Model Tests on Contrarotating Propellers 



and cause damage there. This problem was avoided by reducing the diameter of 

 the aft propeller. This reduction was based on the expected slipstream contrac- 

 tion at design condition. In addition, this reduction is attractive with regard to 

 efficiency, because for equal screw loadings a five-bladed propeller has a 

 smaller optimum diameter than a four-bladed propeller with equal blade area 

 ratio. The sets of contrarotating propellers were designed in such a way that 

 one set is representative for tanker application and another set for cargo liner 

 application. Three additional sets complete the systematic series. The partic- 

 ulars of the propeller models are given in Table 1 and in Fig. 2. 



Table 1 



Principal Characteristics of Table 1 Screw Models 



of the Contrarotating Propeller Series 



Tests were carried out in the towing tank to determine the open-water 

 characteristics of the series. The open-water test results were faired and 

 plotted in the conventional way using the coefficients % = T/pn^D'', Kg = Q/pn^D^, 

 and T7o = ( J/277 )K^/Kq as functions of the advance coefficient J ^ V^/nD. The dia- 

 gram is given in Fig. 3. In this diagram each set of contrarotating propellers 

 is considered as one propulsion unit; the thrust T and the torque Q are based on 

 the sum of the thrusts and torques respectively of the forward and aft screw. 

 The diameter D denotes the tip diameter of the forward propeller. In addition, 

 the ratio of the aft propeller thrust to the total thrust T^^ ^/T and the ratio of the 

 aft propeller torque to the total torque Q^ft/Q are presented in Fig. 3. 



For design purposes, various practical results can be derived from Fig. 3. 

 In the case where the power P and V^ and n are given, the determination of the 

 optimum diameter from a point of view of efficiency of the contrarotating pro- 

 peller system can be solved by plotting -q^ and 5 (S = 101.27/j) as functions of 

 the coefficient Bp (Bp = 33.08 K^ Vj^^^). 



As a comparison the optimum curves for efficiency rj^ and diameter coeffi- 

 cient S of the contrarotating propeller series and the B 4-70 screw series are 

 given in Fig. 4. Screws of the B 4-70 screw series are usually applied behind 

 single-screw ships [10,12]. 



137 



