the sound upward or downward. Therefore, over the longer distances required for detecting submarines, the sound may interact many times with the rough ocean bottom and surface, resulting in scattering and losses. In deeper waters, sound may be refracted downward and away from a surface ship’s sonar system, drastically re- ducing the detection range against a submarine.
During World War II, the RCN observed that in Canadian waters, U-boats could escape detection simply by diving. To increase understanding of the ocean’s thermal structure off
Figure 3. Operation CABOT staff. Naval Research Establish- ment (NRE) staff included (left to right) Norbert Lyons, John Longard, William Mackasey, Bill Ford, Bryce Fanning, and two others (names unknown). Photo courtesy of Canada, De- partment of National Defence.
Canada’s East Coast, the NRE undertook multiple survey programs beginning in 1943. On the West Coast, oceano- graphic surveys were initiated by John P. Tully of the Pacific Oceanographic Group in 1936 and continued jointly with the PNL staff beginning in 1948. Oceanographic surveys and trials to understand the effects of ocean conditions on sonar performance were undertaken in the Pacific Ocean, Bering Sea, and Western Arctic (Chapman, 1998; Canadian Meteorological and Oceanographic Society \\\\\\\[CMOS\\\\\\\], 2014).
In 1950, W. L. Ford led a team aboard the NRE ship HMCS New Liskeard (Figure 3) that was part of a six-ship survey of the northern boundary of the Gulf Stream known as Opera- tion CABOT, a joint undertaking with several American re- search groups. The considerable dataset acquired on the sur- vey (700 bathythermograph slides and 800 water samples on the New Liskeard alone) greatly increased the understanding of Gulf Stream dynamics (Ford et al., 1952; Stommel, 1965).
The surveys resulted in NRE memoranda produced for the RCN and the Royal Canadian Air Force (RCAF) on water conditions off the East Coast of Canada. Longard also de- veloped an extension to the 1942 US Navy code used to de- scribe the ocean temperature structure to accommodate the stronger gradients observed in Canadian waters. The codes could be plotted on a map to give a three-dimensional pic- ture of water conditions. Longard’s extended code was ad- opted around 1948 by the US Navy under the name “NRE code” and used for about 10 years (Longard, 1993).
28 | Acoustics Today | Summer 2018