Ducted and Contrarotating Propellers on Merchant Ships 



i , (2) The other sets of contrarotating propellers are comparable with 



the conventional propeller. The main difference between P1250-P1251 and the 

 other contrarotating propellers is a larger diameter (see Table 7). The results 

 seem to indicate that the optimum diameter in behind condition is larger than in 

 open water. Compare Figs. 14 and 19. Another difference is, however, that the 

 propellers P1250-P1251 have a circulation distribution, which gives more load 

 near the blade tips, i.e., a larger value of p^ 95/^0 7' further, the blade area 

 ratio is smaller. 



(3) The gain in efficiency for the best ducted propeller P1315 D6 is 

 about 7.5%, compared to a conventional propeller at the same number of revs. 



(4) The relation between the efficiencies of the different ducted pro- 

 pellers is approximately the same, when tested in open water (Fig. 16) and in 

 the behind condition (Fig. 19). Thus, duct D5 gives lower efficiency than duct 

 D6 and for the same duct the propeller, which is designed together with the 

 duct, generally has the best efficiency. 



The different propulsive coefficients calculated by established methods are 

 given in Table 7. By way of comparison the effective wake fraction and associ- 

 ated factors are determined, assuming thrust as well as torque identity. As 

 shown, great differences between the two methods are obtained, both for the 

 contrarotating and the ducted propellers. A comparison with the results of the 

 conventional propellers indicates that the thrust -identity method generally gives 

 more consistent results. . ,, .. 



A comparison of Fig. 19 with Fig. 14 shows that, for contrarotating propel- 

 lers, a great part of the final gain in efficiency, which is obtained at the self- 

 propulsion tests, must be attributed to improvement of the propulsive coeffi- 

 cients Vff (i.e., w and and 77^. If the thrust-identity assumption is used, both 

 7)1] and r)g are larger than for a conventional propeller, in spite of the slightly 

 higher thrust deduction factor ±. See Table 7. 



The reason for the slightly higher thrust deduction factor i in the contra- 

 rotating case might be the fact that the thrust coefficient Kj/J^ is smaller in 

 this case than for the corresponding single-propeller case. 



Also on the relative rotative efficiency vr of the contrarotating propellers 

 an influence of the thrust coefficient can be noticed, working in the direction that 

 77^ increases as the thrust coefficient decreases. This might be one of the ex- 

 planations of the fact that the optimum diameter in behind condition seems to be 

 larger than in open water. 



The gain in efficiency for the ducted propellers compared with a conven- 

 tional propeller at the same number of revs. (Fig. 19) was lower than could be 

 expected from the open water tests. Fig. 16 [(^7-7-/7'')^/^ = 2.3]. The main rea- 

 son for this is lower values of the hull efficiency tj^ (lower wake fraction wj 

 and higher thrust deduction factor 1). The relative rotative efficiency 77^ was 

 similar to that of a conventional propeller, and the thrust of the duct Kjj/K^j.vfas 

 about the same as in the open- water tests. 



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