Fully Cavitating Propeller for a Hydrofoil Ship 



the pull correction would amount to 13.5 Ibf for a 1.1/8-inch diameter shaft, 

 which is highly significant when compared with the total force measured on the 

 dynamometer, which in this case was just over 100 Ibf. Since the pressure at 

 the tunnel end of the shaft is always low with low cavitation numbers, the pull 

 correction does not vary too much; but it becomes a relatively large proportion 

 of the total force at the higher J values when the thrust approaches zero. 



It can be inferred from the above pressure measurements at C and B, and 

 from the fact that the difference between them is greater than the pressure drop 

 across the simulator, that the inflow velocity to the screw at point C is greater 

 than the upstream velocity, i.e., the inflow at this point is accelerated. While 

 this may not be the case at the outer radii, it does not confirm the statement 

 that it is possible for decelerated inflows to accompany fully cavitating propel- 

 ler operations. 



Referring again to Fig. A-1, it can be seen that the axial forces P^ and P^ 

 on the surfaces of the hub and downstream fairing cone X , and the annular area 

 of the upstream hub surface Y, have also been included in the measured shaft 

 force or thrust, and the net propulsive force arising from these pressure forces 

 is given by (P^ - P ) . In considering this force, it may be noted that whereas 

 the flow, and hence the pressure forces over the hub, downstream fairing, and 

 outer surfaces of the simulator near the propeller, are correctly produced on 

 the model in relation to the ship, there exists a little doubt as to the relative 

 magnitude of the force Py in the model tests. This arises because the condi- 

 tions in the gap between the propeller hub and wake simulator could not be cor- 

 rectly produced in the model arrangement when using a downstream shaft. 



A few tests were made on a model screw in which the gap between the 

 screw and the simulator was varied from about 0.6 to 3 percent of the screw di- 

 ameter, and it was found that the shaft axial force was unaffected while the gap 

 was small but began to fall off when the gap reached 3 percent of the screw di- 

 ameter. Subsequently, all tests were conducted with a gap varying from about 

 0.6 percent to 1.5 percent of the diameter, and in this range the thrust did not 

 vary with gap size. 



Finally, in connection with the measured tunnel thrust, the pull correction, 

 and the pressure forces acting over the hub, it should be pointed out that the re- 

 sistance of the craft should include the flow over the pods as far downstream as 

 the propellers, plus the pressure forces acting over the pod surfaces in the 

 propeller-pod gaps, both, of course, being determined with the propellers 

 operating. 



TUNNEL WALL EFFECT 



No corrections have been made to the results to account for the constraint 

 imposed on the flow by the presence of the tunnel walls. The tunnel used in 

 these experiments had a slotted-wall working section and, like an open jet tun- 

 nel, this is expected to reduce the corrections from this source to small values 

 as it does with noncavitating propellers. Reference 32 summarises the existing 

 data on fully cavitating propellers in relation to wall effects, but the results are 

 obscure and could not be used for making corrections. 



1015 



