SCREW-PROPELLERS. 



lution only round the cylinder. This arrangement was subse- 

 quently modified, and two convolutions and a half of a double- 

 threaded screw were used instead of one complete convolution of a 

 single-threaded screw. This plan has been occasionally varied, a 

 smaller fraction of a convolution being sometimes used. 



It is found in practice that the amount of the slip in general 

 varies from one-tenth to one-twentieth of the pitch ; that is to 

 say, the actual velocity of the screw through the water is from 

 one- tenth to one- twentieth less than it would be if the screw 

 worked through a solid, or as an ordinary screw in its nut. 



66. The screw-propeller is usually fixed upon an axis parallel 

 to the keel of the vessel, and mounted in a space in the dead wood 

 between the stern-post and rudder-post. It is usually suspended 

 on a short shaft, carried by a metal frame, having a rack on each 

 side, in which endless screws work, by means of which the frame 

 supporting the propeller can be lifted out of the water, so that the 

 screw can be repaired if required or a new one introduced without 

 putting the vessel into dock. 



To enable the water to react in a manner analogous to that in 

 which the nut reacts upon the common screw, the thread requires 

 to be much deeper than, if the screw worked in metal or wood, 

 and the pressing surface to be proportionally larger. Accordingly 

 screw-propellers are always made with much smaller central bodies, 

 and a much deeper thread than the common screw. They are also 

 made as large as possible in diameter, extending generally from 

 the keel to a point nearly level with the surface of the water. Thus 

 the diameter of the screw is little less than the draft of the vessel. 



67. To convey some idea of the forms of screw-propellers, we 

 have represented in the annexed figures the forms of some of the 

 propellers most generally adopted. 



In fig. 17 is represented a perspective view of Smith's screw-propeller, 

 with two threads or blades, as finally adopted in her Majesty's steamer 



Fig. 17. 



Fig. IS. 



Fig. 19. Fig. 20. 



"Rattler." This is the form of the screw now most generally adopted in the 



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