22 REPORT— 1869. 



Theoretically it is of no importance whether we consider the ship in mo- 

 tion and the water at rest, or the ship at rest and the water in motion in an 

 opposite direction. Practically the conditions are modified by the consider- 

 ation that a stream of water almost always has a sloping surface, in which 

 case a resolved part of gravity is one of the active forces*. Besides this, 

 streams useful for experiment are restricted in depth and width, and the 

 conditions of narrow and shallow channels introduce foreign considerations 

 of a very complicated character. 



Propulsion. 



We do not consider it advisable in the present Report to enter into the 

 question of propellers, except so far as may be necessary to the choice of ex- 

 periments. 



All propellers, except sails, tow-ropes, and punt-poles, do their work by 

 the reaction arising from their driving a stream of water in the opposite 

 direction to the ship's motion, or to their stopping or reversing streams already 

 flowing in that direction. This is the case with oars, paddle-wheels, screws, 

 and water-jets alike. But while they thi;s have one principal action in com- 

 mon, they are wholly different in their detailed effect upon the currents and 

 waves which accompany the ship, and in the way in which these currents 

 and waves react upon them. Thus the oars of a row-boat send two streams 

 aft, at such a distance from the sides of the boat as to interfere very little 

 ■with, and to be very little interfered with by, the waves and eddies due to 

 the boat's motion. In the screw-propeller, on the other hand, a large pro- 

 portion of the wake current is either stopped or reversed by the action of the 

 screw, which also interferes with, and is it itself reacted upon by, the wave 

 of replacement. These interferences are so large in amount as not unfre- 

 qiiently to mask the whole of the slip, from the reaction of which the pro- 

 pulsion is obtained, giving rise to the phenomenon of apparent negative slip. 

 For a theoretical account of what is supposed to take place under these cir- 

 cumstances, we refer to the following Papers in the ' Transactions of the In- 

 stitution of Naval Architects,' and the discussions wliich took place upon 

 them : — 



Eankine, " On the Mechanical Principles of the Action of Propellers." 



Froude, " Note on the above Paper," vol. vi. for 1865, p. 13 et seq. 



Eeed, " On Cases of Apparent Negative Slip," vol. vii. for 1866, p. 114 

 et seq. 



Eankine, " On Apparent Negative Slip." 



Froude, On the same. 



Eigg, " On the Eolations of the Screw to its Eeverse Currents," vol. viii. 

 for 1867, p. 68 et seq. 



Eigg, " On the Eeverse Currents and Slip of Screw-propellers," vol. ix. for 

 1868, p. 184. 



See also Bourne, ' On the Screw-propeller,' second edition, chap, iii., and 

 Eankine, ' Shipbuilding : Theoretical and Practical,' pp. 88, 89, 247, and 

 259. 



"We consider it to be beyond doubt that the theoretical investigations of 

 this part of the subject have been extended in advance of the point at which 

 fresh experimental foundations ought to be laid for them. 



* This remark is due to Bourgois. See his Memoir, sitp. cii. p. 3. 



