Ducted and Contrarotating Propellers on Merchant Ships 



0.65 



o Conventional propellers, S5PA 4 60 



cf - " - -"- designed, project 



• Contra-rotating propellers 



75 100 125 150 175 200 



Number of revs, jmin 



Fig. 20 - Container-vessel project. Results of 

 self-propulsion tests. 



in open-water efficiency of the order of 5-7% can be expected. This gain, to- 

 gether with the better value of the relative rotative efficiency, makes the im- 

 provement as large as 12% in the present case, in spite of the lower value of the 

 hull efficiency. 



7. CAVITATION AND VIBRATION STUDIES 



As mentioned above, in addition to the efficiency it is also necessary to dis- 

 cuss cavitation and vibration properties when comparing the merits of different 

 propeller arrangements. Although these questions have not been finally treated 

 in connection with the investigation mentioned above, some preliminary experi- 

 ments have been carried out in order to illustrate the situation. The experi- 

 ments include cavitation studies in uniform flow with all the propellers, cavita- 

 tion experiments in irregular flow distributions with some of the contrarotating 

 propellers and introductory studies of local pressure variations on the afterbody, 

 induced by contrarotating propellers. 



7.1. Cavitation Inception in Uniform Flow 



Some indication about the danger for cavitation with different propellers 

 might be obtained from the studies of incipient cavitation. 



Tanker project. For the tanker project three different sets of contrarotat- 

 ing propellers have been tested, as mentioned in Sec. 6. They differ primarily 

 with regard to diameter, radial pitch distribution, and blade area ratio (see 

 Table 7). Curves for incipient cavitation phenomena, including tip vortex, back, 

 and face cavitation, are presented in Fig. 21. All the curves are fairly favour- 

 able in comparison with corresponding curves for conventional SSPA 5.60 



1293 



