Unconventional Propulsion -Silver leaf 



Table 1 m. o.^.-f ^ 



Propulsive Efficiency and its Assessment 



(b) Comparisons based on the conventional quasi -propulsive coeffi- 



cient 



^D 



take account of most interaction effects between the hull 



(c) 



and the propulsion device, and thus give a far better indication 

 than thrust efficiency r;^ of relative hydrodynamic efficiencies. 

 However, the quasi-propulsive coefficient does not penalize losses 

 due to the drag of appendages associated with the propulsion de- 

 vice, and these can vary significantly for different devices. 



The most satisfactory basis for comparing hydrodynamic efficien- 

 cies is a qualified propulsive efficiency t]^-^ based on the useful 

 propulsion power ?^ related to the resistance of the naked hull. 

 Although it may not always be easy to identify unambiguously and 

 acceptably the resistance of the appendages defining the factor kf^ 

 in the relation t^^j^ = \^[^rj^ , this should always be attempted. 



(d) The thrust and propulsive efficiencies of a propulsion device are 

 linked by a hull interaction factor r]^ , and the relation tj^ - t^^t^j^ 

 provides a useful way of separately comparing the direct and the 

 interaction effects of different propulsion devices. 



(e) Transmission efficiency should be included in any complete per- 

 formance comparison of propulsion devices; the overall factor 7? 

 or T^j^ is a more comprehensive index of relative propulsive effi- 

 ciency than any hydrodynamic efficiency criterion alone. 



These relations between the thrust efficiency -q^ and its components are 

 summarized in Table 2, and Fig. 1 gives values of real jet efficiency rj-r for the 

 wide ranges of thrust loading c^ and head loss coefficient K^ over which marine 

 propulsion devices are now required to operate. 



Specific Power and its Implications 



All the propulsive efficiencies considered here are based on a useful power 

 output directly related to the resistance overcome. While this can be logically 

 justified, it is irrelevant to the ship designer for whom hydrodynamic efficiency 

 is more usefully defined by the power required to propel a specified displace- 

 ment at a stated speed. This can be simply demonstrated by the not-infrequent 



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