van Manen and Oosterveld 



5 percent LBP at aft FP. The tanker model was tested at the loaded and the 

 ballast condition; the cargo liner model was tested only at the loaded condition. 



The results of the resistance and self -propulsion tests are presented in 

 Tables 4 through 6. Figs. 9 and 10 show the performance predictions for the 

 tanker and the cargo liner. Table 7 compares the results of the propulsion tests 

 with the ship models equipped with the conventional screws and the contrarotat- 

 ing propellers. Table 7 shows that application of contrarotating propellers on 

 both ships gives a significant reduction in DHP. The DHP of the tanker model 

 with contrarotating propellers is about 4.5 percent less in the loaded condition, 

 and 8 percent less in the ballast condition, when compared to the model with the 

 conventional screw propeller. The contrarotating propellers behind the cargo 

 liner require about 6.5 percent less DHP than the conventional screw propeller 

 in the loaded condition of the ship. The gain in trial speeds, due to application 

 of contrarotating propellers, is at maximum power absorption (16,000 DHP): 



tanker in loaded condition, 0.12 knot; 

 tanker in ballast condition, 0.30 knot; 

 cargo liner in loaded condition, 0.21 knot. 



The tanker with the conventional screw arrangement suffered from air sucking 

 into the propeller plane in the ballast condition, whereas this phenomenon did 

 not occur when the contrarotating propellers were fitted to the model. This 

 must be attributed to the smaller diameters of the contrarotating propellers. 



An analysis of the various propulsion factors shows that the wake fraction 

 was larger for the contrarotating propellers than for the conventional screws. 

 This is due to the smaller diameters of the contrarotating propellers. In the 

 case of the tanker the thrust deduction factor did not differ very much. This 

 factor was somewhat larger for the cargo liner with contrarotating propellers 

 than with the conventional screw. For the tanker, the increase in propulsive 

 efficiency due to contrarotating propeller application was principally obtained 

 by a better hull efficiency, whereas for the cargo liner this increase was ob- 

 tained by both a better hull efficiency and a higher open-water efficiency of the 

 contrarotating propellers. More detailed data must be made available, however, 

 to give a complete explanation of the obtained reduction in DHP. 



Cavitation Tests 



Cavitation tests were conducted in the 40-cm-diameter slotted wall cavita- 

 tion tunnel with flow regulator of the Netherlands Ship Model Basin (13,14), 

 simulating the full-load operating conditions. The axial wake distributions be- 

 hind the two models, as measured in the deep-water basin by means of a pitot- 

 tube were simulated in the tunnel. The results of the velocity surveys in the 

 way of the propeller are described in Fig. 11 for the tanker and the cargo liner. 



The results of the cavitation tests are presented in Figs. 12 and 13. From 

 an examination of the various test results it can be concluded that the conven- 

 tional screw and the forward propeller of the contrarotating propellers are quite 

 comparable as far as blade cavitation is concerned. This holds as well for both 



144 



