206 



suspended from the top of a tower. A load of 1500 pounds was suspended from 

 the free lower end of the cable and permitted to rotate. When rotation stopped 

 and equilibrium was reached the cable had unlaid 320 turns and had lengthened 

 16 inches. Repeated applications of the same load resulted in additional unlay- 

 ing and elongation each time. This elongation caused by rotation results in sub- 

 jecting the core to far more than its designed share of the total load and results 

 in its eventual destruction. This occurs because the outer strands accomodate 

 themselves to the increased length by assuming a lay helix of larger pitch. On 

 the other hand the already shorter core strand is twisted even tighter by the 

 lengthening of the rope lay. This permits the core to lengthen only by tensile 

 elongation and yield of its component wires. 



To avoid this kind of design deficiency a coreless three-strand wire 

 rope was constructed and tested in the same way. In contrast to the test record 

 of the seven-strand wire, the three-strand wire made only 40 turns and was 

 elongated only two inches under the initial loading. The amount of rotation and 

 elongation diminished with repeated loadings until a condition of equilibrium was 

 reached. 



As a result of these tests hydrographic winches on Scripps Institution of 

 Oceanography vessels have been equipped with 3/l6 inch 3x19 wire rope. This 

 cable has functioned satisfactorily and has a much greater life than seven strand 

 cable of the same material. It appears that now most cable retirennent will re- 

 sult from corrosion damage. One length of 3/l6 inch 3x19 cable of galvanized 

 improved plow steel has been in service more than 18 months. It should be 

 pointed out that the larger wires with thicker zinc coatings of this cable have 

 greater corrosion resistance than the relatively small, thinly clad wires of the 

 7x19 aircraft cord. It is true that there has been a sacrifice of some flexibili- 

 ty, but this does not appear to be an important loss. 



Tensile tests of the 3x19 cable have shown it to be as strong or stronger 

 than comparable aircraft cord. Cost of the new cable is about 60 percent of the 

 cost of aircraft cord of the same diameter and quality. 



DISCUSSION: Roger Revelle 



The desire to take as long a core as possible is a very natural one, for 

 what lies far beneath the sea floor challenges both the imagination of the geolo- 

 gist and the ingenuity of the inventor. But in the design and use of bottom sam- 

 pling equipment the objectives of investigation should be kept in mind. Consid- 

 eration of these objectives indicates that an ideal coring device should: 



1. Take as long a core as possible. 



2. Cover as large an area as possible. 



3. Disturb the sediments and the overlying water as little as possible. 



4. Collect any kind of material on the bottom, be it mud, sand or rock. 



5. Record or preserve the original orientation of the material. 



No corers have yet been developed which fulfill all these requirements 

 and the aim of most designers has been to satisfy only one of them. Conse- 

 quently it is necessary to use several different collectors to obtain satisfactory 

 samples for different purposes. 



