The next step is to draw a flattened elevation or development of one of the blades, in order to give the actual curves of its outline, and afterward its thickness at various points. Draw a horizontal line from c and f ([Fig. 41]), and through this a centre line. This will give the length of the blade from the boss, and the centre line of the propeller shaft may be added below. Then take the lengths a b and c d from [Fig. 40], and set them off on [Fig. 43], as shown, joining all four points. This figure would be the true outline of the blade if there were no curves. The actual outline is found by drawing the curves according to the dimensions.
Fig. 42. Propeller lines complete
Lay out the propeller, as shown in [Fig. 42], which will give the elevation of the blades, all being alike.
To find the area of a propeller blade, mark it off in parallel lines, say 3 in. apart, and note the width at the centre of each portion. Add the widths together, and divide by the number of widths. This will give the mean width, which must then be multiplied by the length of blade to obtain the area. If the measurements are all in inches, the result should be divided by 144 to give the area in square feet, and then be multiplied by the number of blades to give the total area.
Fig. 43. Angle of propeller blade
To measure the pitch of a propeller, lay it down on a level surface, hold a straight edge level across centre of blade with a square up from the lower edge, as in [Fig. 43]. Measure the distance B and H and the radius R from the centre to the part where the measurement is taken; then B : 2wR :: H to pitch, P or P = 2wRH/B. The measurements may be made in more than one place and the average taken, as the blades are sometimes twisted slightly.