Davis and English 



all given in terms of wake factors which are the local axial, tangential, and ra- 

 dial velocities divided by the ship speed, or 



Vg v^ v^ 



^a = "v"' f tg = T^ and f ^ = -^. 



When considering the fluctuating forces and moments that might be pro- 

 duced by the screw operating in a nonuniform flow, the inflow angle /3 is highly 

 significant, since this angle incorporates both variations in axial and tangential 

 velocities. This angle is defined as, 



tan [i = ^ = t 



Qr + T, V X77 7, 



tg ^jT- + tg 



J \ 



for an anticlockwise -turning propeller looking forward, and using the sign con- 

 vention for ftg shown in Fig. 5. The variation of d at the zero-pitch zero yaw 

 attitude of the pod is shown in Fig. 6, while Fig. 7 shows the results of harmoni- 

 cally analysing this /3 variation. Clearly, the amplitude of the third harmonic, 

 which could be of significance in producing undesirable vibration at blade pas- 

 sage frequency with a three -blader, is relatively low, lower in fact than the 

 sixth harmonic and only slightly larger than the ninth harmonic. Thus the at- 

 tempt at minimising the harmonic content of the wake corresponding to multi- 

 ples of the blade number has been quite successful and should ensure freedom 

 from serious thrust and torque fluctuations. The second and fourth harmonics, 

 which are important in relation to the production of fluctuating shaft bending 

 moments and which must be withstood by the shaft bearings, are higher than the 

 third harmonic. However, from the magnitude of the wake distribution there is 

 no reason to suspect serious excitation from this source. 



The wake simulator used in the No. 1 water tunnel was of simple construc- 

 tion and is shown mounted upstream of a fully cavitating screw in Fig, 8. Due 

 to limitations imposed by the size of the tunnel, the simulator had to be con- 

 tracted in the length dimension; but nevertheless a reasonable simulation of the 

 axial flow was found possible, as may be seen from Fig. 9. Adjustments to the 

 intensities and widths of the wakes shed by the arms of the simulator were made 

 by adding wire gauze to the arms. The intensity of the main foil wake could not 

 be reproduced before the arm of the simulator began to cavitate, and therefore 

 this particular wake shadow was not sufficiently intense. 



DESIGN STAGES AND PERFORMANCE CHARACTERISTICS 



The initial diameter chosen for the Bras d'Or screws was 3,67 ft. This was 

 based mainly on satisfying the requirements in the full-power high-speed condi- 

 tion, and on the desire to keep the screws as small as possible in order to obtain 

 high rpm and hence a moderate gear cartridge and pod diameter. Little propel- 

 ler data were available in the early stages from which the estimates of full- 

 power low-speed operation could be made, but as the project progressed and 

 more models were tested it became clear that it would be necessary to increase 

 the diameter to 4,0 ft to meet the low-speed high-thrust requirement at takeoff 

 and the VDS body towing requirement at foilborne speeds, 



968 



