The Vibratory Output of Contrarotating Propellers 



It is interesting to compare the dynamics of a conventional gearing and an 

 epicyclic gearing, since the latter may lead to a simple and less expensive in- 

 stallation (6). 



Conventional Gearing— The turbine installation with a twin reduction gear 

 and two contrarotating propellers is shown in Fig. 10. The block diagram shown 

 in this figure describes the dynamics of the system. The two propellers are 

 represented by a single inertia Ij or 1 2, each being the sum of the mechanical 

 and hydrodynamical inertia effects. Additional feedback signals represent the 

 static characteristics of the propellers. It is assumed that one torque-rpm rela- 

 tion exists, without mutual interactions of both propellers. This assumption is 

 acceptable when the ship speed and the ratio between the speed of both propellers 

 have a constant value. The following equation can be derived: 



dn 



[Ij + A2(I G+ i^) + b2(I G+ I2)] -^ + [A2Fj(ni) + B^¥^{n^)]nj 3 Q^ - AD^ - BD, 



This equation gives the dynamic relation between the torque generated in the 

 turbine, the propeller load fluctuations Dj and D2 generated by ship motions and 

 orbital water motions, and the rotational speed of the engine and propellers. 



TURBINE 



1^6-7- I 2 1 1 



■--J^ 



vi^il 



-L-J. CON 

 ~ 1 p 



TRA ROTATING 

 PROPELLERS 



Fig. 10 - Low-frequency analysis of contrarotating 

 propeller arrangement with conventional gearing 



247 



