Sailor Speak of the Week – XBT



  • eXpendable BathyThermograph. A device that measures the water temperature at various depths which can be used to predict differences in sound velocity and sonar performance.


Unknown. Modern times.


A bathythermograph is an instrument that measures and logs the water temperature. An expendable version is usually a small device that functions in the same manner, but it’s thrown overboard and can transmit a signal back to the vessel.

Water temperature is important for submarines and in Anti-Submarine Warfare (ASW). Submarines that dive deep can get below the thermal layer (AKA thermocline) which is a temperature gradient of colder water below the warmer waters near the surface. It’s difficult for sound to propagate through the thermal layer. One way for ASW surface vessels to hear beneath this layer is to use a Variable Depth Sonar (VDS). Hence, why using some form of bathythermograph is important for knowing the temperature of the waters.


Crowell, J. (2003, October 31). Naval Terminology, Jargon and Slang FAQ Part 2 – N through Z. Retrieved from

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