Page 469 



ECHO SOUNDING 



5235 



the 312 Fathometer, although the reeds have different vibration frequencies. The 

 tachometer is enclosed in a case to protect the reeds from injury. It is equally effective 

 as a speed indicator when sounding in either fathoms or feet. If the motor speed changes 

 from any cause, the reeds indicate the direction and degree of change, thus aiding in the 

 readjustment of the governor or in the use of manual control. The various values for 

 the seven reeds in this tachometer are given in table 13. 



5235. Batteries 



One heavy-duty 12- volt storage battery, or two heavy-duty 6-volt batteries, of 

 200-ampere-hours capacity or more, should be used to furnish the power for the instru- 

 ment. This capacity should suffice for 15 hours' continuous operation. The current 

 drain of the battery is 11 amperes. The battery voltage can be checked constant!}^, 

 while the instrument is in use, by means of the voltmeter on the cabinet. The battery 

 voltage may vary between 9 and 15 volts without interfering with the operation of the 

 instrument. 



After each day's operation the battery should be recharged. The charging current 

 should be about 15 amperes and it should be charged for about the same number of 

 hours the instrument was in operation. At the end of the charging period a hydrometer 

 test should be made to see if the battery is sufficiently charged. Since the charging rate 

 and hours of charging required will depend on the age and condition of the battery, it 

 will be necessary to alter them from time to time. Facilities should be arranged so that 

 the batteries can be recharged without removal from the launch. If it is desired to 

 charge the batteries while the instrument is being operated, the battery-charging unit 

 must be arranged so that it does not interfere with the operation of the instrument; for 

 this pm-pose it may be necessary to use a filter between the charger and the battery. 

 In remote regions a spare set of batteries should be carried in each launch. 



5236. Installation of Acoustic Units 



The method of securing the fish (see 5231) to the side of the launch will vary with 

 the dimensions of the vessel, making it impracticable to design one supporting structure 

 for use on launches of various sizes and designs. The following description of an installa- 

 tion used on one of the launches of the Coast and Geodetic Survey is intended to illus- 

 trate one of the possible ways of supporting the fish. This method with slight modifica- 

 tions might be used on launches of different design. 



With reference to figure 103, the two vertical supports are l)^-inch extra-heavy 

 brass pipe, threaded at each end. The lower ends screw into the pipe fittings on the 

 fish and the upper ends are screwed into 1%-inch brass tees. A 1}^- by 3-incli brass 



