Ducted and Contrarotating Propellers on Merchant Ships 



(3) If A^A^ is decreased from 1.10 to 1.00, which means that the dif- 

 fusor angle decreases from 8.5° to 3°, a loss in efficiency of about 2% is 

 obtained. 



(4) On heavily loaded ducted propellers, higher efficiency is obtained 

 for large duct lengths. This is probably due to the fact that larger values of 

 AyA^ can be used with longer ducts. 



(5) K a ducted propeller is tested at loads considerably lower than the 

 design load, the flow separates from the duct external surface and the efficiency 

 decreases. 



All the ducts calculated at SSPA are intended to have a shock-free entrance at 

 the design point. The angle a^ between nose -tail line of duct profile and propel- 

 ler shaft and the camber fj^/L of the different ducts are shown in Fig. 15. Cor- 

 responding values for the ducts tested by van Manen (8) are also given. 



It is interesting to note that the angle a^ is larger for the ducts tested by 

 van Manen than those designed at SSPA. If, for a given total load, a^, of the duct 

 is larger than the value valid for shock-free entrance, a peak of low pressure is 

 developed on the external surface of the fore part of the duct. Since this low 

 pressure must be followed by a pressure increase for a flow-accelerating duct, 

 too -large values of a^ mean an increasing risk for separation on the external 

 surface of the duct [compare point (5) above]. r-v ■ <c\ 



PROPULSIVE COEFFICIENTS 



6.1. Projects Investigated 



Most of the results discussed in this section have been obtained from self- 

 propulsion tests with models for two projects, one tanker of about 150 000 tons 

 DW and one fast container vessel of about 12 000 tons DW. Main dimensions of 

 these two projects are given in Table 6. In the case of the tanker two conven- 

 tional propellers, calculated for different numbers of revs., were tested as well 

 as three sets of contrarotating and six different ducted propellers. In the case 

 of the container vessel only two conventional propellers, designed for the same 

 number of revs., and two sets of contrarotating propellers were tested. The 

 different stern arrangements of the tanker and the container vessel are shown in 

 Figs. 17 and 18. From the figures it can be seen that in both cases conventional 

 afterbody shapes were used. 



Table 6 



1287 



