ratio which lowers the hydrodynamic drag. Steel is immune to fish bite 

 but very susceptible to abrasions and corrosion. Its high in-water 

 weight makes steel armoring depth limited. 



Cable with metallic armor is quite stiff and requires a large bend- 

 ing radius. Titanium has been considered as a possible armoring material 

 because of its high strength, but it generally is cost prohibitive for 

 most applications (see Corrosion). 



In general, synthetic materials have a number of advantages over 

 their metal counterparts. These advantages are: (1) high strength-to- 

 weight ratio; (2) relatively good flexibility; and (3) resistance 

 to corrosion. Use of synthetics in the ocean environment, however, has 

 been limited compared to the use of improved plow steel for cable 

 manufacturing . 



From Table II-l and Figure II-2, it can be seen that the synthetic 

 material "PRD-49" has a tensile strength about 1.6 times greater than 

 improved plow steel and a specific gravity which is only 18.7 percent of 

 improved plow steel. However, "PRD-49" has a modulus of elasticity about 

 one half that of improved plow steel. This characteristic may be unde- 

 sirable in applications where minimum elongations are a requirement. In 

 working cable applications where bottom breakout force is involved, how- 

 ever, the lower modulus may be an asset. An important negative factor of 

 "PRD-49" is its present very high cost when compared to improved plow 

 steel or the other materials. 



The synthetic, "Parafil", shows promise in cable applications. 

 "Parafil" is high-strength polyester filaments densely packed in a tough 

 polyethylene sheath. The completely parallel construction makes the 

 cable inherently torque-free and, thus, the cable will not rotate under 

 load nor will it kink. It also exhibits several advantageous properties: 

 (1) minimal creep (=16 percent tension reduction after 100 weeks of load- 

 ing to 80 percent of breaking strength); and (2) thermal stability. 



Figure II-4 shows a comparison of the modulus of elasticity of the 

 various materials used as strength members with the modulus of elasticity 

 of copper" as manufactured for cables. It can be concluded from this 

 figure that for the same unit elongation, stainless steel, improved plow 

 steel, and "PRD-49" Type III will carry a larger tensile stress than 

 copper when all these materials are compared in the form of a straight 

 wire or yarn. On the other hand, "PRD-49" Type IV, Fiberglas Type ECG-75, 

 Dacron, Parafil (Type A, C and D) and Nylon will carry less tensile 

 stress than copper for the same unit elongation. Thus, this figure illus- 

 trates why a straight solid conductor should not be used for large load- 

 carrying E-M cables, particularly if the synthetic materials with moduli 

 of elasticity less than for copper are used. However, since copper 

 usually is used in the form of braids and twisted multiple conductors, 

 the design of an electro-mechanical cable can be such that all of the 

 tensile load is carried by the strength members. 



Types of Construction 



The double layer, contrahelically-wound, external armoring described 

 as the standard E-M cable is the basic type of construction for the 

 strength member. The two contrahelically-xround layers can be manufactured 



