404 HYDRAULICS AND ITS APPLICATIONS 



that the mean effective angle of the screw blade should be 45, this 

 making the pitch equal to about twice the extreme diameter, and that 

 the true slip should be about 12 per cent. The blade area, projected on 

 a plane perpendicular to the axis of the screw should then be equal to 



8*9 R 

 -j^ square feet, where R = resistance of ship in pounds at the 



'' 8 



maximum velocity TV 



Power Necessary for Propulsion. 



If R = resistance of ship in Ibs., and V s its velocity in feet per second 

 we have 



Work done in propelling vessel = R V s foot-pounds per second. 



7? V 



.'. Horse-power to overcome resistance = * 



ooO 



The I.H.P. of the ship's engines will be considerably more than this, 

 because of mechanical friction losses in the engine and propeller shaft 

 and, to a still greater extent, because of the inefficiency of the propeller 

 itself. In general it may be taken that from 45 per cent, to 60 per cent, 

 of the energy developed in the engine cylinders is utilised in doing useful 

 work. 



Since R is approximately proportional to V s z , the horse-power will vary 

 as V s 3 , except at very high speeds, where the wave-making resistance 

 becomes abnormal. 



ART. 116. JET PROPULSION. 



When a jet of water escapes from an orifice in the side of a vessel, the 

 force necessary to produce this outflow is equal in magnitude to the flux 

 of momentum per second across the vena contracta of the jet, and acts in 

 the direction of flow of the jet. This force necessitates an otherwise 

 unbalanced pressure or reaction on the side of the vessel opposite to the 

 orifice (p. 110), which reaction tends to move the vessel in the opposite 

 direction to that of the jet. 



Advantage has been taken of this in a system which has been applied 

 (though with only moderate success) to the propulsion of large vessels. 

 Here a supply of water is drawn into the boat, usually through a vertical 

 pipe opening amidships, by a centrifugal pump driven by the main 



1 For a further investigation into the action of the propeller the reader is referred to a 

 paper on the action of propellers by Professor Rankine, " Transactions Tnst. Naval Architects," 

 .vol. 6 ; also papers by Mr. Froude in vols. 6, 8, and 19 of the same Transactions, and to 

 White's " Natal Architecture," p. 543. Also to papers in the " Proc. Inst. C.E.," vol. 102, p. 74; 

 vol. 122, p. 51 ; vol. 165, p. 293 ; and in the " Transactions Inst. Naval Architects," 1897, p. 241. 



