become visible by their empty cores, and may be seen as interlacing helices following in the track of the extremity of each blade, like adherent strings of sea-weed.[1]
The vortices from the external extremities of the blades are all of the same "hand" and consequently tend to wind round one another; they may be conceived to break up into spiral groups and perhaps sub-groups, as they are left behind in the propeller race, after the manner indicated already in the case of the aerofoil (Fig. 86).
§ 218. On the Design of an Aerial Propeller.—A few simple rules may be formulated for the design of an aerial propeller; these rules will be applicable mutatis mutandis to the marine propeller.
(1) From the conditions, assess the probable value of (usually about 10 degrees), and (Fig. 136) plot the efficiency curve from the equation (§ 204). Any arbitrary scale may be employed.
(2) Decide on "discard point"; that is, the minimum percentage of maximum available efficiency, and so determine blade length.
(3) Draw the thrust grading curve, (Fig. 136), as in § 213 (Fig. 133). At this point the designer has to exercise his judgment; it is perhaps best to draw a trial curve freehand, the object being a smooth curve beginning and ending at zero, but in general character to simulate the truncated wedge form based on the Newtonian theory; then let fall perpendiculars from the conjugate points of equal efficiency, and draw radial lines through the origin to suit the freehand curve as nearly as possible; then correct the freehand curve to pass through the intersections.
(4) From the thrust grading curve (Fig. 136) derive the load grading curve the ordinates being calculated by multiplying the thrust ordinates by the corresponding values of sec (Fig. 136).
- ↑ The author has, for example, observed such air-core vortices from the after-deck of twin-screw S.S. New York.
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