INTRODUCTION TO SONAR 





Figure 5-25. 



71.127 

 Expendable components and launcher. 



the drop required, the slower the speed neces- 

 sary. Thus, the ship is increasingly vulnerable 

 to submarine attack. The BT is often lost be- 

 cause of parting of the wire. It is subject to 

 damage caused by striking the bottom or other 

 objects. Because of faulty slides, or a faulty 

 stylus, several lowerings may be required be- 

 fore an acceptable trace can be obtained, thereby 

 prolonging the ship's exposure to attack. Rough 

 weather may produce such hazardous conditions 

 for operating personnel that a lowering cannot 

 be made at all. Additionally, the cost of pro- 

 viding each ship with three different types of 

 BTs must be considered, plus spares— each one 

 requiring its own grid and individual calibration. 

 For many years the Navy has been seeking 

 a replacement for the mechanical BT that would 

 be cheaper, more accurate and reliable, and 

 safer to use. After extensive evaluation, an 

 expendable bathythermograph (XBT), figure 5-23, 

 system has been accepted and is being intro- 

 duced into the fleet. The cost of the XBT system 

 is considered reasonably low. The extra fuel 

 consumed in maneuvers required for a mechani- 

 cal BT lowering often costs more than the 

 expendable components of the XBT. Also, the 



71.119 

 Figure 5-26. — BT recorder. 



88 



