USS Bowfin (SS-287)
The ears of the Boat
On a fully submerged submarine of World War II, radar was rendered useless, and observations through the periscope were, by necessity and design limitations, extremely limited. The submarine had to depend chiefly on listening.
Sonar (SOund Navigation And Ranging) became the ears of the boat by using hydrophones to detect and track targets. There were two main types of sonar listening gear on a World War II submarine like USS Bowfin - sonic and supersonic.
Sonic (JP) gear is useful for picking up targets at great distances. Sounds appear more natural and are more easily recognized than those produced by supersonic gear. The hydrophone in sonic gear is mounted topside. When a sound wave hits the front of the hydrophone, the long metal tube changes slightly in size. This sets up an electric current in wires coiled around its wooden core. Sound cannot hit the back of the tube very strongly because it is protected by a rubber baffle. Hand-operated and electrically controlled mechanisms turn the hydrophones in any desired direction.
JP is the Navy term for sonic listening gear. The J means that it can be used for listening only. The second letter P merely indicates the model.
Supersonic gear is superior for catching sounds used by escort vessels in searching for submarines, and can be used to send out sounds to determine the range of a target. Two supersonic hydrophone projectors, the QB and the JK/QC, are mounted at the bottom of shafts which extend through the hull under the forward torpedo room. Lowering these shafts puts the two projectors below the keel.
JK/QC is the Navy term for one type of supersonic gear. The JK half of the combination projector is for listening only; the QC half can also be used for sending out sounds into the water.
QB designates the other type. As indicated by the letter Q, the QB projector can send out as well as receive sounds.
The JK/QC combination projector is mounted portside. The JK face is just like QB. The QC face contains small nickel tubes, which change size when a sound wave strikes this face.
Change in shape of the salt crystals or in the size of the metal tubes generates a small electric current in connecting cables, which is strengthened and changed by the receiver-amplifiers so that it is heard as sound in the phones or speaker.
Most information taken from Submarine Sonar Operator's Manual, November 1944.
Beginning with her eighth war patrol, Bowfin also carried the top secret FM sonar (QLA), which was used to detect mines.