316 



calculations is valid for a hydrofoil of length 

 c = 10 cm, wherein the pressure distribution was 

 calculated by means of the profile theory for 

 incompressible flow with the completion of shock 

 pressures caused by the compressibility of the 

 water. With these calculations one question re- 

 mains unsolved: Up to which negative values can 

 the local pressures on the profile really decrease 

 in natural water? On the full scale propeller of 

 the "Sydney Express" the local pressure gradients 

 are probably steep and reach negative pressures, 

 causing bubbles with a diameter of 10 ym, or less, 

 to cavitate. Regarding the measurements, bubbles 

 with diameters of about 10 ym to 20 ym were still 

 recorded in the results from Test 47 (Figure 15) up 

 to Test 65 (Figure 16) . For un)cnown reasons, how- 

 ever, from Test No. 65 on nuclei with a diameter 

 of less than 20 ym frequently could not be measured. 

 On the other hand one has to consider that, the 

 smaller the nuclei concentration Zo becomes, the 

 smaller the biibbles enlarged by cavitation. 



It is apparent from these remarks that it would 

 have been desirable to record bubbles or nuclei 

 with a diameter below 5 ym. But this was impossible 

 even with a 4 Watt laser which delivers 900 mW on 

 the green line. Therefore, it has to be admitted 

 that not all bubbles, which possibly are partici- 

 pating in the cavitation process, could be detected. 

 The question arises whether this will be possible 

 without any doubt in the future and if it is 

 necessary or not. Also the following aspects would 

 have to be considered: the required laser intensity 

 is limited; the exact local pressure distribution 

 on the propeller blades is difficult to determine 

 and on the other hand the tensile stress that can 

 actually be supported by the sea-water is quanti- 

 tatively unknown. 



Before closing this paragraph a personal impres- 

 sion in connection with the bubble sizes should be 

 mentioned which is supported by the collection of 

 photographs in Figure 8 and by numerous additional 

 pictures and propeller observations on the "Sydney 

 Express": The propeller will always find in the 

 flow a sufficient number of small nuclei leading 

 to cavitation. Therefore, the fullscale cavitation 

 will always be more stable than the model cavitation 

 with its smaller negative pressures and its different 

 nuclei distribution. 



The white foam on the cavitation pictures of the 

 full-scale propeller clearly indicates a large 

 number of nuclei, which have led to cavitation and 

 grown together. 



8 . SUMMARY 



The comprehensive laser scattered light measure- 

 ments on the "Sydney Express" showed the following: 



1. The nuclei spectra measured in a seaway in 

 the Indian Ocean are quite different: In the range 

 of the nuclei diameter of 20 - 40 ym either a 

 relative or an an absolute maximiom of nuclei was 

 measured. (Figure 16) . The motions of the ship, 

 especially the pitching motion, are in this con- 

 nection as decisive as the wave motions on the sea 

 surface (Figures 18 and 19) . The nuclei of this 

 range (diameter: 20 to 40 ym) , consist of bubbles, 

 since the scattered light method, carried out at 

 the same time with the Aminco-colorimeter did not 

 show any difference between unfiltered and filtered 

 water. 



2. In good weather conditions the absolute 

 maximum of the bubbles with a diameter between 20 

 and 40 ym (Figure 20) disappears. The nuclei of 

 smallest diameter show the largest nuclei concen- 

 tration. It probably consists of bubbles and 

 suspended particles, as the comparison with micro- 

 scope- and Coulter Counter measurements has shown. 



3. Measurements made at different speeds 

 (Figures 21 and 22) have again resulted in an 

 absolute maximum at a diameter of 20 to 40 ym for 

 the smallest ship speed at 12 kn. These nuclei 

 certainly consist of bubbles, since the Aminco mea- 

 surement in this case also did not show any dif- 

 ferences . 



4. Measurements in shallow water show an 

 absolute maximum at a diameter between 20 and 40 

 ym. The majority of these nuclei consists of 

 suspended particles, as the Aminco scattered light 

 measurement have shown. These suspended particles 

 probably do not contribute to cavitation, since the 

 comparison of propeller excited pressure fluctuation 

 measurements between deep and shallow water shows 

 practically no difference (Figure 7) . 



5. The ship's vibrations caused by the propeller 

 do pose a big problem for measurements of this type. 

 The insensibility of the laser against vibrational 

 stresses, however, after it was stiffly connected 

 with the ship, was suprisingly good. Even the 



high loading caused by the temperature did not 

 create any bad effects in the laser. 



6. Future laser measurements should possibly 

 anticipate diameter ranges below 5 ym. A more 

 precise determination of suspended particles 

 requires a greater effort than the present method. 



7. Further results of this trial will be 

 published later. 



ACKNOWLEDGMENT 



The comprehensive measurements on the "Sydney Express" 

 represented a project of the Sonderforschungsbereich 

 98 "Schiffstechnik und Schiffbau" (Special Research 

 Pool 98 "Marine Technology and Naval Architecture") 

 to which Det Norske Veritas (propeller observation) 

 contributed. The project was sponsored by the 

 Deutsche Forschungsgemeinschaf t. 



The authors wish to express their gratitude to 

 Hapag-Lloyd who made the "Sydney Express" available 

 for this investigation. Many thanks are expressed 

 to Captain W. Scharrnbeck, the Chief Engineer H. 

 Zwingmann, and the whole crew of the "Sydney Express", 

 who, by their excellent co-operation, made possible 

 the measurements and good results. 



The authors are grateful to: Ing. (grad) L. Hoffman 

 (Hamburg Ship Model Basin - electronics, programming 

 and evaluation); U. Steidlinger and W. Folkers, 

 (Institute for Shipbuilding, Hamburg), and F. Meier 

 (Technical University Munich) - all three provided 

 for the mechanical construction and repair on board; 

 Mrs. U. Schmidt (Institute for Shipbuilding, Hamburg , 

 - for drawing the diagrams) ; Miss A. van Blericq 

 (Hamburg Ship Model Basin - for the translation of 

 the German original into English); and Mrs. I. Jurschek 

 (Institute for Shipbuilding - for typing the manu- 

 script) . Dr. R. Doerffer (Institute for Hydrobiology 

 of the Hamburg University, SFB 94) recommended the 

 Aminco scattered light device and made suggestions 

 concerning oceanography. 



