Wereldsma 



The choice of the propeller combination is based on efficiency considera- 

 tions and on expected favorable vibrational behavior. A four-bladed propeller 

 was chosen for the forward propeller. To avoid impinging of cavitating tip vor- 

 tices of the forward propeller on the blades of the aft propeller the diameter of 

 the aft propeller was reduced. This reduction is based on the expected slip- 

 stream contraction at the design condition of the system. It was decided to have 

 a five-bladed aft propeller. The particulars of both propellers are given in 

 Fig. 2. 



PITCH DISTRIBUTION 

 PERCENT 



4-BLADED LEFT HANDED FORWARD PROPELLER 



5-BLADED RIGHT HANDED AFT PROPELLER 



Fig. 2 - Plan of the pair of contrarotating propellers 



This combination is expected to have a reduced vibratory output. The 

 lateral effects, mainly generated by the five-bladed propeller, are expected to be 

 reduced due to the more uniform inflow, being the wake of the forward propeller. 

 Also the total axial excitation, mainly generated by the forward propeller, is 

 expected to be relatively small, due to the distribution of the power between two 

 propellers. 



The measurements have been carried out for the following condition: 



ship speed 

 propeller speed 



20.78 knots, 

 121 rpm. 



238 



