WATER DENSITY AND ITS APPLICATIONS 



less buoyant. Therefore, under these conditions, a submarine must 

 pump ballast during the dive in order to maintain trim. The amount 

 of water to be discharged depends on the size of the submarine and 

 on its compressibility, the latter varying with the type of construction 

 and, to a lesser extent, with individual vessels. 



As previously pointed out, temperature may decrease from the 

 surface downward or in the water underlying a mixed surface layer. 

 With decreasing temperature the density would increase downward. 

 The sea may be thought of as a series of horizontal layers, one below 

 another, in which the density is either uniform or increases down- 

 ward by a greater or lesser amount. 



A layer of increasing density gives a diving submarine more 

 support and tends to counteract the compression effect. It is 

 conceivable that the two effects may just balance, so that a diving 

 submarine will remain in trim all the way down. On the submarine 

 bathythermogram card, Figure 5, are printed isoballast lines, which 

 show the amount of temperature change with depth that will, by its 

 effect on the density of the water, exactly balance the compression 

 effect of a submarine of the type for which the card was prepared. In 

 any layer where the temperature-depth trace parallels the isoballast 

 lines, the diving submarine will remain in the same state of trim 

 throughout the layer. If the temperature is more nearly uniform, so 

 that the trace crosses the isoballast lines toward the right, a diving 

 submarine will get heavier and will have to discharge ballast to re- 

 gain trim. In a strong gradient that crosses toward the left, it will 

 be light and will have to flood ballast. 



200 



300 



400 



30 



40 



50 



eo 



70 



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90 



Figure 5. Submarine bathythermogram showing effect of 

 temperature gradients on ballasting operations. 



It is common to find temperature conditions in the sea of the 

 kind shown in Figure 6, in which there is a surface layer of mixed 

 water and an underlying layer with a sharp decrease in temperature. 

 Suppose a submarine is in trim at periscope depth (Position A) and 

 makes a dive with no ballast changes. As it goes through the mixed 

 layer it gets heavier and sinks more rapidly. But the temperature 

 gradient below gives it more buoyancy again, and it finally comes 

 to trim at Position B, where the temperature trace intersects the 

 same isoballast line that passed through Position A. This is an 

 example of a very quick and efficient, dive that makes full use of 

 temperature and density conditions. It would be much less efficient 

 to dive in trim, discharging ballast while in the mixed layer, and 

 flooding again below it. However, if there were no temperature 

 gradient below the mixed layer, it would be more efficient, as well 

 as safer, to remain more or less in trim all the way down. Thus, the 

 correct diving procedure depends on knowledge of the vertical 

 temperature structure of the water. The above example is simplified 

 as it is not commonly possible to dive without making any ballast 

 changes at all. Nevertheless, the general principle holds in almost 

 any case. It is possible to dive more efficiently in water of known 

 temperature structure than in an unknown situation, because when 

 the diving officer knows the total amount of ballast change that will 

 be needed, he can make the proper adjustments at an even rate 

 through the entire operation. Hence, he will not be stopped by a 

 sharp density gradient or forced to increase the noise output of the 

 submarine in the effort to get through the layer. The time saved 

 diu-ing such a dive may be as much as 10 or 15 minutes. 



To obtain proper knowledge of the temperature structure of the 

 water, it is necessary for the submarine to make frequent exploratory 

 dives. Density conditions are best coped with in an emergency if a 

 recent "BT" record is available. How often such dives are needed 

 depends on the variability of temperature conditions where the 

 submarine is operating. 



The usefulness of density layers is too obvious to escape any 

 submariner. For them it is a fortunate coincidence that from both 

 the acoustic and diving standpoints density layers provide the best 

 possible protection in evasion. The so-called "layer effect" reduces 

 the echo and listening ranges. A submarine submerged well below 

 the top of a density layer is more difficult to detect. It is also 

 apparent that a submarine in the middle of such a layer requires little 

 effort to maintain constant depth. If it rises it will be in water of 

 less buoyancy and will tend to sink again. If it goes deeper it will 

 encounter more buoyant water. Hence, it is not only easy to maintain 

 quiet operation, creeping or balancing with the motor stopped, but 

 it is also easier to maintain control of the ship. 



30 



100 



300 



400 



30 



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DEGREES FAHRENHEIT 

 50 60 70 



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90 



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200 



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- 400 



90 



Figure 6- Submarine bathythermogram showing diving 

 operations in a mixed layer with an underlying 

 negative gradient. 



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