displacement amplitudes tested. Also, the DAS and Uniline cables were very different in terms of their 

 response characteristics. For example, the log decrements for the Uniline cable decreased by about 

 0.001 as the amplitude dropped from 0.1 inches to 0.02 inches; this was independent of the tension. 

 On the other hand, the change in structural damping of the DAS cable depended on the tension. For 

 tensions less than 200 pounds the log decrement increased by as much as 70-90 percent for amplitudes 

 between 0.02 and 0.1 in., whereas the increase was 30 percent or less for tensions above 200 pounds. 

 Some of the differences are undoubtedly due to the onset of slack cable conditions, but it should be 

 noted that the DAS cable damping increases were always dependent upon the tension even when the 

 cable was essentially taut. This discussion pertains only to the taut or nearly taut cases for which the 

 mounting struts were perpendicular to the cable axis; a similar analysis for other strut orientations 

 and/or lower tensions was not attempted because of the uncertainties involved. 



This discussion has shown the difficulties encountered in attempting to measure the structural 

 damping of marine cables. One inescapable conclusion, however, is that the structural damping of 

 cables is small as shown here, in Fig. 3.10 and in reference C2. This small structural damping places 

 most cable strumming conditions well toward the left-hand portion of Fig. 2.2 where the reduced damp- 

 ing is very small and hydrodynamic forces predominate in controlling the strumming response. 



C.2 Hydrodynamic Added Mass and Damping. The added mass effect due to the unsteady relative 

 motion between the surrounding fluid and the cable is customarily treated as being proportional, by the 

 added mass coefficient C^^, to the mass of the fluid displaced by the body. Therefore the ratio of vir- 

 tual mass density in water to the body's mass density in air is given by 



— = 1 + CJS (CI) 



Pa 



in which S is the specific gravity of the cable. It is assumed that the fluid loading in air is negligible. In 

 addition, the ratio of the in-air resonant frequency, Xi, to the in-water resonant frequency, fw, at the 

 same load and mode shape is given by (using the taut string approximation, as in Appendix A) 



1/2 



A 



fw 



Pw 

 Pa 



142 



(C2) 



