EXAMPLE 4: APPENDED BODY OF REVOLUTION WITH FOUR RAKED PROPELLERS 



In this example the thrust deduction analysis is applied to compare 

 four different propellers fitted to the same hull. The hull is an appended 

 body of revolution, represented by DTNSRDC Model 5224-2. This model is 

 identical to DTNSRDC 5224-1 described in Example 3 except for a slight modi- 

 fication to accommodate a large propeller hub (see Figure 18). 



The four propellers are wake-adapted designs with equal diameters and 

 thrust loading coefficients. A comparison of principal characteristics is 

 given in Table 6 and propeller geometry is presented in Tables 7 through 10. 



The radial distributions of bound circulation and hydrodynamic pitch shown 



18 



in Figure 19 were derived from lifting-line calculations using Lerbs' 



criterion for optimum loading. The propellers are distinguished by varying 

 amounts of skew with corresponding "skew back" along the respective geo- 

 metric pitch helices. As a result, the blade vake differs considerably 

 among the four propellers, as illustrated in Figure 20. Physically, it was 

 expected that these differences in blade axial positions would strongly 

 influence the thrust deduction. 



Calculated and measured values of thrust deduction are compared in 

 Table 11. Both the theoretical and experimental results exhibit a pro- 

 nounced decrease in the interaction force with increasing propeller rake. 

 However, the computed values of 1-t are substantially lower than measured 

 in each case. This discrepancy is very likely due to difficulties in the 

 experiment owing to the weight of the propeller models.* Comparison with 

 the result of Example 3 provides evidence that the measured values of t are 

 too high. 



The calculated contributions of the stern appendages to the thrust 

 deduction are given in Table 11. These vary from 19 to 24 percent of the 

 total, which is comparable to the result of example 3. In Figure 20 the 

 rake distribution of propeller 4567A is shown as a dashed line and may be 

 compared with propeller 4487. Since propeller 4487 is further aft on the 

 body and has a larger diameter, the thrust deduction fraction should be 

 lower. This is, in fact, predicted from theory (0.059 versus 0.09), while 

 the experiments yield a higher result for 4487 (0.109 versus 0.09). 



*These models were approximately 5 times heavier than typical models due 

 to material (bronze instead of aluminum) and size. 



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