Pien and Strom-Tejsen 



the body of revolution is appreciable in comparison with the blade rotating speed, 

 the streamlines will become spiral lines. 



After the two cases discussed, it is relatively easy to discuss the general 

 case with the propeller working behind a surface ship, where the wake field 

 does not have axial symmetry. In this condition, the blade loading is periodic. 

 However, from the viewpoint of ship powering, we are only interested in design- 

 ing a propeller which will produce the required total circumferential average 

 thrust or torque. Hence, we can replace the wake field behind a surface ship by 

 an "equivalent" wake field having an axial symmetry where the wake strength at 

 any radius is equal to the circumferential average of the original wake field. 

 The propeller is then designed as in the case behind a body of revolution. Hence, 

 in this case, the propeller operating condition is quite different from the condi- 

 tion for which it is designed. The actual performance of the propeller has to be 

 calculated after the propeller has been designed, as described in the next section. 



If the performance is not satisfactory with regard to alternating propeller 

 forces and pressure distribution over the blade, then such design conditions as 

 number of blades, amount of rake or skew, blade area, and blade contour may 

 require changes. 



Propeller Performance Prediction 



The previous section discussed the design of a propeller in the behind con- 

 dition and indicated that the performance in a circumferentially varying wake 

 field is not fully known. In this section we will describe how to calculate such 

 performance. The purpose of the calculation is twofold: to obtain assurance 

 that no operating trouble will arise from propeller cavitation or propeller- 

 induced hull vibration, and to gain more insight regarding such factors as rela- 

 tive rotative efficiency and effective wake. 



Information on the pressure distribution over the blade at various blade po- 

 sitions is vital if we are to determine the possibility of propeller cavitation or 

 propeller- induced hull vibration for the particular hull- propeller combination. 

 If the load variation is too severe, the propeller under consideration may have 

 to be redesigned. Perhaps the average radial load distribution needs revision 

 or the radial blade area distribution and the area ratio should be changed. Also, 

 the rake or skew might not be the optimum as viewed in the light of the particu- 

 lar wake distribution at hand. All these questions can be answered by conducting 

 a performance prediction computation. 



It is assumed that the wake field in the absence of the propeller is known. 

 Usually this information is obtained by a wake survey in the propeller plane. For 

 an accurate prediction, however, it is very desirable to have a wake survey car- 

 ried out in three different planes — one near the blade leading edge, one near the 

 blade trailing edge, and one in between these two planes. If the wake suvey is 

 carried out in one plane only, we are compelled to assume that there is no varia- 

 tion of wake velocity in the axial direction. 



114 



