Fig. 7. The Snodgrass current meter. 



although the actual integration from the velocity 

 and direction records would be impossibly tedious 

 in practice. Richardson's modification of the 

 Snodgrass meter, with its digital recording sys- 

 tem, may provide a solution but it remains to be 

 proved that the discrete sampling is being done 

 at a sufficiently high frequency. Wo suitable 

 internally- integrating meters are known. 



Errors Due to Pendulous, Elastic-Cord and Rotary 

 Types of Oscillation 



Pendulum action is an effect which is not 

 well documented. One worker reported observing 

 unexpectedly high apparent currents at depths of 

 the order of 100 to 200 meters which he believed 

 due to such resonances. A calculation of reson- 

 ant lengths shows a suspension 100 meters in 

 length resonant to a 20-second period. However, 

 the period of ship's roll is in the region of 



5 to 10 seconds, corresponding to lengths of 20 to 

 80 feet, and one would anticipate a greater likeli- 

 hood of resonances in this region. The increased 

 damping of greater wire lengths also diminishes 

 the likelihood of long-period resonances . Meters 

 mounted in the span of a taut wire generally will 

 be more stiffly supported and responsive to shorter 

 periods . Experimental work would be desirable to 

 detect such phenomena. 



Another form of error due to a short rapid 

 pendulous motion is "jitter," a situation occur- 

 ring when an electrically registering rotor is 

 stalled or almost stalled on the contact. The 

 pendulous motion causes the rotor to oscillate 

 back and forth across the contact, producing rapid 

 contact closures which are seen as rapid forward 

 motion by the registering mechanism. Richardson 

 has pointed out that a related but not so serious 

 behavior occurs with current meters in his mooring 

 system fi Apparently there is a rotary oscillation 

 of the cylindrical current meter body about the 

 rotor caused by surges which induce torque gradi- 

 ents in the helically- laid supporting rope. For- 

 tunately, the register that counts turns is in a 

 form which permits the excess counts to be 

 detected. There is a suspicion that such rota- 

 tions may affect the reference compass and cause 

 errors in direction. Richardson also points out 

 that the Russian practice of mounting current 

 meters on an arm projecting from the supporting 

 cable must exaggerate errors due to cable rotation. 



Errors Due to Vertical Motion 



The errors due to up-and-down motion of the 

 current meter arise from k causes : (l) asym- 

 metrical water flow about the rotor generated 

 by the body of the current meter, (2) direct 

 sensitivity of unhoused rotors to vertical motion 

 due either to front-to-back asymmetry in the pro- 

 peller blades or to a form of turbine action 

 which occurs in horizontally oriented bucket 

 wheels, (3) porpoising and (K) constant tilting 

 of the current meter due to water drag which 

 exposes a projection of the face of the meter to 

 vertical motion. 



The first cause is difficult to avoid. In the 

 Ekman current meter the propeller has been housed 

 in a horizontal tube which undoubtedly removes 

 much of the effect. However, the writer has 

 directed tests in which meters of the Ekman type 

 were cycled up and down through a 5-foot motion 

 every 5 seconds to simulate the rolling of the 

 BROWN BEAR. Under these circumstances the rotor 

 ran backward at a rate corresponding to about 

 0.25 fps. The Ekman-Merz meter similarly treated 

 ran forward at the equivalent of . 6l fps . 



The Price current meter (Fig. 8) which has an 

 unhoused bucket-wheel ran forward at about 2.2 fps 

 in a similar test. A fraction of the effect prob- 

 ably was due to porpoising. One would expect the 

 Snodgrass current meter to be relatively insensi- 

 tive to vertical motion since all aspects of the 

 rotor and housing are nicely symmetrical with 

 respect to such motion. 



Ikk 



