WATER DENSITY AND ITS APPLICATIONS 



Similar to the preceding example, the main in-flow takes place at 

 the bed with heavily silt-laden water being carried into a relatively 

 quiet basin, and deposition ensues naturally. 



At sometime during the rising tide the salinity in the estuary 

 exceeds that of the basin, with salinity increasing continuously until 

 high water. This density difference tends to increase the inertia 

 forces and thus strengthens the silt-bearing flow into the basin. 

 Conversely, during the latter part of the ebb the salinity of the basin 

 exceeds that of the estuary and the out-flowing density current 

 assists the normal ebb flow from the basin. However, the total out- 

 flowing current is not sufficient to remove the silt deposited during 

 the rising tide. 



DENSITY AND SUBMARINE APPLICATIONS 



As an instrument designed for subsurface operation, submarines 

 are required to dive from the surface into deeper layers of water. 

 If the temperature and salinity of sea water were uniform from sur- 

 face to bottom, these operations would be relatively simple. These 

 two variables, however, change to cause varying densities which 

 complicate subsurface operation. Such a complication is the de- 

 crease in buoyancy of a submarine at great depths, caused by hull 

 compression. This must be compensated for by adjustments in bal- 

 last which the submarine must make under water. The presence of 

 a density gradient serves to complicate this adjustment. 



In addition to this, strong density gradients may hamper the 

 ascent or descent of a submarine as well as impair the attempt to 

 remain in balance or trim at a particular depth. The effect of 

 density can very well be seen when one considers that a change in 

 density of 0.001 changes the buoyancy of a fleet-type submarine by 

 as much as 5,400 pounds. A change in salinity of one part per 

 thousand alters the density by 0.00078 and thus changes the buoyancy 

 of the submarine by 4,200 pounds. The role of density is of critical 

 importance in certain phases of submarine operations. 



Early investigations of sound conditions in the oceans led to the 

 development of the bathythermograph, and it quickly became appar- 

 ent that the whole problem of sound transmission in sea water was 

 highly complex. Echo and listening ranges might be limited by any 

 one of half a dozen or more factors. Much research was necessary 

 before it was possible to devise a simple and accurate method of 

 translating the basic temperature-depth curve of the bathythermo- 

 graph into a practical prediction of sonar conditions. 



Basically, bathythermograph literature is divided into two main 

 groups, prediction manuals and charts. 



Prediction manuals are designed so the probable limits of sonar 

 ranges can be determined from any particular bathythermogram to 

 enable efficient tactical operations. The submariner obtains informa- 

 tion useful to him on the best depth for evasion, probable ping, 

 listening ranges, and the most efficient diving procedure. Perhaps 

 the most important use is for predicting ballast adjustments so that 

 diving can be accomplished as quietly as possible and with a mini- 

 mum of lost time. 



The above-mentioned charts include sonar charts showing average 

 echo ranging conditions, average diving conditions, and bottom 

 sediment charts for shallow water sonar work. Charts on average 

 conditions are of less tactical value than a bathythermogram 

 obtained at the time and place needed because conditions are 

 generally too variable to permit accurate predictions on the basis of 

 averages. On the other hand, they provide a perspective unobtainable 

 from a small number of bathythermograms and hence are useful not 

 only to the observer for determining how often bathythermograph 

 readings should be made but also for more strategic purposes. It is 

 generally agreed that submarines can change depth more quickly 

 and more silently if the diving officer understands the changes in 

 density in the superficial layers of the ocean. 



Maintaining efficient diving operations would be simple if the 

 buoyancy of sea water was uniform. Since it is not, the variations in 

 density that occur make each dive a separate problem requiring 

 slightly different tactics. For example, if the submarine is in trim 

 at periscope depth, its overall density is approximately the same as 

 that of the surrounding sea water. Consequently, it has no great 

 tendency either to rise or sink and such small movements as occur 

 are readily corrected with slight changes in the angle of the diving 

 planes. As the vessel travels at periscope depth it may move into 

 water of greater or lesser density. An increase in temperature makes 

 the water expand so that it is lighter. As density also depends on its 

 salt content, changes in density require reballasting to bring the 

 submarine back into trim. These lateral density changes are 

 relatively slight in most cases, however, and it requires no great 

 effort to keep the vessel on a horizontal course. 



If a submarine in trim at periscope depth dives in water of 

 uniform density it gradually gets out of trim because the increasing 

 pressure at greater depths compresses the hull, making the submarine 



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