Wo attempt has been made to derive the more 

 complex response of the taut-wire mooring since 

 there has been so little standardization that it 

 is more difficult to put plausible values on the 

 constants of the system. However, the response 

 will differ only in degree from those illustrated. 

 If the submerged buoy has a large buoyancy com- 

 pared to the drag forces on the lines and surface 

 float, it is likely that it may be the most rigid 

 system, even for a meter near the surface. For 

 a meter at the submerged buoy or in the span of 

 the taut wire it definitely has an advantage. Wo 

 analysis of Richardson's system is offered since 

 the extensive experimental results now forth- 

 coming from current meters on this moorage will 

 form a better basis for an analysis of errors. 

 This system may compare favorably with the taut- 

 wire mooring. 



It can be seen from the above that phenomena 

 occurring when the moorage is slack are completely 

 lost. Those velocities which occur during the 

 elastic stretching of the moorage are attenuated 

 and shifted in phase to a degree directly related 

 to the ratio between the effective extensibility 

 of the moorage and the linear transport of water 

 during a half -cycle. 



Rotary motions also are subject to similar 

 distortions. If the radius of the streamline of 

 rotary motion is less than that of the stretched 

 moorage the motion is lost, except for any effect 

 of the drag of the cable on the bottom. If the 

 radius of the true motion is somewhat larger only 

 a fraction of the velocity and transport will be 

 observed. 



A more hopeful situation exists if there is a 

 relatively steady current, involving large trans- 

 ports, upon which the fluctuations are superim- 

 posed. If the moorage is stretched so as to 

 remove most of its elasticity it will yield rela- 

 tively little to fluctuations along the line of 

 the moorage and these fluctuations may be fairly 

 faithfully represented. Lateral components 

 involving small transports, however, will be 

 severely attenuated or absorbed. It is easily 

 demonstrated that a moorage stretched to a radius 

 of 1,000 feet under a current of 1.0 fps will 

 respond to a small change of direction at a rate 

 which will attain 90/0 of the change in 2,300 

 seconds or 38 minutes, during which time the 

 direction record in the meter changes corres- 

 pondingly slowly. 



All of the stray motions which have been 

 described in this section are slow enough for 

 the meter to accurately register the relative 

 velocity with the consequent advantage that the 

 integral of all cyclic transports is zero over 

 one cycle so that the distorted velocities at 

 least can be removed from the results. In the 

 next sections effects will be treated which do 

 not sum to zero and which can contribute to 

 spurious determinations of long term transports 

 or average velocities . 



Meter Response to Rapid Changes in Horizontal 

 Motion ~~ ~ ~~ ~ ~ 



Rapid transient horizontal motions, real and 

 artificial, are presented directly to the current 

 meter by wave motion or indirectly by wave induced 

 platform movement or natural turbulence of the 

 water. Current meters are subject to a number of 

 spurious responses and response failures which can 

 lead to some confusion. Again the effects are 

 the most serious when the real currents or their 

 means are small compared to the real or artificial 

 transients. 



When a surface-floated platform rides in the 

 waves it undergoes complex motions due to the 

 waves. It is subject to the particle motion of 

 the waves which moves the platform back and forth 

 with wave period. This motion, combined with 

 the restraint of the moorage and the rolling of 

 the platform moves the current meter suspension 

 back and forth or, more generally, in a small 

 highly irregular loop, usually elongated. The 

 effect is amplified if the meter is suspended 

 from a boom extending far from the metacenter of 

 a ship. This motion, insofar as it is transmitted 

 to the current meter, is undesirable. If the 

 meter responded accurately the motion could be 

 recorded and integrated to zero. For various 

 reasons the current meter does not respond accu- 

 rately. The errors may be classified as due 

 either to failure in directional response or non- 

 ideal velocity response. Both will be treated 

 as though the suspension were fixed and the water 

 fluctuating but the treatment applies equally 

 well to the reciprocal situation. Failure in 

 directional response will be illustrated by 

 reference to two general types of current meter. 

 One has a propellor-like rotor intended to be 

 exposed to the current from the front only and 

 it carries a tail fin or other orienting device 

 to turn the entire meter to face the current. 

 The other has a rotor presumed equally sensitive 

 to flow from all orientations, e.g., the Savonius 

 rotor. Direction, if obtained, is derived from 

 a light vane, of short time-constant, rotated 

 with reference to an internal compass. 



In the first type of meter there is a definite 

 limitation in directional response. Take for 

 example a meter with a flat tail fin and the 

 center of the fin at a radius, r, from its center 

 of rotation. This meter is subjected to a step- 

 wise reversal of velocity at magnitude, v, and 

 lies with an angle between the source direction 

 of the current and the projection of the meter 

 axis through the tail fin. On a flat fin it may 

 be assumed that the component of the velocity 

 normal to the fin turns the current meter with 

 no slip and no inertial forces. The normal com- 

 ponent, of course, is zero when the meter is at 

 0° and 180° incidence, which is unreal because 

 it leads to an infinite period of rotation, 

 whereas the tail fin is certainly started on its 

 way in these indeterminate regions by stray motion 

 or turbulence. A more realistic estimate may be 

 obtained by integrating the equation between 

 limits of 3° and arc cos O.632; the latter a not 



lk2 



