520 isaacs [chap. 24 



This cascade persists for about one-third of a wavelength, or 100 ft in the 

 example. 



Obviously a design adequate to withstand this motion will withstand the 

 extent of motion of the common swell, but not necessarily its ceaseless 

 repetition. 



The comber, then, imposes the stress and excursion requirements on the 

 mooring, and the common waves and swell impose the requirements for 

 preventing chafing. 



It is essential that the surface float has sufficient restraint so that it is pulled 

 slowly through the comber and is not carried to its terminus ; otherwise it is 

 likely to be struck by the subsequent comber, and a succession of these events 

 can transmit very high stresses to the mooring system in general. 



In addition, a very lightly restrained surface float can "surf-board" and 

 broach-to on a comber, and be carried even farther than the terminus of the 

 comber. Hence, the next comber may strike it in the most vulnerable situation, 

 that is, when it is drifting back into position and possibly broadside. 



In storms in the deep sea, the dominant waves and winds are most likely to 

 be in the same direction. Consequently, the wind stress can be considered as 

 imposing an initial load and restoring force on the surface float that is carried 

 out of position by a breaking wave. This condition does not obtain near fast- 

 moving storm centers of course, but a mooring has survived the close passage 

 of the eye of one of the fast-moving "chubasco" of Mexican water, where wind 

 velocities were estimated at about 80 knots. 



C. Other Environmental Factors 



Other environmental factors that influence the design of a mooring are : the 

 electrical conductivity of sea- water, the low oxygen tension, sharks, seaweed 

 rafts, ice, seals and sea-lions, rough-bottom debris and boat traffic. 



The electrical conductivity makes it mandatory that no dissimilar metals be 

 in electrical contact in sea-water. 



The oxygen tension decreases with depth to a minimum in the deep ocean. 

 The oxygen tension rarely falls below 0.4 ml O2/I. in the open ocean, but it may 

 drop to 0.15 ml O2/I. or lower in trenches and basins. Some metals, including 

 some stainless steels, are inadequately protected at such tensions. 



Sharks are voracious creatures of the deep sea that will strike or bite at any 

 object that appears attractive. A small float on a line or a tag-end of line or tape 

 on an instrument cable frequently is bitten off or damaged by sharks. It is 

 essential that every component appear continuous with the system to minimize 

 shark bites. That is, tags, pigtails and small floats of different color from the line 

 should be avoided. 



Seaweed rafts appear close in to coasts, usually at oceanic fronts. A surface 

 pennant will become fouled by such rafts, and if the raft is large enough it will 

 carry the mooring away. Nipa rafts of immense dimensions are present in some 

 parts of the South Pacific. Where this problem is likely to be severe, and the 



