Fig. 8. The Price current meter. 



The term "porpoising" refers to the tendency 

 of the meter to nose upward when rising and down- 

 ward when falling. This action occurs most 

 severely on current meters which are mounted in 

 trunnions and carry improperly designed horizon- 

 tal tail fin surfaces . One meter that porpoises 

 is the Price meter but it already is so sensi- 

 tive to vertical motion that the additional effect 

 is unimportant. Another in which the effect can 

 he most serious is the modified Roberts current 

 meter, one model of which both horizontal and 

 vertical fins have several times the area of 

 those in the standard meter of Fig. 9- Some 

 horizontal fin is necessary in this instrument to 

 balance the pressure moments of the large rotor 

 during vertical motion but the writer has 

 observed the meter to turn its nose nearly ver- 

 tically downward and upward during moderate 

 heaving of the vessel. The standard Roberts 

 meter also exhibits porpoising to a significant 

 degree. 



It is important to recognize that heaving 

 motions may be transmitted along the supporting 

 cable to much greater depths than are horizontal 

 disturbances . 



Fig. 9- The standard Roberts current meter. 



CONCLUSIONS 



A pessimistic picture has been painted of our 

 ability to measure ocean currents on a continuous 

 basis with suspended current meters and existing 

 techniques. The problems of stray motion and 

 limited dynamic response are extremely serious . 

 Generally speaking, small short period fluctua- 

 tions will be difficult to find. The large, long 

 period transients may be measured with fair accu- 

 racy but probably not well enough to determine 

 mean currents in those areas where the transients 

 far exceed the means. There is a great need for 

 a current meter of rapid response in both direc- 

 tion and velocity which is insensitive to stray 

 motion and will integrate accurately to zero all 

 of the undesired cyclic motions. 



Of existing current meters the most promising 

 is the Snodgrass current meter, in its continu- 

 ously recording form, although it is far from 

 perfect. Something will have to be done to con- 

 vert the form of the record into one which either 

 internally integrates or is easy to integrate 

 later from the record. Richardson's modification 

 of the Snodgrass meter appears to be producing 

 useful information and solving many of the 

 problems . 



Of existing mooring systems the stiff taut- 

 wire moor with a small surface float serves fairly 

 well the region from the submerged buoy downward, 

 but not the upper section. A close competitor 

 may well be the Richardson moor. It may be that 

 early work with continuously moored systems in 

 the more difficult areas may have to be done with 

 bottom mounted meters. 



The present discussion has tended to emphasize 

 situations which are serious because they are 

 easier to illustrate. It is readily granted that 

 in particular experiments much less serious con- 

 ditions may exist. In such cases the experimenter 

 must accept the burden of proving that his 

 results are real and meaningful. 



1^5 



