Hadler and He eke r 



which also gave by far the best overall performance. However, with the third 

 propeller it was not possible to get on the plane (i.e., over the resistance hump), 

 although this type has proved to be very successful in normal applications on 

 many planing craft. 



As already stated by the authors, the blade shape is very important and our 

 experience seems to confirm that airfoil-type sections are not suitable for this 

 working condition. For race boats, where the propeller is more deeply im- 

 mersed at lower speeds, the vertical forces generated by the propeller have 

 probably also some effect on the overall performance (trim). 



From some tests with Propeller 3 in the Vosper Cavitation Tunnel, run with 

 free surface at atmospheric pressure, the fully ventilated condition could be 

 studied at low advance coefficients to simulate the acceleration phase in the 

 hump region. The cavities were so large that hardly any water could be accel- 

 erated; only very small thrust (and torque) was measured. 



REPLY TO DISCUSSION 



J. B. Hadler and R. Hecker 



Mr. Volpich has added significantly to the historical aspects of this paper. 

 It should be noted that Mr. Volpich' s comments pertain to the low-speed shallow- 

 draft application of this type of propeller, where little or no ventilation occurs 

 on the blade. 



Captain Bindel requested performance information for the backing condition 

 with the ship going forward. To our knowledge no work has been done in this 

 area as yet. It certainly must be investigated before engineering application can 

 be made on an operating vehicle. 



Professor Kruppa's remarks pertain largely to possible vibration problems 

 on partially submerged propellers. Our paper was concerned primarily with 

 steady-state performance, and thus we have not treated the problem of vibratory 

 forces. We most certainly would agree with Professor Kruppa that significantly 

 increasing the number of blades offers one of the best means for reducing the 

 vibratory forces to an acceptable level. To this end we are conducting ongoing 

 research on an eight-bladed propeller to establish the effects of high number of 

 blades upon performance both steady- state as well as vibratory. 



Mr. Suhrbier notes the difference in performance at low advance coefficients 

 for similar propellers with different types of blade sections. His experiences 

 help point out the fact that little is known about interference effects of the cavities 

 between propeller blades, particularly at low advance coefficients. We must ob- 

 tain considerably more knowledge in this area if successful designs are to be 

 achieved for the propulsion of sophisticated, high-performance craft which have 

 a marked resistance hump to traverse in the process of achieving their design . 

 speeds. 



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