"•■'■■ ' n Wereldsma 



The low-frequency interaction is of reduced significance. The high-frequency 

 interaction (mutual blade encounter) results in a lateral excitation. Critical 

 conditions of shaft whirling and propeller blade resonance have to be avoided. 

 For the case of a propeller combination with equal blade numbers the interaction 

 is expected to be important in torsional and axial directions, having frequencies 

 equal to the sum of the blade frequencies of both propellers and its multiples. 



3. For a proper determination of the resulting vibratory propeller motions 

 and accompanying forces the mutual hydrodynamic interaction coefficients of 

 both propellers have to be estimated in addition to the normal hydrodynamic 

 propeller coefficients. For the high-frequency excitations generated by the mu- 

 tual blade encounter the internal elasticity of the propellers affects the hydro- 

 dynamic coefficients (propeller blade elasticity). 



4. Compared with the conventional gearing of a turbine installation the epi- 

 cyclic gearing will result in a broader range of excitation frequencies as a re- 

 sult of a stronger rpm modulation due to seawave disturbances. 



REFERENCES 



1. Report of the ITTC Propeller Committee 1966, Tokyo 



2. Wereldsma, R., "Some Aspects of the Research on Propeller-Induced 

 Vibrations," International Shipbuilding Progress, 1967 



3. Manen, J.D.V., "The Choice of the Propeller," Marine Technology, New 

 York, 1966 



4. O'Brien, T.P., "Contra- Rotating Propellers for Large Tankers," Inter- 

 national Marine Design Equipment, London, 1967 



5. Wereldsma, R., "Dynamic Behaviour of Ship Propellers," Publication 255 

 of the Netherlands Ship Model Basin, 1965 



6. Jung, I., "Swedish Marine Turbine and Gear Development," SNAME Sym- 

 posium on Marine Power Plants, Philadelphia 1966 



252 



