ENTRAINED AIR 



455 



Figure 7. Underwater photograph of white water under hull of a PT boat traveling at 9.5 knots. 



stroyed and thus removed from the ocean. Even 

 granting the validity of this physical assumption, the 

 removal of bubbles cannot be expressed mathemati- 

 cally by a vanishing bubble density. Inasmuch as the 

 nimiber of bubbles reaching the surface per imit time 

 and per unit area equals the product of the bubble 

 density and their average velocity upward, a vanish- 

 ing bubble density implies a vanishing number of 

 bubbles reaching the surface and thus does not cor- 

 respond to the physical situation envisaged. In addi- 

 tion, the decay of turbulence as the wake ages may 

 also have to be considered. 



27.3 



ENTRAINED AIR 



The fact that sailing ships have a conspicuous wake 

 suggests that a good deal of air is trapped along the 

 waterline of any vessel under way. Such air might 

 materially contribute to the mass of bubbles ap- 



pearing in the wake of vessels propelled by engines. 

 For instance, if Figure 5 could be relied on, the hull 

 wake on the starboard side of the USS Idaho (BB42) 

 would be even stronger than the stern wake. Of 

 course, nothing is known about the extension in depth 

 of the respective foam masses. 



Qualitative tests ° showed that echoes from the 

 wake of a barge towed by a tug alongside could be 

 detected with an NK-1 type shallow depth recorder 

 ranging downward from a launch carried across the 

 wake. However, it was found that this wake was more 

 acoustically transparent than the wakes of vessels 

 propelled by screws and therefore probably had a 

 shorter lifetime. 



These conclusions were confirmed by experiments 

 in which sailing furnished the motive power. The 

 ship used was a 104-ft yacht; measurements were 

 made as described for other ships in Section 31.3. 

 The wake when using sail was never found to be 



