SUMMARY AND CONCLUSIONS 



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 carefully con- 

 trolled model experiments on a twin-screw, transom-stern hull. The 

 objective of these experiments was to obtain systematic accurate experi- 

 mental data showing the effects of hull pitching and waves on periodic 

 and time-average blade and bearing loads under carefully controlled 

 experimental conditions so that the effects of ship motions and waves on 

 periodic and time-average blade and bearing loads could be isolated. 

 The experiments were conducted under steady ahead operation in calm 

 water with no ship motions, in calm water with forced sinusoidal pitch- 

 ing of the hull, in regular waves with no ship motions, and in regular 

 waves with forced sinusoidal pitching of the hull at a frequency equal 

 to the wave frequency of encounter over a range of phases between the 

 pitching motion and wave encounter. An error analysis indicates that 

 the experimental results are sufficiently accurate to support the con- 

 clusions drawn. The periodic blade loads were calculated using trochoi- 

 dal wave velocity profiles, and a representation of the propeller based 

 on a quasi-steady method. 



The experimental results show the following: 



a. The amplitudes of the periodic blade loads are significantly 

 modulated hull pitching motions and wave encounter. 



b. The time-average blade loads per propeller revolution vary 

 significantly with wave encounter but only slightly with hull pitching 

 motion. 



c. The peak blade loads per revolution vary significantly with 

 hull pitching motions and wave encounter. 



d. The individual influences of the wave velocity profile and the 

 induced velocities due to vertical hull motions can be linearly super- 

 imposed for transom stern configurations. 



The results show that the hull significantly alters the amount of 

 modulation of the shaft frequency loads due to both the periodic ver- 

 tical 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 propel- 

 ler. The quasi-steady representation of the propeller is sufficient for 

 this application because the frequencies of encounter of the waves and 

 of the hull pitching motions are low relative to the propeller rota- 

 tional speed; i.e., the reduced frequency is low. Therefore, for engi- 

 neering purposes, the modulation can be estimated by simple trochoidal 

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

 multiples derived from the experiments presented in this paper. 



The experimental results show that the first eight shaft rate 

 harmonics of blade loads are modulated and increased by hull pitching 

 motions and waves relative to the respective values in calm water without 

 hull pitching. Comparable modulations and increases in bearing loads 



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