What's a "BT Drop"

I  have often been tempted to believe that the Navy has invented some particularly fiendish ways to get even with sonarmen.  One of these is a device called a  bathythermograph, or BT for short.  The BT looks somewhat like a rocket, but is actually a recording thermometer.

While the BT is being lowered to a depth of three hundred feet, the depth and water temperature are recorded on a smoked glass slide which can be read later on a graph.  When a boxful of slides has been accumulated, it is sent to  somebody in the Pentagon or somewhere, who probably laughs and throws the box of slides into the trash.

BT drops are routinely done every four hours while the ship is at sea, regardless of weather conditions. (I once lost a BT during a typhoon; it was found on the      fantail the next day.) Usually, two sonarmen will be sent to make a BT drop; one day , I was sent alone, and during general quarters, at that.

When the BT was down somewhere near 300 feet, mount 55 suddenly swung around and stopped with the end of the barrel pointing directly over my head, then BLAMM, BLAMM, blamm about five times.....I was deaf as a rock for three days.

Unfortunately, this was not an isolated incident; many ping-jockeys can tell the same story.  The Navy's second torture device is known as the FXR gear, (pronounced "foxer").  This consists of a series of parallel bars, which are bolted together in pairs a fraction of an inch apart.  When towed behind the ship, the bars vibrate and set up an outlandish racket which allegedly lures acoustic torpedoes away from the noise of the ship's screws.  This device was not a bad idea for its time.  The only difficulty was in retrieving the foxer gear using a hand-cranked winch.

On one occasion, "Leif" Ericson and I were sent aft to crank the fxr back  aboard.  We were convinced that the tow cable had suddenly stretched to a length of at least several hundred miles when the ship suddenly sat down on her haunches and started making turns for about thirty six knots.  That evening saw two very tired sonarmen ,both of whom had very low opinions of the people on the bridge.

http://www.steaminsteve.info/1956to1958.htm