Table I. Wire angle and times for 

 increase in velocity of £ 



response in current meter subjected to a step 



and the horizontal velocity of movement of the 

 meter by 



dx 

 dt 



1.2 



k tan 9 o 

 cos 9„ 



tan 9 



cos 9 



(2) 



For 9O70 response, 6 is chosen as the value cor- 

 responding to v = 1.18 v . This equation is 

 integrated numerically for angles greater than 

 10°, but at smaller angles it may be approxi- 

 mated by the equation 



where k = 



2gW 

 PCpA 



t= i 



vfF 



49 



:7T 



00 



W = weight of meter + terminal weight 



+ lower third of cable in water, lbs, 



p = density of sea water, 64 lb/ft^, 



Cp = drag coefficient, assumed 1.0, 



A = area of meter + weight + lower third 

 of cable, ft 2 , 



9 = angle wire makes with vertical, 

 radians, and 



9 = initial wire angle. 



The current meter is able to sense only t he 

 slip, which at time zero is the initial velocity, 

 v , and approaches 1.2 v asymptotically. 



To solve the above equation for time, t, dx 

 is replaced by its equivalent L cos 9 d9 where 

 L is the wire length. The time to attain a given 

 wire angle is 



t = i 



cos 9 



d9 



{kJQo 1P f tan Qp ^j 

 V cos 9 Q V 



(3) 



f tan 9 

 cos 9 



which, on numerical integration, gives 



t ._ 5-4 L^ (5) 



v/T 



The times for 50$ response are about 0.27 a s 

 great and for 95$ response are 1.3 times greater. 



From Table I it may be concluded that there 

 are severe restrictions on the ability to detect 

 short term transients when using long suspensions. 

 For placement of deep current meters one would 

 prefer to fasten the meter in the span of a taut- 

 wire mooring with a stiffness much greater than 

 that corresponding to 150 pounds of buoyancy. 

 The buffering offered to deep current meters 

 against short term fluctuations of the surface 

 platform is evident . 



It is of interest that the most important 

 contribution to the area in the preceding calcu- 

 lation comes from the wire itself. Table II shows 

 the areas and weights of wire used in the pre- 

 ceding problem. Since the times increase in pro- 

 portion to the wire diameter, every effort should 

 be made to keep the suspending wire as small as 

 possible if excessive lag times and wire angles 

 are to be avoided. 



139 



