FACTORS IN PROPELLER SELECTION

 

A common misconception in selecting a propeller is that it is only necessary to specify diameter and pitch.

Although these factors are the most critical as mentioned earlier there are many other characteristics that must be considered.  

From the desktop of

Leonardo Montoya

If, for instance, you simply want a propeller of 61.25 inches in diameter and with a 56 inches of pitch, on opening the manufacturers catalo you would discover six or seven very different types of propellers available in these dimensions. Among other things, you have to consider the number of blades, blade area, blade thickness, section shape and so on.


Attempting to select a propeller on the basis of pitch and diameter alone is like walking into a hardware store and asking simply for a ¾- inch, number 8 screws. The shopkeeper would immediately ask you if you need a wood screw, sheet metal screw, or a machine screw. A round head, oval or flat. It is equally important to specify the correct type of propeller.


Before you can properly specify and order the most suitable propeller for your application, you must specify most of the following factors, listed roughly in order of importance:

  1. Diameter
  2. Pitch
  3. Number of blades 
  4. Hand (Left-hand or right-hand turning)
  5. Propeller shaft diameter and keyway
  6. Blade area (usually using Mean-Width Ratio or Disc-Area Ratio)
  7. Supercavitating or standard noncavitating blades
  8. Blade section shape
  9. Skew Angle
  10. Rake
  11. Blade Thickness
  12. Hub Diameter

Items 1 thought 6 must be specified for every propeller and every installation. Items 7 thought 12 are of greater importance for differing types of craft and in solving specific problems.





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