The two holes, at the two angles at the ends of the board, are for the purpose of making right and left hand propellers, as it is desirable to use two propellers with the A-shaped model. Two propellers with the deltoid model are not so necessary.
After the twist is made and the blade properly secured in position it should be allowed to thoroughly dry, and afterwards, if it is coated with shellac, will not untwist, as it is the changing character of the atmosphere which usually causes the twisted strips to change their positions. Shellac prevents the moist atmosphere from affecting them.
MATERIAL FOR PROPELLERS.—Very light propellers can also be made of thin, annealed aluminum sheets, and the pins in that case will serve as guides to enable you to get the desired pitch. Fiber board may also be used, but this is more difficult to handle.
Another good material is celluloid sheets, which, when cut into proper strips, is dipped in hot water, for bending purposes, and it readily retains its shape when cooled.
RUBBER—Suitable rubber for the strips are readily obtainable in the market. Experiment will soon show what size and lengths are best adapted for the particular type of propellers which you succeed in making.
PROPELLER SHAPE AND SIZE.—A good proportion of propeller is shown in Fig. 89. This also shows the form and manner of connecting the shaft. The latter A has a hook B on one end to which the rubber may be attached, and its other end is flattened, as at C, and secured to the blade by two-pointed brads D, clinched on the other side.
Fig. 89. Shape and Size.
The collar E is soldered on the shaft, and in practice the shaft is placed through the bearing hole at the end of the frame before the hook is bent.
SUPPORTING SURFACES.—The supporting surfaces may be made perfectly flat, although in this particular it would be well to observe the rules with respect to the camber of large machines.