0.5 



ACTUAL TOWED 

 SPEED 



SPEED INDICATED BY ROTOR 

 (ONE REV. AVG.) 



130 140 



TIME (SECONDS 



Fig. 12. Response of rotor CS-2 to irregular changes in tow carriage speed. 



CONCLUSIONS 



We consider it presumptuous to assume that 

 there is such a thing as the "best" current 

 meter. A current meter, just as any other 

 instrument, must be evaluated on the basis of 

 the user's requirements. It is a good instru- 

 ment if it satisfies the particular needs with 

 high reliability and within the user ' s allowable 

 limits of accuracy, cost, time and manpower. It 

 is certain that the Savonius rotor current meter 

 is a good instrument for many applications . In 

 practically all cases its characteristics and 

 limitations must be recognized if not accounted 

 for at the data analysis stage. 



With reasonable manufacturing control the 

 standard Savonius rotor current meter should 

 operate reliably in the range of 0.05 to 3 or 

 k knots. Turbulent flow encountered in the 

 natural regime may be expected to bias rotor 

 performance; much yet remains to be done before 

 this bias can be quantitatively estimated. 

 Normally, measurement of highly turbulent or 

 rapidly varying flow with the rotor probably 

 should be avoided. Rotor output is likely to 

 be greater in the natural environment than in 

 a tow tank so the meter will register a higher 

 mean current speed than actually present . 



Marine fouling has a marked effect on rotor 

 output even when not very severe. The rotors 

 should either be kept clean or a correction 

 factor greater than unity applied. Presence of 



an anti-fouling aerosol similar to petroleum 

 jelly has little effect on performance causing 

 a slight increase of output. 



Above about 0.2 knots steady state performance 

 is essentially unaltered by changing the length 

 or number of tiers of vanes in the rotor. At 

 speeds approaching the threshold output will be 

 altered inversely proportional to surface area 

 of the rotor plates . Reduction of rotor diameter 

 increases efficiency, presumably due to reduc- 

 tion of surface area, particularly of the tier 

 separators . 



The Savonius rotor is a highly inertial device. 

 Response to acceleration can be several times 

 faster than to deceleration (when the flow past 

 the meter is stopped) and response factors are 

 strongly dependent on the magnitude of speed 

 changes . 



ACKNOWLEDGMENTS 



Support for these studies was jointly fur- 

 nished by the Hytech Division of Bissett-Berman 

 Corporation and the Office of Naval Research 

 under Contracts Nonr2119(10 at the A. & M. College 

 of Texas and Nonr21l6(l) at the Scripps Institu- 

 tion of Oceanography. George Barlow of SI0 

 assisted with the testing and N. E. J. Boston of 

 Texas A. & M. directed much of the data analysis. 



12^ 



