PROPULSION OF VESSELS 



235 



illustration. Take a piece of joist or lath D, which should be 

 as straight as possible and place it so as to touch one of the 

 blades at any distance, as b, from the axis A B, taking care to 

 hold it parallel to the axis. Next take a carpenter's square, 

 shown at E, and place it on the lath and against the blade, so 

 that the point at which the square touches the blade will be 

 the same distance from the axis as is the lath. Measure the 

 distances a, b, and c; a is the distance from the square to the 

 point at which the lath touches the blade, and c the distance 

 from the point at which the square touches the blade to the 

 lath. The distances a and c may be obtained in a different 



manner, if considered more convenient, thus: Place the screw 

 propeller so that one blade is horizontal. To a piece of string 

 about 10 ft., or more, in length tie two nuts; place the string 

 over the blade, with the nuts hanging down, at the distance 

 from the shaft axis at which it is desired to find the pitch, 

 taking care to place the string so that both parts hanging down 

 are the same distance from the axis. The distance the two 

 parts are apart is the distance a. To find c, hold a lath against 

 the blade and both vertical parts of the string; while holding 

 the lath parallel to the shaft axis the distance c can be measured. 

 The pitch of the propeller may then be calculated by the 

 6.2832 ba 



formula 





