extract from the complexities of motion those com- 

 ponents which are of interest for the purpose at 

 hand and (2) sufficient distribution of the mea- 

 surements in time and space. Attention often 

 must he concentrated upon a particular component 

 of motion because the entire field of motion is 

 too complex for present understanding or because 

 a current meter designed for one range of veloci- 

 ties may not function in a range which is grossly 

 greater or smaller. A current meter designed for 

 measuring the particle velocities of waves may be 

 of little use for general purposes and one which 

 might measure the low velocities in deep water 

 may not serve at higher velocities. Furthermore, 

 one must understand what he wants. When near- 

 surface measurements are made, either by drift 

 bottles or with the GEK, the result obtained is 

 sensitive to a few feet difference in the depth 

 of immersion of the measuring device. There has 

 been a tendency to dismiss as inaccurate those 

 measurements made quite near to the surface, 

 explaining the errors as due to "windage." The 

 writer believes that such differences likely 

 represent real gradients in velocity with which 

 we are not yet prepared to deal. We are more 

 comfortable describing currents which represent 

 greater volumes of flow or which are representa- 

 tive of the forces acting on more deeply immersed 

 objects like ships or buoys. 



If a mean is desired then the period of mea- 

 surement must be long enough to obtain a good 

 mean and if the result is to represent a volume 

 rate of flow over a large section, the complete 

 section must be investigated in width and depth 

 with due regard to the variations caused by 

 weather and other variables. Too much wishful 

 thinking has been done of the type which purports 

 to describe a sine curve by means of one point 

 measured at random. 



Exploration, of course, is legitimate and 

 necessary. What is objectionable is any tendency 

 to assign wide validity to isolated measurements. 



THE EREORS IN CURRENT METERS HUNG FROM A SHIP 

 OR BUOY 



To again clarify the limitations placed on 

 this discussion it is emphasized that attention 

 is being concentrated on current meters which 

 can be suspended from a ship, buoy or other plat- 

 form. For convenience, considerations are 

 limited to meters whose velocity sensors are 

 rotors but the same principles apply, almost 

 without exception, to other types of meters 

 similarly suspended . Other devices such as the 

 geomagnetic electrokinetograph, parachute drogue 

 and Swallow float are omitted. These still have 

 their spheres of usefulness but at present they 

 appear too slow, inaccurate or expensive (in 

 terms of ship time) to solve the problem of mea- 

 suring transient flows in the detail which will 

 be necessary in the future. No attempt will be 

 made to catalog the various types of current 

 meters as this has been done by a variety of 

 investigators . 2> 3> ^ 



It is scarcely necessary to point out that' the 

 current meter, even in ideal behavior, measures 

 only the water motion relative to it so relative 

 motion of the meter introduces an error. If the 

 direction element fails to respond precisely 

 another error results. These errors are discussed 

 under two somewhat overlapping headings: (l) 

 those errors which can occur without any motion 

 of the supporting platform and (2) those due to 

 motions of the support in company with failures 

 of the meter and its suspension when exposed to 

 rapidly changing flows. The latter have been 

 called "dynamic errors . " 



ERRORS WHICH CAN OCCUR WITHOUT MOTION OF THE 

 PLATFORM 



The errors in current meters hung from a ship 

 or buoy arise mostly from the motions of the 

 platform butthere are at least 5 sources of error 

 even if the platform is fixed. These are: 

 (l) distortion of the near-surface flow by the 

 platform, (2) deviations of a magnetic compass 

 in the meter by iron on the platform, (3) elasti- 

 city and hence distorted response of the long 

 suspension, (k) dynamic errors of the current 

 meter itself and (5) indirect error which results 

 from the error in depth due to wire-deflection 

 when the meter carries no depth element. 



The first error may readily be demonstrated 

 by simultaneous comparison of current meters near 

 the bow and near the waist of a vessel, or of 

 meters on opposite sides of a vessel, at depths 

 less than perhaps 2 or 3 times the ship's draft. 

 It requires little imagination to picture a 

 variety of distortions due to the large body of 

 the ship at various angles relative to the cur- 

 rent. Such effects may extend laterally for 

 distances of the order of one ship's length as 

 one quickly discovers when using a drift-pole 

 paid out on a measured line from the stern. The 

 effects are particularly serious in small currents 

 (less than one knot) when wind and the yawing of 

 the vessel may make the vessel lie at a sizable 

 angle to the current. In measurements made from 

 a freely drifting vessel the distortions may be 

 less serious unless there are important velocity 

 gradients in the sea between the surface and 

 twice the depth of the keel or the vessel is 

 drifting rapidly with respect to the water. 



The deviations of a magnetic compass by iron 

 on the ship are well known. Yet, meters con- 

 taining magnetic compasses have been used blithely, 

 close to the vessel, with no determination of 

 the errors. Errors up to 12 in the indications 

 of a current meter hung near the surface at the 

 stern of the wooden vessel BROWN BEAR have been 

 demonstrated . ? On steel ships the errors are 

 much more serious. The writer has found errors 

 of the order of 100° at a depth of one -quarter 

 ship's length and 2 keel depths below the surface 

 on a 275-foot ice-breaker. 



The effects of a long suspending cable are a 

 time lag in the response to changes in current 



137 



