=-■-■•' •• '. ■ - ; - t' Pallabazzer 



which can be written in dimensionless form as 



.^=1 , a^^j--—— (25,) 



and accordingly 



In Ref . [22] the feasibility of truncated nozzles for underwater propulsion was 

 presented, showing that practically ideal performances can be actually obtained. 



4.2 Efficiency 



As can be seen from the literature, the efficiency of a propulsor in marine 

 environment is the most discussed parameter, because of the difficulty of taking 

 into account so many variables. In this work, two efficiencies have been intro- 

 duced, that is, (a) the conventional propulsive efficiency v^: 



"•^ 1 V„ 2 + a 



-^nK'-^.') l^~ .. (27) 



" CO 



where the gas contribution has been neglected. Its value for a truncated nozzle 

 is 



2a 



V 



■pc 



2 (27') 



V 



^- - 1 



V2 



(b) a.n overall efficiency Vg, defined as the ratio between the propulsive power 

 SV„ and the chemical energy inflow ae^, = jgma(a'+ a" + a^S) u^{a\ a" and 

 ttjS are the fuel rate fractions) 



SV aVj 



"^^^ T:^^ ] : ■ (28) 



Since for underexpanded nozzles all the quantities at denominator cannot 

 change because of the supersonic flow, the value of ^g for truncated nozzles is 



crV^ a 



C 00 c 



^^^ " jgy(a' + a" + ajS)Hj " "^e ~ ' (2^') 



This efficiency is the only one which could permit a comparison among com- 

 pletely different propulsive devices, such as the water jet and the hydrojector, 

 since any mechanical power is excluded from the performances. 



1128 



