545 



HYDROGRAPHIC MANUAL 



Page 506 



545. The Advantages of Two Echo-Sounding Instruments 



It has been found advantageous to have two echo-sounding instruments on vessels 

 engaged in extensive hydrographic surveys, so that each may be used to its best ad- 

 vantage and so there will be no loss of time due to instrument failure. Most echo- 

 sounaing instruments are designed to operate to best advantage within a certain depth 



range and, in addition, some types of instru- 



DORSEY FATHOMETER No 3 ] mcuts wLll glvc morc dctailcd information 



INDICATOR - . 



than others, when soundnig over irregular 

 bottom. 



For surveys in shoal and moderate 

 depths the Dorsey Fathometer No. 1 and the 

 808 Fathometer have proved a useful com- 

 bination. Because of its greater accuracy, 

 the Dorsey Fathometer No. 1 is useful when 

 sounding over regular bottom. But where 

 the bottom is irregular, characterized by 

 sudden shoals, rocky bottom, or substrata, 

 the 808 Fathometer is used advantageously, 

 because the f athogram contains a permanent 

 record which may be carefully and deliber- 

 ately scanned for these features at any future 

 date. 



It is common practice to operate both 

 of these instruments simultaneously when 

 sounding, reading the depths on the Dorsey 

 Fathometer and referring to the fathogram 

 for bottom features of special importance or 

 interest that might be missed by the fathom- 

 eter attendant. The Dorsey Fathometer 

 No. 1 is frequently used for sounding in 

 depths beyond the range of the 20-fathom 

 scale, and instances have occurred in the past where the rapid changes of depth have 

 confused the fathometer attendant so that he added the wrong multiple of 20 fathoms 

 to the dial reading. In such cases the fathogram of the 808 Fathometer is invaluable 

 as a means of verifying the depths. 



Another combination of echo-sounding instruments that has proved valuable is 

 the Dorsey Fathometer No. 3 and the Hughes Veslekari. The recording feature of the 

 Veslekari instrument supplements the Dorsey Fathometer in the same way the fatho- 

 gram of the 808 Fathometer supplements the Dorsey No. 1. In deep water where the 

 bottom is extremely irregular, as in submarine valleys, the soundings of the Dorsey 

 Fathometer operating on the 312 Fathometer sonic system are often difficult to interpret 

 due to multiple reflections from slopes, but the fathogram of the Veslekari instrument 

 will show a pattern of soundings that usually can be analyzed to give a more nearly 

 correct value of the depth. The Veslekari instrument is a useful supplement to the 

 312 Fathometer for the same reasons. 



In order to avoid serious loss of time in isolated regions because of failure of the 

 echo-sounding equipment, a Dorsey Fathometer No. 3 and a complete 312 Fathometer 



Figure llO.- 



-Intereircuit connections between 312 Fathometer 

 and Dorsey Fathometer No. 3. 



