234 PROPULSION OF VESSELS 



slightly rounded. Very often part of the anterior portion 

 near the hub is also cut away. 



Radially Expanded Pitch. Sometimes the surfaces of the 

 blades are not truly helical; as usually found, the pitch near 

 the tip is greater than the pitch near the hub. Such a propeller 

 is said to have a radially expanded pitch. The reason for 

 constructing the blade in this manner is this: Since the part of 

 the blade near the hub strikes the water at nearly a right angle, 

 it acts chiefly to churn the water, and since the water near the 

 periphery is thereby disturbed, the tip of the blade acts on 

 water in motion. By increasing the pitch at the tip, it is 

 supposed that the resistance at all parts of the blade is more 

 nearly equalized. 



Axially Expanded Pitch. The blades are sometimes con- 

 structed in such a manner that the anterior portion of the blade 

 has a finer pitch than the posterior portion. Such a blade is 

 said to have an expanding or axially expanded pitch. The 

 object to be attained by it is as follows: The anterior por- 

 tion of the blade, striking on water at rest and encountering 

 the resistance due to a solid body moving through water at 

 rest, sets the water in motion, driving it astern. Therefore, 

 the posterior portion acts on water in motion. By expand- 

 ing the pitch to the same extent, further motion is given to 

 the water by the posterior portion, and it is supposed that the 

 resistance at all parts of the blade is thereby equalized, the 

 same as with radially expanded pitch blades. 



Surface Areas. The actual area of the surface on the 

 driving side of a propeller blade is known by various names, 

 as the developed blade area, the helicoidal blade area, or simply 

 the blade area. When referring to the total blade area, it is 

 usually spoken of as the developed propeller area, the helicoidal 

 propeller area, or simply, the propeller area. The area of a 

 blade projected on a plane at right angles to the propeller shaft 

 is called its projected area; the projected area of all the blades 

 is the projected propeller area. The area of the circle described 

 by the tips of the blades is the disk area of the propeller. The 

 pitch ratio is the ratio of the pitch of the propeller to its diameter. 



Measurement of Pitch. In practice, the pitch of a pro- 

 peller may be found quite closely in the manner shown in the 



