Prospects for Unconventional Marine Propulsion Devices 



enclose the rotor in a long duct which does not develop thrust; it is then conven- 

 ient to introduce a further factor v^ to allow for the ducting and other losses in 

 the system apart from those at the rotor itself. The factor for the system losses 

 can be combined with the ideal jet efficiency to give a "real" jet propulsive effi- 

 ciency vj , and these different factors are by definition directly related thus: 



Although the real jet propulsive efficiency ^j has been much used, particularly 

 in analyses of waterjet systems, it is a convenience which does not have physi- 

 cal coherence, since it combines an ideal fluid jet efficiency with a factor domi- 

 nated by viscous losses in ducting, while the corresponding losses in the rotor 

 are included in the pump factor Vp . - . 



The ideal jet efficiency rj^ depends only on the ratio of the mean jet inlet 

 velocity to the velocity at the nozzle or jet exit, decreasing sharply as this jet 

 velocity ratio k increases. The real jet efficiency Vj depends on the head loss 

 in the system (excluding pump losses) as well as on the jet velocity ratio; as 

 this loss tends to zero t?. - r,^ . Two further coefficients are useful in compara- 

 tive analyses; these are the thrust and power loading coefficients c^ and Cp , re- 

 spectively, in which the thrust and the power are related to the disk area at the 

 rotor and the speed of advance. The thrust loading coefficient C^ is directly re- 

 lated to the jet velocity ratio, so that the ideal jet efficiency vi can be ex- 

 pressed either in terms of thrust loading Cj or jet velocity ratio k . Further, 

 when consistent units are used throughout, these loading coefficients are re- 

 lated to the thrust efficiency 77^ thus: 



It is often convenient to separate the power losses in the transmission be- 

 tween engine and propulsion device from the other losses in the system; this 

 leads to a quasi -propulsive coefficient which conventionally is related to the ef- 

 fective horsepower of the hull with appendages, but which should more properly 

 be related to the useful propulsion power based on the resistance of the naked 

 hull alone. However, the overall efficiency is a more comprehensive index of 

 relative performance than the quasi -propulsive coefficient; since alternative 

 propulsion devices may necessarily have different transmission systems, such 

 as geared or direct drives, it can be misleading to ignore the transmission 

 losses in comparing the real efficiencies of different propulsion devices. 



Table 1 summarizes these factors which affect the assessment of propul- 

 sive efficiency, and emphasizes the differences between the conventional effi- 

 ciency factors and those proposed here. 



The principal conclusions of this analysis are: 



(a) The thrust efficiency 77^- of a propulsion device defines its per- 

 formance only in unreal isolated conditions. Hence, comparisons 

 of the hydrodynamic efficiency based on thrust efficiency are in- 

 adequate and can be misleading. 



889 



