and an attenuation in the peaks of the recorded 

 flow. A more important contribution to the same 

 effect is introduced by the elasticity of the 

 moorage. This subject as well as the dynamic 

 behaviors of current meters will be discussed 

 below under "Dynamic Errors . " 



The problem of error in depth is more or less 

 self explanatory. Its importance depends upon 

 the gradient of velocity with depth and would be 

 especially serious in situations where there are 

 relatively sharp changes, as in the region of 

 the Cromwell Current . 



DYNAMIC ERRORS 



The dynamic errors associated with current 

 meters have been classified into 5 types : (l) 

 those due to slow, more or less random movements 

 of the platform, (2) those due to the elasticity 

 of long suspensions and the elasticity and slack 

 in moorings, (3) those due to dynamic failures 

 in the meter itself which prevents the accurate 

 following of rapid transients, (k) those due to 

 pendulous or elastic-cord types of oscillation 

 of the suspension excited by the rolling and 

 heaving of the platform or by turbulence and 

 (5) those due to vertical motions of the meter. 



Effects of Slow Movements of the Platform 



The effects of slow movements of the platform 

 will be discussed first. Platforms anchored on 

 the surface on relatively long and elastic anchor 

 lines undergo a complicated cycle of motions even 

 in constant currents. There is the oscillation 

 about a center near the bow, called yaw , oscil- 

 lation about a center near the anchor, called 

 swinging , both superimposed upon a fore-and-aft 

 (riding) movement due to cyclic tightening and 

 relaxing of the anchor cable. The platform moves 

 with each change of current and with each gust of 

 wind. The current meter dangling below on a wire 

 experiences these motions with some time delay 

 and attenuation due to the elasticity of the sus- 

 pension. In depths of 100 meters or less, on 

 small vessels the writer has found yaw, swinging 

 and riding nearly small enough to ignore at 

 velocities of one knot. In deep water with 

 larger ships and different weights and scopes of 

 anchor wire this conclusion might well require 

 modification. A long series of observations has 

 been made on the ARMAUER HANSEN in depths of 

 1,800 to ^,000 meters. Cursory examination of 

 these data shows fluctuations in the bearing of 

 the anchor cable of the order of one or two 

 degrees at water velocities approaching one knot, 

 whereas in more common conditions when velocities 

 were about 0.5 knot the scatter is about -5°. 

 More extreme fluctuations occurred at lower 

 velocities . 



assuming a sinusoidal variation in the velocity 

 of yaw. It is found that the amplitude of yaw is 

 *9 feet and the spurious velocity at the middle 

 of the yaw is 0.25 fps. The use of a buoy as a 

 platform will reduce the yaw greatly. Swinging 

 and riding still remain, however, and it is likely 

 that their combined effects are at least com- 

 parable to the velocity calculated above. A 

 strong and gusty wind blowing across the direction 

 of current flow can severely accentuate the motion 

 of the ship at low water velocities. 



Reduction in Dynamic Response Due to the 

 Elasticity of Long Suspensions 



The discussions of this section apply equally 

 well to the effects of rapid stray motion at the 

 top of the current meter suspension and to the 

 dynamic response of the meter and its long sus- 

 pension to real transients in the flow. It will 

 be shown that long suspensions buffer the current 

 meter against short rapid movements of the plat- 

 form but diminish the ability of the meter to 

 record transients in the velocity at depth. The 

 response of a current meter to a step-change in 

 velocity is derived first. 



Table I shows the time required to attain 90$ 

 response to a sudden 20$ increase in velocity for 

 a typical current meter suspended on a light wire 

 under a number of conditions. The initial wire 

 angle also is shown to give some feeling for the 

 depth error involved. The velocity of 3-0 fps at 

 10,000 feet is admittedly unrealistic, but the 

 reciprocal situation of currents being measured 

 from a ship drifting at 3-0 fps is quite a real 

 possibility. 



Table I has been calculated by a gross simpli- 

 fication of a complex problem. The meter area 

 and weight have been lumped together with the 

 area and weight of the lower third of the wire 

 and assumed concentrated at the resulting center 

 of area, at which point the water is presumed to 

 act. The remainder of the wire is assumed weight- 

 less and without drag. The waterforce has been 

 taken proportional to the square of the slip 

 velocity and the drag coefficient as unity. The 

 meter and terminal weight together are taken to 

 have a frontal area of one square foot and a 

 weight in water of 150 pounds . The supporting 

 wire is assumed to have a diameter of 0.1875 

 inch. Inertial forces are neglected and the 

 velocity of the meter at any time is taken as the 

 difference between the new water velocity, v 2 , 

 and the slip necessary to maintain the wire angle, 

 9 , at that instant. 



The slip is given by 



v = f 



tan 9 

 cos 9 



(1) 



Consider the spurious error due to yaw alone 

 calculated on the assumption of a swing of +5 

 during a period of about 2 minutes at a point 

 100 feet from the bow of a lit-0-foot vessel and 



138 



