THE EFFECTS OF HULL PITCHING MOTIONS AND WAVES ON PERIODIC 

 PROPELLER BLADE LOADS 



Stuart D. Jessup and Robert J. Boswell 



David W. Taylor Naval Ship Research and Development Center 



Bethesda, Maryland 2008A 



ABSTRACT 



Fundamental investigations were made of the effects of periodic 

 hull pitching motions and waves on the periodic loads on propeller 

 blades and bearings. These periodic loads were measured during care- 

 fully controlled model experiments in which the periodic hull pitching 

 motions, regular waves, and relative phase of the hull pitching to the 

 wave encounter were systematically and independently varied. The peri- 

 odic blade loads were calculated using trocltoidal wave velocity pro- 

 files, and representation of the propeller based on a quasi-steady 

 method . 



The results of both theory and experiment show significant modula- 

 tion of the amplitudes of the periodic blade loads with hull pitching 

 motions and wave frequency of encounter. Further, the experiments con- 

 firm the theoretical assumption that the individual influences of the 

 wave velocity profile and the induced velocities due to vertical hull 

 motions can be linearly superimposed. The influence of the hull sig- 

 nificantly modifies the amount of modulation of the shaft frequency 

 loads due to both the periodic vertical motion of the propeller and the 

 trochoidal wave velocity profile in the absence of the hull. However, 

 trends of shaft frequency loads are well predicted by simple periodic 

 variations of the velocity into the propeller, and a simple quasi- 

 steady representation of the propeller. Trends of the results are 

 shown to be consistent with available full-scale data. Therefore, for 

 engineering purposes, the modulation of blade loads due to waves and 

 hull motions for transom type hulls can be estimated by simple trochoi- 

 dal wave velocity profiles, quasi-steady propeller theory, and constant 

 multiples derived from the experiments presented in this paper. 



I. INTRODUCTION 



The mechanisms by which rough seas and resulting ship motions 

 influence the time-average and periodic loads on propeller blades and 

 propeller shafts and bearings are complex. Factors include the in- 

 creased time-average propeller loading due to increased hull resistance 

 and the increased periodic loading resulting from the influence of the 



