thrust block. Any smoothing of the wake will therefore not only improve the hydrodynamic 

 performance of the propeller but also reduce one of the causes of hull vibration. 



Knowing the wake components at any point, it is possible to calculate the forces on 

 the section of the propeller blade at that point on the assumption that they will be the same 

 as those it would encounter in a steady flow of the same pattern and, by summation, the total 

 force and moment on the whole propeller. This method of analysis is called the "quasi- 

 steady" method, and Figure 85 shows the variation in total thrust for a 4-bladed propeller 

 behind the three 0.70 Cg models calculated in this way. Much theoretical work is in pro- 

 gress directed towards taking into account the dynamic effects of variations in wake velocity- 

 the so-called "unsteady" method-but in the meantime the "quasi-steady" method is com- 

 monly used for comparative qualitative calculations when considering the effects of possible 

 changes in propeller design. Wake diagrams of the type given will be useful in this respect. 

 The longitudinal and tangential velocity components around any circumferential line in the 

 propeller disk can be analysed into harmonic components, and the relative magnitudes of 

 these will have an important influence on the vibratory thrust and torque forces. The wake 

 pattern should therefore be considered as one factor whenever any decision is to be made in 

 the choice of number of propeller blades. 



ifO 50 60 



BLADE ANGLE JN DEGREES {$) 



Figure 85 - Thrust Fluctuation 



XI-29 



