are required to reduce the taped chronograph recordings of the data 

 output from the Roberts Meter to a summary of current speed and 

 direction at various times. 



D. RECENT INSTRUMENTS 



Newer developments which show promise for wide- scale application 

 to Hydrographic Office surveys are: (1) the various neutral buoyancy 

 drifting acoustic sources and (2) the electromagnetic underwater log 

 which is currently under investigation. 



1. Neutral buoyancy acoustic sources 



The use of free-drifting acoustic sources that can be adjusted 

 to hover at any depth and be tracked with hull-mounted hydrophones 

 has interesting possibilities but at present appears prohibitively expen- 

 sive except for very specialized applications. These units cost approxi- 

 mately $200 each and are not recoverable. 



2. Electromagnetic underwater log 



The U. S. Navy has underwritten the development of an underwater 

 log that utilizes electromagnetic principles. The sensor is designed to 

 measure flow with an accuracy of ± 2 percent and has a threshold value 

 of 0.1 knot. It has potentialities as a current meter, but its conversion 

 will involve considerable development. 



3. Savonius rotor current meter 



This instrument has been developed recently for use in sensing 

 ocean currents of very low drift. The estimated range of current 

 measurements is 0.05 to 3.0 knots. 



E. CONCLUSIONS AND RECOMMENDATIONS 



Of the meters owned and/ or tested by the Hydrographic Office, the 

 most versatile is considered to be the Roberts Meter. At this Office 

 a method has been developed and tested to feed the output from the 

 Roberts Meter into a recorder which would print speed and direction 

 directly onto a paper tape or other permanent form of recording or 

 through an electric typewriter. Records in this form also are suitable 

 for analysis by computers, and development of computer programming 

 for such analysis is recommended. Further, improvements in cali- 

 bration and mechanics should be. pursued at the Hydrographic Office 

 to try to obtain the maximum potential performance from the Roberts 



VI- 6 



