The making of a propeller, although it appears a difficult task, is not as complicated as would appear, and with the object in view of making the subject readily understood, an explanation will be given of the terms "Diameter," and "Pitch," as used in the art.
The Diameter has reference to the length of the propeller, from end to end. In calculating propeller pull, the diameter is that which indicates the speed of travel, and for this reason is a necessary element.
Thus, for instance, a propeller three feet in diameter, rotating 500 times a minute, has a tip speed of 1500 feet, whereas a six foot propeller, rotating at the same speed, moves 3000 feet at the tips.
PITCH.—This is the term which is most confusing, and is that which causes the most frequent trouble in the mind of the novice. The term will be made clear by carefully examining the accompanying illustration and the following description:
In Fig. 76 is shown a side view of a propeller A, mounted on a shaft B, which is free to move longitudinally. Suppose we turn the shaft so the tip will move along on the line indicated by the arrow C.
Now the pitch of the blade at D is such that it will be exactly in line with the spirally-formed course E, for one complete turn. As the propeller shaft has now advanced, along the line E, and stopped after one turn, at F, the measure between the points F and G represents the pitch of the propeller. Another way to express it would be to call the angle of the blade a five, or six, or a seven foot pitch, as the pitches are measured in feet.
Fig. 76. Describing the Pitch Line.
In the illustration thus given the propeller shaft, having advanced six feet, we have what is called a six foot pitch.
Now, to lay out such a pitch is an easy matter. Assume, as in Fig. 77, that A represents the end of the blank from which the propeller is to be cut, and that the diameter of this blank, or its length from end to end is seven feet. The problem now is to cut the blades at such an angle that we shall have a six foot pitch.
Fig. 77. Laying out the Pitch.