Schmiechen 



in terms of magnitudes to be determined in propulsion tests alone, M^ denoting 

 the momentum outflow from the propeller outside the near field of ship and pro- 

 peller, and F denoting a towing force, if any. For propellers discharging above 

 the free surface, the measuring procedure is particularly simple; see Sec. 4.2. 



In order to avoid the difficulties encountered in the determination of the 

 propeller inflow details depending on the choice of the propeller boundary the 

 following procedure is suggested. Instead of the hull, ideal, and jet efficiencies 

 as defined before in terms of the inflow momentum velocity, we may introduce 

 the effective efficiencies 



' V* - ^"^ ~ ^^ ^ (87) 



^IDEAL = ^ ^\ (SS) 



and 



2 1 



^'JET 



in terms of the inflow energy velocity. 



m 2/^2 - ei 



2 1 



(89) 



This description of the propulsive performance in terms of energy rather 

 than momentum has particular advantages. It avoids not only explicit, but even 

 implicit references to the interaction forces between propeller and hull, however 

 they may be defined. Further on the value of the mass-specific energy, e.g., is 

 only weakly dependent on the location of the propeller boundaries and conse- 

 quently the same holds for the effective efficiencies (87)-(89) and the effective 

 performance characteristic 



^hyd = ■^eff'^^'^hull ^^ideal^ 

 = *^ (^ideal) • 



6. CONCLUSIONS 



This outline of ideas and procedures concerning the definition and evalua- 

 tion of performance criteria of pulse-jet propellers will not be complete without 

 due reference to further extensions and generalizations. 



As the basic principles are not restricted to propulsion systems, the deduc- 

 tions are euqally valid for reaction motors and brakes, if the quantities intro- 

 duced are considered as algebraic. Two major disadvantages of a unified expo- 

 sition, which has been envisaged, are that the efficiencies defined would assume 

 any positive and negative value and that the word-language would be extremely 

 clumsy. 



While in a completely formalized presentation, making use of some sort of 

 operational notation, these drawbacks may be fully compensated for by the ad- 

 vantages gained, they are prohibitive in the present context. The disadvantage 

 of the suggestive language chosen is certainly that it may suggest exactly the 



1100 



