236 PROPULSION OF VESSELS 



in which P- pitch of screw propeller; 

 a = depth of blade; 

 b = distance from center of shaft where width and 



depth of blade is measured; 

 c = width of blade. 



The distances a, b, and c should all be taken in inches, and 

 the measurements for pitch should always be taken on the side 

 of the blade that strikes the water when propelling the vessel 

 ahead. 



EXAMPLE. If a screw propeller blade 6 ft. from the center 

 of the shaft is 22 in. deep and 41 in. in width, at right angles 

 to the shaft, what is the pitch? 

 SOLUTION. Applying the formula, 



6.2832 X 12 X 6 X 22 

 P = = 242.75 in. = 20 ft. 2J in. 



Determining the Kind of Pitch. To determine whether a 

 screw propeller is a true screw, two or more measurements 

 of the pitch should be taken on different parts of the blade at 

 the same distance from the axis. Another set of measure- 

 ments should be taken at some other distance from the axis. 

 If the pitches calculated from these measurements agree closely, 

 the propeller is a true screw. 



To determine whether the pitch of the screw is radially 

 expanded, calculate the pitch at two or more distances from 

 the axis; if the pitch increases toward the tip of the blade, 

 the screw propeller is of radially expanded" pitch. 



To determine whether the screw has an expanding pitch, 

 the pitch must be calculated for the anterior and posterior 

 portions of the blade. The pitch for the posterior portion 

 should be the coarser; and, if calculated for any distance 

 from the axis, the pitches of the anterior portion, as well 

 as those of the posterior portion of the blade, should agree, 

 provided that the axial measurements arc taken in the same 

 planes passing through the axis. 



Required Pitch of Propeller. The pitch required for a 

 screw propeller may be found by the formula 



