UNUSUAL TWO-PROPELLER ARRANGEMENTS 



T. Munk and C. W. Prohaska 



Hydro- og Aerodynamisk Laboratorium 



Lyngby, Denmark 



INTRODUCTION 



Many of the problems connected with the screw propulsion of ships origi- 

 nate from the irregularity of the wake of the ship. When a propeller is working 

 in an irregular wake, the change of the inflow velocity and the angle of incidence 

 may give rise to cavitation or vibrations. 



These difficulties are less pronounced when twin-screw propulsion is used, 

 but then only a small part of the energy in the friction wake is utilized and this 

 causes a decrease in the hull efficiency. In many cases this decrease will not 

 be fully compensated for by the increase in propeller efficiency due to the re- 

 duced loading of the two propellers. 



THE INTERLOCKING PROPELLER ARRANGEMENT 



The difficulties here are thoroughly treated by P. C. Pien and J. Strjz^m- 

 Tejsen in Ref. 1 which discusses several stern arrangements with regard to 

 total efficiency and the ability to reduce cavitation and vibration. In addition, 

 a new stern arrangement is proposed. 



In this arrangement, the two propellers of a normal twin-screw system 

 are moved aft to the longitudinal position of a normal single- screw propeller 

 and inward until the distance between the shafts is less than the diameter of 

 the propellers, which therefore overlap in the centerline zone. This should 

 combine the advantages of the twin-screw system, which are high propeller 

 efficiency due to the reduced propeller load and minimal generation of cavita- 

 tion and vibration due to the smooth wake field, with those of the ordinary 

 single- screw system, which are low appendage resistance and high hull effi- 

 ciency due to the high viscous wake just behind the ship. 



Tests were carried out with the system mounted on a tanker model and the 

 results compared with results from previous tests with other propulsion sys- 

 tems on the model. It was evident that the interlocking propeller arrangement 

 offers great advantages. 



1521 



