PROPULSION OF VESSELS 233 



the path of the point mentioned above, its path will be the 

 surface of a true screw, provided the angle that the line makes 

 with the axis remains constant. From this, it follows that 

 a true screw is one in which the advance of any point, in the 

 direction of the axis, at any distance from it, for any part 

 of a revolution, but the same in each case, is the same. By 

 causing lines making equal angles with each other and the 

 axis to rotate about the axis in a helical path, a multiple- 

 threaded true screw will be generated, having the same pitch 

 as a single-threaded true screw generated by a line following 

 the same helical path. 



Consider a four-threaded, right-handed screw, generated 

 by the lines A , OB, O C, and O D, in the accompanying 

 illustration. These lines represent the intersections of the 

 four helical surfaces with a 

 plane E F perpendicular to the 

 axis. Assume the helical sur- 

 faces to be cut by a plane, asGH, 

 parallel to the first and inter- 

 secting the axis at another point. i^^^-^it^^^ M /* \~' 

 Then, O A A' O', O B B' O', D ^ A ^~IB'\D W 



OCC' O', and O D D' O' will be 



the helical surf aces of the blades ^_^ .^. . . 



<5f a four-bladed. right-handed C^' c C JH C 



screw propeller. If pieces of 

 metal are shaped to conform to these helical surfaces and 

 if these pieces of metal, which are called blades, are fas- 

 tened to a hub, which in turn is keyed to a shaft rotated 

 by an engine, the screw propeller in its simplest form is 

 obtained. It the screw propeller is revolving in the direc- 

 tion of the arrow, the portion of the blade that strikes the 

 water first, which will be near the plane G H, is known 

 as the anterior portion of the blade; and the portion that 

 is near the plane E F, as the posterior portion. That part 

 of the blade that is near the periphery A A' is known as 

 the tip. In practice, screw propellers are hardly ever made 

 of the shape shown. Generally the anterior portion of the 

 blade is rounded off toward the tip, as shown by the dotted 

 line on the blade O A A' O'. The posterior portion is also 



