Performance Criteria of Pulse-Jet Propellers 



uniform inflow to the propeller, although this is not likely to occur in practice. 

 As basic model we may imagine a propulsion device of advance speed V with 

 respect to a large body of fluid but the propeller acting in a wide stream of ve- 

 locity wV in that fluid, no matter how this stream may be generated. Actually 

 this model excludes interaction phenomena proper. We are still considering 

 free-running propellers changing only the reference conditions; the propeller 

 does not change the stream it is acting in. 



To account for this change we have to distinguish between the propulsive 

 efficiency and the propeller efficiency as defined earlier. As the case of a fluid 

 stream generated by a ship hull, i.e., a wake, w denoting the wake fraction, is 

 of particular interest, the propulsive efficiency 



^EFF = KV/P , - (71) 



i.e., the effective efficiency of the propeller, R denoting the resistance of the 

 vehicle equal to the net thrustof the propeller delivered under service condi- 

 tions, whatever the definitions of both magnitudes may be. 



With the thrust deduction / - 



1 - t = R/T (72) 



and the hull efficiency - c, .■ ; 



^HULL = (1 - t)/(l - W) , . , .,^: • .,- ; (73) 



the effective or propulsive efficiency is obtained in the conventional form 



■'^EFF " "^'HULL ■^PROP " ^'^' 



For specified uniform wakes the thrust deduction, and therefore the hull and the 

 effective efficiencies, may be determined theoretically, if the thrust deduction 

 due to friction is neglected in a first approximation, when practical application 

 is concerned. 



While for wakes Wq of the same pressure, i.e., the same potential energy, 

 as the surrounding fluid, the hull efficiency is simply 



^HULL= 1/(1 - %) ' (75) 



in any other case a thrust deduction has to be taken into account. It may be de- 

 termined from a comparison of the actual propeller with a dynamically equiva- 

 lent propeller outside the regime of modified pressure, i.e., outside the near- 

 field of the ship, both producing the same race far downstream, supposing that 

 such a propeller may be constructed. The conditions of equivalence are the 

 equalities of mass flows encountered and power transmitted to the wake at the 

 one and the other pressure level, explicitly, 



Oo = (76) 



and 



(E, - E,), . E^ - El . (77) 



1097 



