Prospects for Unconventional Marine Propulsion Devices 



Table 2 

 Thrust Efficiency and its Components 



Situation in which, at constant displacement, a change in propulsion device leads 

 to an increase in ship resistance (r or R^ ) and a proportionately smaller in- 

 crease in propulsion power P^ ; then, though propulsive efficiency as measured 

 by tid or -q^.^ will increase, the designer will not consider it an advantage that a 

 higher power is required. 



Specific power is a parameter which can give guidance in such circum- 

 stances. Defined in engineering units as horsepower per ton-knot, it is related 

 to an equivalent nondimensional parameter thus: 



Specific Power 



SV 



6.88 R/A 



V 



in which power p is in hp, speed V in knots, and displacement A and resistance 

 R are in tons; if desired, the resistance-displacement ratio R'A can be replaced 

 by the reciprocal of the familiar lift-drag ratio L D . When the power P is taken 

 as engine output Pb , then the overall efficiency -q should be used if R is taken 

 as total ship resistance, and r]^ if the naked resistance Rj^ is the basis. Simi- 

 larly, if the power is taken as the dhp P^ , then either rj^ or r/^j^ should be used, 

 as appropriate, in calculating the specific power. 



It has been found useful to relate specific power p/AV to a speed coefficient 

 such as v/A^'^ which does not involve more than ship speed and displacement. 

 Figure 2 is a plot of specific power in terms of such a speed coefficient; the 

 data, partly derived from published information which may not be precisely de- 

 fined and partly from other sources, are for a very wide range of types of ships 

 and other marine craft, including large tankers, passenger liners, high-speed 

 patrol craft, hydrofoil ships, and amphibious and nonamphibious hovercraft. 

 Plots such as this show that in general, as expected, specific power increases 

 with speed coefficient, and also suggest that, for each speed coefficient, there is 

 a minimum specific power corresponding to the "best" performance yet achieved. 



891 



