5 



if 



force acting on the whole nozzle AS is lbs. per lb. of water 



per second flowing through the screw. Turning then to the 

 remainder of the nozzle, to the left of SS, the water passing 

 through the screws ultimately looses its extra velocity, and in 

 so doing, produces a backward pressure (viz. from £ to A') on 

 that part of the nozzle. In the case of a pure and simple nozzle 

 immersed at a great depth in an infinite ocean the backward 

 and forward pressures would be equal, and the neck of the 

 nozzle at SS would be continually lengthened by the separation 

 of the forward and after parts, AS and SA f , of the nozzle by the 

 forces acting to separate them, were there not a disarrangement 

 of the motion of the surrounding water near SS, whereby such 

 changes of pressure or " head " would occur as to produce an 

 equal tendency to prevent the separation of AS from SA', and 

 the whole tube of flow ASA' would be, on the whole, acted on 

 by no forces, and therefore remain of fixed form in a fixed 

 position, with the water flowing past and in it. When, however, 

 there is a propeller at SS setting the water revolving the motion 

 of the water in the part SA' of the tube of flow will be different 

 from what it was in AS, and additional forces will exist, due to 

 centrifugal pressures, which may be (and in the case of the pro- 

 pulsion of a ship are) able to keep the obstruction (that is the 

 ship) in its place in the nozzle AS, and to provide in addition 

 for the expenditure of energy required by the ship and nozzle 

 ASA' causing waves to spread continually out into the ocean 

 (as they must do when the ship, &c, are near the surface) and 

 to overcome frictional and other subsidiary losses of power. 

 The extra thrust, due to the circumstance of the ships body 

 forming an obstruction in the nozzle AS is familiar under the 

 title of " added resistance " in screw vessels. 



In one sense, the efficiency of the screw is the ratio 

 work expended on thrust _ 

 Total work done, 

 but this is not the same as the commercial efficiency, the latter 

 . work to keep obstruction in place 



Total work done 



