PROPULSION OF VESSELS 237 



in which P = pitch, in feet; 



Fj = speed of ship, in feet per minute; 



S a = apparent slip, in per cent., expressed decimally; 



N = revolutions per minute; 



Required Diameter of Propeller. For the diameter of a 

 screw propeller, Seaton gives the formula: 



in which D = diameter of screw propeller, in feet; 

 H = indicated horsepower; 

 P = pitch of screw propeller, in feet; 

 N = revolutions per minute; 



C = a constant ranging from 17,000 for slow freight 

 steamers to 25,000 for fast-running light 

 steamers, as torpedo boats and fast steam 

 launches. 



EXAMPLE. Find the diameter of a screw propeller for a 

 steam launch with an engine of 10 H. P., the screw having 

 a pitch of 4 ft. and making 200 R. P. M. 

 SOLUTION. Applying the formula, 



25,000X-^ = 3.5 ft. 



When the rule gives a diameter that is impossible for the 

 conditions, either P or N, or both, must be varied. Making 

 either or both of these values larger will give a smaller diameter 

 of screw; conversely, making either or both of these values 

 smaller gives a larger diameter of screw. The rule is intended 

 for screw propellers: with four blades; if three blades are to 

 be used, the diameter should be increased about 10%; and if 

 two blades are to be used, about 20%. The pitch ratio varies 

 in practice between 1.1 and 1.6. 



Blade Area. The total blade area of four-bladed propellers 

 ranges from 35 to 45% of the disk area; in three-bladed pro- 

 pellers, it ranges between 27 and 33%, and in two-bladed 

 propellers, between 20 and 25%. The value to be chosen 

 should vary with the pitch ratio, using a low total blade area 

 for a low pitch ratio and increasing the value as the pitch ratio 

 is made greater. 

 17 



