Stretch 



Stretching is a propensity of wire rope which must be taken 

 into account in selecting and using marine cables. When drops to 

 precise depths are required, a lengthening of the cable could re- 

 sult in erroneous data. Also, long cables could be weakened to a 

 point where critical instrumentation could be lost or subsurface 

 buoys would gradually rise to the surface as the cable unwound. 



There are two types of wire rope stretch: (1) elastic stretch, 

 which is a function of the elastic properties of the material of 

 which the rope is made, and which is recoverable in accordance 

 with Hooke's law; and (2) constructional stretch, which is caused 

 by the progressive adjustment of the individual wires moving to 

 seat themselves more firmly into the cable structure. Construc- 

 tional stretch is not recoverable and will manifest itself, during 

 the first few times the cable is loaded, to a degree depending upon 

 the severity of the loading. 



Corrosion and rope stretch, as discussed above, may be 

 considered mechanical problems, which depend largely upon the 

 composition and construction of the wire rope. There are other 

 aspects of the corrosion and wear problems, such as electrolytic 

 corrosion and the effects of sheave and drum action, but these all 

 point mainly to proper selection and maintenance of the cable. 



Rotation 



A further problem which exists in the use of wire rope in 

 oceanographic operations is that of rotation. When a weight is 

 suspended from a ship, the cable supporting it becomes a free 

 body able to rotate at will. Such rotation causes an unlay of the 

 wires composing the cable and could, if permitted to proceed far 

 enough, reduce its strength capability to the breaking point and 

 cause the loss of both cable and load. The personal hazards in- 

 volved are obvious . A further undesirable effect of the rotation is 

 that it opens the cable structure and allows entry of sea water, 

 thus hastening corrosion of the core and inner wrap wires. 



