Unusual Two-Propeller Arrangements 



tests were repeated with only one propeller working and the other removed and 

 compensated for by a tow-rope force. 



The atixiliary propeller arrangement was tested with a right-hand as well 

 as a left-hand auxiliary propeller in combination with a right-hand main 

 propeller. 



For comparison, self -propulsion tests were also carried out with a normal 

 single screw and with normal twin screws turning outwards. 



The stress at the root of one propeller blade was measured in a number of 

 the tests. 



In Table 2 a list is given of the total number of tests carried out until May 

 1968. Further tests, however, are underway. 



Stock propellers were used for all the tests. The diameter of the single - 

 screw propeller was chosen as optimum for a number of revolutions for the 

 ship in agreement with that of large slowly running diesels. The diameter of 

 the interlocking propellers was taken as 0.9 times that of the single-screw pro- 

 peller. This diameter gave an optimum number of revolutions somewhat below 

 that of the single -screw propeller, but this was found permissible, as a gear 

 in any case is necessary for the connection of the two shafts, and some reduc- 

 tion of the number of revolutions will then be natural. 



Two sets of stock propellers were used as interlocking propellers. Both 

 had pronounced rake and were therefore not quite suitable for the purpose. 



In the auxiliary propeller system the stock propeller with the smallest 

 diameter was used as an auxiliary propeller. This propeller was not very suit- 

 able, since the diameter was too large and the pitch ratio and the developed 

 blade area ratio were too high. 



Data for the propellers are given in Table 3. 7 



TESTING METHODS 



The resistance tests were carried out in the normal way. 



The self-propulsion tests were carried out with a tow-rope force accord- 

 ing to the Hughes friction line for a form factor of 1.36 for fully loaded condi- 

 tion and 1.28 for ballasted condition, and with c^ = 0.15 • 10"^. 



Except for the auxiliary propeller arrangement, torque and thrust were 

 measured by mechanical dynamometers. In the two-propeller cases the two 

 dynamometers were connected to one motor to ensure uniform running of the 

 propellers. 



The stress at the root of one propeller blade was measured by means of 

 strain gauges. These were placed on each side of the propeller blade to pre- 

 vent signals caused by temperature expansion. The strain gauges were wired 

 through a hollow shaft to a unit which was placed on the shaft inside the model 

 and which consisted of the remaining resistances of a Wheatstone's bridge, an 



1525 



