Fully Cavitating Propeller for a Hydrofoil Ship 



APPENDIX 



WATER TUNNEL TESTING TECHNIQUES 



All the hydrodynamic testing for this project has been conducted in the No. 1 

 water tunnel in the Ship Division, NPL, and in the course of testing a number of 

 factors have come to light which are considered worthy of comment. These 

 mainly refer to the corrections that have been applied to the raw measured data 

 in order to determine the predicted ship values. Because it is not customary to 

 use water tunnel results when predicting conventional displacement ship propul- 

 sion from models, these corrections assume a greater importance in the model 

 testing of fully cavitating propellers where tunnel testing is necessary. 



WATER SPEED MEASUREMENT 



The largest single factor affecting the results has arisen through the use of 

 the upstream wake simulator. In order to set the water speed in the tunnel 

 working section, the flow downstream of the walce simulator was first measured 

 with a pitot rake and then this volumetric mean flow was plotted against the 

 pressure drop occurring in the upstream contraction. Subsequently, when the 

 screws were being tested the tunnel water speeds were set by using this cali- 

 bration. 



At any early stage in the test programme one of the fully cavitating propel- 

 lers was tested both with and without the wake simulator. When tested without 

 the simulator, the upstream bluff end of the screw hub was shielded from the 

 oncoming flow by attaching a fairing cone to the hub which rotated with the 

 screw. This procedure is customary when testing propellers with a downstream 

 drive shaft. However, in this case, due to the large amount of hub taper which 

 is necessary to fair in with the pod shape, the upstream fairing cone was neces- 

 sarily ill-shaped in order to keep it of reasonable length. Testing the screw in 

 this manner gave quite large differences compared with the results of testing 

 behind the wake simulator, the thrust without the simulator being less than when 

 it was tested with it. This was considered to be due to the large pre-swirl in 

 the direction of the screw rotation caused by the rotating fairing cone, a view 

 that was supported by the nature of the slight amount of cavitation that occurred 

 on the upstream cone. Subsequently, no further tests were conducted without the 

 presence of the wake simulator. 



SHAFT PULL CORRECTION 



A correction to the measured values of thrust has been necessary due to the 

 differential pressures acting on the ends of the downstream drive shaft. One 

 end of the shaft is in atmospheric air, while the other end is in the low pressure 

 of the working section. Normally, without a wake simulator, the pressure acting 

 on the propeller end of the shaft is taken to be the water tunnel working section 

 pressure. However, with the wake simulator that was used, it was necessary to 

 measure the actual pressure between the upstream end of the rotating screw and 

 the stationary downstream end of the wake simulator. The arrangement tested 



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