Performance of Partially Submerged Propellers 



and 



_ Vs 



\ = ;^l/2 ■ - ■ 



where ^ is the linear ratio of full-scale to model propeller. 



In the open-water testing of fully submerged propellers it is not necessary 

 to observe the Froude law of comparison; hence, the practice of running these 

 tests at constant rpm just high enough to achieve an adequate local Reynolds 

 number to ensure turbulent flow (usually greater than 5 ■ 10^ at the 0.7 radius). 



It would appear from these tests that the same minimum-Reynolds-number 

 requirements will have to be maintained for the open-water testing of partially 

 submerged propellers. In addition there is the requirement of o- (or Froude) 

 scaling. Fortunately, the base drag coefficient decreases as the square of the 

 velocity; thus, there must be some a- value beyond which any further increase in 

 test speed is unwarranted. Some of the low advance coefficient data at the 

 higher test speeds in this series has cr^ values, based upon tip velocity, of less 

 than 0.005 (root section a values were considerably larger). The good collapse 

 of test results in this region adds support to such a suggestion. 



The advance coefficient of transition from base-vented to fully vented op- 

 eration is also subject to a scaling as demonstrated by the speed dependency 

 shown in Table 3. These results along with the transition value obtained from 

 the tests have been plotted as a function of o-^, based upon the propeller-tip 

 velocity in Fig. 27. Above and to the right of the line is the base-vented region, 

 and below and to the left is the fully vented region. An extrapolation of these 

 results to a lower a value would indicate that there is a probable maximum j 

 value at which transition would occur. If a is one of the dominant scaling laws 

 then it would be expected that geosims would provide a comparable curve. At 

 this writing a similar test has not yet been made on Propeller 3767 to check the 

 validity of this hypothesis. 



Assuming that this is the proper relationship for scaling transition, the re- 

 sults of the lowest test spots from Propeller 3767 are shown in this same figure; 

 hence, it becomes apparent why transition was not achieved on the tests on this 

 propeller. 



SUMMARY 



As stated in the introduction, the results of these experiments tended to 

 raise as many questions as they answered. It must also be remembered that the 

 experiments represent the results of only three propellers, all with similar-type 

 sections and all with a small number of blades; hence, the conclusions should be 

 construed as tentative, pending a wider base of experimental and theoretical 

 knowledge. 



Summarized below are the major findings from these experiments. 



1481 



