The Vibratory Output of Contrarotating Propellers 



From this experience it is possible to give indications about changes in the 

 fluctuating forces to be expected due to changes in important parameters such 

 as blade number and hull shape (1,2). Also the effect of propeller geometry has 

 been studied, and guidelines for a favorable vibration level can be given in the 

 design stage. All this information, however, refers to conventional single- screw 

 propulsion systems and moderate power absorption. 



Since the general line of ship development tends toward larger units with 

 increased power, efforts have been made to overcome practical difficulties such 

 as limited propeller diameter, which restricts the efficiency and maximum 

 power absorption of single- screw ships. To maintain a favorable propeller ef- 

 ficiency and to prevent cavitation damage for these high-powered units, ducted 

 propellers and contrarotating propellers are seriously considered and have 

 proved to be beneficial (3,4). - 



For vibration analysis of these type of propulsion systems, however, it is 

 unacceptable to extrapolate knowledge and insight about the dynamic aspects of 

 conventional propellers. Therefore, instantaneous force measurements have 

 been carried out on a set of contrarotating propellers operating behind a fast 

 cargo liner. In this paper the results of this investigation are reported and in- 

 troductory considerations on the dynamic analysis of "contrarotating-propeller- 

 shaft-engine systems" are given. 



THE SHIP AND PROPELLERS AND THE 

 MEASURING TECHNIQUE 



The combination of two contrarotating propellers has proved efficient when 

 operating behind a fast cargo liner. The measurements were performed with a 

 model of that ship type (scale 1:24). The body plan of the ship is given in Fig. 1. 

 The particulars of the ship are as follows: the length at DWL is Lj, = 160 m, the 

 ratio of L^ to the molded breadth B is Lq^/b - 7.13, the ratio of b to the draft t is 



B/T = 2.53, and displacement/Lj^T = 0.5879. 



Fig. 1 - Body plan of the investigated ship 



237 



