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When Sonar Wasn’t Enough: The Bathythermograph

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Featured in Ripley's Believe It or Not!


A World War II Submarine Detector

A terror of the seas, the German U-boat struck fear into the hearts of Navies and ocean liners all over the world. Though early submarines could only perform brief underwater missions to strike ships from below, by the time World War II engulfed the Pacific, the high-seas game of cat-and-mouse had become far more deadly.

Underwater sound detection systems had gotten their start in 1917 Britain. Thrown into anti-submarine testing, the American version—SONAR—was quickly adopted by ships in order to sniff out subs. Suspecting a submarine to be in the vicinity and pinpointing them for a counter-attack, however, had great disparity. Aiming a torpedo at a hidden submarine required immense precision.


At the time, sonar readings could be thrown off by variances in depth and water temperature. Thankfully, a secondary sensor was able to fill in the variables. The bathythermograph was a torpedo-shaped device strung behind ships via cable. Operators normally had access to a small shack at the aft of the ship, and manned a winch attached to the device. In rough weather, this area would often be totally covered by waves. If the equipment was deployed, someone would have to man the cables, risking being swept away by turgid water. Because the design hadn’t been well tested, leaving the winch unattended would result in it unwinding until the bathythermograph was lost. Onlookers would often see technicians disappear from sight behind the waves.


Though it was attached to the ship with a cable, the bathythermograph required purely manual readings. To check depth and temperature, technicians would load an oil-covered slide into the torpedo-shaped housing, let it loose, then reel it back in to take a reading.

As these sensors advanced, submarines themselves would eventually make use of them. With bathythermographs mounted to their sides, submarines became capable of attacking ships without ever making visual contact.

Source: When Sonar Wasn’t Enough: The Bathythermograph

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