CHAP. VIII.] 



STEAM NA VIGATION. 



453 



direction of the axis might and ought to be considerably reduced. 

 Much smaller fractions were then employed, and the branches or 

 wings of the propeller were multiplied, though they were often no more 

 than four and sometimes two in number (Fig. 308). The employment 

 of screws with six blades or more, offers more disadvantages than 

 advantages, for the action of one would interfere with that of another. 



FIG. 307. Smith's first model screws : single screw with complete turn ; double screw with half turn. 



It is the extension of the diameter of the blades of the screw and the 

 rapidity of rotation that gives most power to the propeller. 



We have stated how the screw is arranged in its frame behind a 

 ship. We should add that to avoid the resistance which would be 

 offered by the screw in the case of sailing being substituted for steam, 

 arrangements are made either to make it loose, or to take it for a time 



FJG. 308. Screws with two and four wings. 



out of its frame. In the latter case a well is formed in the hinder 

 part of the vessel, the screw is taken off, and raised between sliding 

 boards into the well where it can also be examined and repaired accord- 

 ing to requirements. 



In Griffith's screw propeller probably the best modification of the 

 common screw now in use a hollow sphere is substituted for the 

 central position of the blades, which in the ordinary form absorbs 



