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Asdic, Sonar and Detection Gear
A number of devices were used both to detect submarines, and to allow those same submarines to locate their targets without having to surface or raise the periscope. The earliest, and most basic of these was the hydrophone, sophisticated versions of which are still used today.
Hydrophones are extremely simple in concept. In the most basic form, a directional microphone is extended into the water through the bottom of the vessel. By rotating the microphone, the operator can listen for propeller and other noises that might indicate a submarine or a target. The louder the noise, the more likely it was that the microphone was pointed at the target.
It didn’t take long for the engineers to figure out that a pair of highly directional microphones, separated by a few feet, on a rotating mounting, would be more efficient than a single microphone. With each microphone feeding into a skilled operator’s headset, he could get a more accurate directional fix than by just relying on sound volume.
Between the wars, this concept was developed and refined. The American “JP” submarine hydrophone was a simple, highly effective design. Essentially a long, magnetized iron bar with a coil of wire wrapped around it, the back was covered with a sound attenuating material, so that it became relatively insensitive to sounds coming from behind. This was mounted on a shaft, connected to a handwheel in the forward torpedo room that the operator used to train the device. When necessary, the operator could plug the cable into a power source to re-
The JP head was mounted on the upper forward deck in American fleet type submarines. In this position it could pick up the sound of targets and escorts with minimal interference from the noises the submarine itself was making. Also, being on the upper deck, the JP could be used when the submarine was sitting on the bottom. The boat’s other sonar equipment, which was mounted under the hull, couldn’t.
One device used to aid the operator in zeroing in on a sound source was the “magic eye” on his amplifier panel. This was a special type of vacuum tube, with the top sticking out through the panel. A phosphor coating created a pie shaped image on the top of the tube. Older people are likely to remember these from old FM radios, which used them as tuning meters. As the signal gets stronger, the “pie” gets narrower, until it becomes a line at peak strength. The operator could watch this to confirm what he was hearing.
While hydrophones alone could give a bearing on a target, they were severely limited in capability. Range had to be estimated based on variables such as sound volume, which called for a lot of skill and experience on the part of the operator, and usually a good bit of luck as well. Target speed was more easily estimated, as the operator could count the revolutions of the target’s propellers. As long as good intelligence was available on the target, particular propeller pitch, it was possible to make a good speed estimate.
Presuming a target’s screws have a pitch of three feet (that is, one revolution will move the target forward three feet), if the screw is turning at 100 rpm the target should travel 300 feet per minute, or 18,000 feet per hour. Divide that by 6,076 (the number of feet in a nautical mile) and you get a target speed of just under three knots. This isn’t very fast, but for most submerged submarines before the nuclear age it wasn’t all that slow, either.
A major limiting factor on hydrophones was the speed of their own vessel. Anti-
During World War I, Allied scientists addressed the problem of finding something more efficient than hydrophones. The result of their investigation was called Asdic by the British and Sonar by Americans. (Asdic is an acronym for Anti-
The simplest form of Asdic combined hydrophones with a sound source. The emitter sent out a sound pulse that reflected off the target. The length of time it took for the pulse to travel to the target and return could be used to calculate the range. This is actually the same principle used by an echo sounder to determine depth.
During the inter-
Of course, anti-
Asdic/Sonar was a mixed blessing for submarines. It could be used to give an accurate range and bearing on a target, but it also provided a noise source for the escorts to home in on. Radar has always suffered from the same problem. It warns you that the enemy is coming, but it also tells them where you are. The Sonar head in the picture was part of the Sonar suite in an American fleet type submarine, and could be retracted inside the hull when secured.
American submarine theorists were so confident in the capabilities of submarine sonar that immediate pre-
Once the war started, events changed this doctrine. To begin with, theories aside, none of the captains were ever able to get hits on surface targets using Sonar alone. That didn’t really become practical until wire-
The Japanese did, however, have excellent sound gear, and some highly skilled operators. If a submarine pinged a target, the escorts could hear it and would try to get a reciprocal bearing and attack. For this reason, submarine Sonar is usually used sparingly, generally only single pings for range finding. Submarines do, of course, listen for escorts pinging, which they tend to do much more frequently than submarines. Escorts view active Sonar not only as a way to detect subs, but as a way to warn them to stay away or risk detection and destruction.
Modern submarine doctrine calls for boats to be equipped with powerful, highly sophisticated active Sonar, and for that gear to almost never be used. Most detection still employs the modern version of the venerable hydrophone, though it is now linked to a powerful array of analytical equipment and visual displays that give a much clearer picture than just listening. At the same time, operator skill is still the most important factor. Contrary to what turns up in a lot of techno-
Through World War II, submarines were mainly used in the anti-
During World War I, Germany quickly realized that submarines made idea commerce raiders. Initially the old rules were followed, ships were stopped, and if contraband was discovered the crew was allowed to abandon before the ship was sunk. That lasted until the British began arming cargo ships, after which the U-
Germany was initially even more successful at this in World War II, when the German U-
Today, the primary target for a submarine’s torpedoes is another submarine.
Normally, the system would be used in the passive mode, using the 48 transducers in the Balkon Gerät (balcony gear) mounted under the boat’s bow, which could detect targets at long distances. This was a more advanced form of the phased array Sonar Germany had developed early in the war, and could be considered the direct ancestor of all modern submarine listening gear.
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