315 



20 



i,0 60 



80 



100 -—/xm 

 Diameter 



FIGURE 29. LSL-technique compared with other 

 investigations . 



(1965). In Mullin's report tihe following average 

 percentage in the different size categories for 

 near surface samples (15 m) are given: 500 - 350 

 ym: 2%; 350 - 125 ym: 5%; 125 - 95 ym: 4%; 95 - 60 

 ym: 6%; 60 - 33 ym: 6%; 33 - 10 ym: 18^; and 10-1 

 ym: 58^. The content of organic carbon can amount 

 to 4.5 - 34% of the particulate matter in the 

 different regions of the oceans [see Zeitzschel 

 (1970)]. Zeitzschel continues: "It can be concluded 

 from the results obtained at the Gulf of California 

 and the above mentioned references that small par- 

 ticles, mainly in the range from 1 to 10 um in diam- 

 eter, predominate in offshore surface waters of the 

 oceans." Investigations by Gordon (1970) and Carder 

 et al . (1971), which are compared with our results in 

 Figure 29, revealed the same results. It is obvious 

 tJiat the "Sydney Express" results - ending at a 

 diameter of 10 to 20 ym for reasons of intensity - 

 would probably show strongly increasing particle 

 numbers below this range. This can be seen from 

 the results of Gordon (1970) and Carder et al. 

 (1971) which have been published by Jerlov and 

 Nielsen (1974) . 



The fact that a large number of small particles 

 in sea water show every arbitrary geometrical shape 

 (according to Zeitzschel) also reminds one of the 

 shapes of particles from the water of a cavitation 

 tunnel, shown by Peterson et al. (1975) - Figure 6. 

 These sea water particles of different shapes 

 (diameter 1 to 10 ym) , which according to Figure 29 

 are always available in a high concentration can 

 easily nucleate cavitation, as we know from many 

 investigations [(e.g., Peterson (1972) and Keller 

 (1973)]. 



The problem of the difference between real shapes 

 of the nuclei, detected by the laser beam in the 

 sea water and the diameters evaluated for the 

 measuring results can only be mentioned here. In 

 this connection one should remember that the cali- 

 brations on the "Sydney Express" were performed 

 with latex spheres, whereas the real shape of the 

 nuclei in the seawater is unknown. This problem 

 also arises with the Aminco-method and with the 

 Coulter Counter measurements, the latter working, 

 however, according to the conductivity principle. 



A further uncertainty is probably included in 

 the comparison of results obtained from oceanographic 

 studies carried out with water samples from the 

 open sea and those obtained from laser scattered 

 light measurements carried out in the flow and in 

 the boundary layer of the ship. The low-pressure 

 area of the boundary layer with its vortices of 

 different size most likely have a great influence 

 on the conversion of pore nuclei into bubbles when 

 they are moved from the calm free sea through the 

 boundary layer of the ship and thereby increase. 

 Due to the long running-time along the ship's hull 

 diffusion will also have an effect. 



These physical processes accompanying the growing 

 of the bubbles in the low-pressure areas of the 

 boundary layers and the effect of diffusion could 

 be the explanation for the fact that the lower 

 speeds (12 kn. Tests 79 and 83) show a larger 

 bubble concentration Co (due to the long running- 

 time along the ship's hull) than the higher speeds 

 (21.6 - 21.8 kn. Tests 70 and 90) with a shorter 

 running-time. (See measuring series with different 

 speeds - Figures 21 and 22). Thus - at a ship's 

 speed of about 60 rpm - a characteristic size of 

 bubbles has been formed. The measurements in a 

 seaway (Tests 61 and 65 - Figures 17 and 16) show 

 similar characteristic sizes of bubbles between 20 

 and 30 ym. In a seaway the turbulence is larger 

 due to wave and ship motions . According to Sevik 

 and Park (1973) the turbulence can lead to character- 

 istic bubble sizes in connection with the pressure 

 history. 



All considerations concerning bubble sizes must 

 finally lead to those bubbles participating in the 

 cavitation process. According to the calculations 

 by Isay and Lederer (1977), small bubbles, which 

 can also arise from pore nuclei, will grow faster 

 than big ones (Figure 30) . The result of such 



[r^i 



O.O0A 0.01 0.02 0JD4 0.1 0.2 



Cfiordlenght c=2A 



FIGURE 30. Calculated growtii of a single bubble in 

 a hydrofoil flow [Isay and Lederer (1977)]. 



