Hadler and He eke r 



for the lowest speed of 5.18 ft/sec. The low-speed test clearly shows a sig- 

 nificantly larger torque coefficient and a small increase in the thrust coeffi- 

 cient in the base -vented flow regime. A close examination of the torque results 

 indicates that there is a possible difference for each test speed but this is 

 largely masked by the experimental scatter. An estimate of the propeller blade 

 drag shows that the base drag is quite large for the low speed test due to the 

 relatively low blade velocity, hence, the high torque values measured. Since 

 the base drag coefficient decreases as a function of the local velocity squared, 

 it becomes quite small at the highest test speed. This points to the importance 

 on this type of propeller experimentation of scaling the section o- values. 



A comparison between the K^ and Kg versus J curves for propellers with 

 airfoil sections shows marked difference from those with supercavitating-type 

 sections. Figure 26 from Ref. [3j is typical. It may be noted that at the high 

 advance coefficients the K^ and Kq values are independent of test rpm but as 

 J is reduced, the higher-rpm tests start to show a decrease; ultimately, there 

 is the transition point where there is a large drop in Kp and Kq, the magnitude 

 of which is sensitive to the test rpm. To explain the differences we should know 

 how ventilation develops on an airfoil-type section. At the higher advance co- 

 efficients the foil is at a low enough angle of attack that it is fully wetted, thus 

 K^ and Kq are independent of test speed. As the advance coefficient is reduced, 

 a vented cavity probably forms from a point near the maximum thickness. 

 Eventually the advance coefficient will be reduced to the point that a fully vented 

 cavity forms from the leading edge. It is at this point that there is a rapid 

 change in the lift, which results in the drop in the Kp and Kg values. The large 

 drop in lift observed on the airfoil section can be accounted for by noting that 

 when a fully vented cavity develops on an airfoil section it acts as a supercavi- 

 tating section with negative camber. Unpublished measurements of side forces 

 on wedgelike and ogival surface-piercing struts show much more radial change 

 in side-force coefficients for those struts with convex curvature on the pressure 

 face. The preceding comparison points rather clearly to the desirability of us- 

 ing section shapes that do not result in convex curvature when operating fully 

 vented and that have a minimum tendency towards cavitation inception at the 

 leading edge. 



COMPARISON OF TEST RESULTS 



So that comparisons can be more easily made between the performance of 

 the propeller at different test conditions, the system of ng and J versus cj 

 have been used in Figs. 19-22. The thrust coefficient c^ is based on the 

 submerged-disk area. These curves rather clearly show that the maximum ef- 

 ficiencies in the partially submerged condition are comparable to those for the 

 fully wetted condition. The lift-to-drag ratio of base-vented sections is greater 

 than when fully wetted, but the blade entry and exit losses must reduce their ef- 

 ficiency to the point where they are comparable. It should also be noted that the 

 range of C^ value over which a given propeller can operate efficiently is much 

 narrower than for the fully submerged condition. The extent of the operating 

 range is comparable to that of the supercavitating propeller. 



These curves, particularly those on Propellers 3767 and 3768 show that for 

 the base-vented condition there is partial collapse of the data for the range of 



1476 



