Thursday, May 13, 2010

Propeller Blade Area Ratio

Propeller Blade Area Ratio are usually explained in three terms Projected Area Ratio; Developed Area Ratio (DAR) and Expanded Area Ratio. In the design of marine screw propeller, the DAR has to be determined usually from the cavitation calculation. To get the value of the DAR, there are some propeller design calculations that a naval architect or marine engineer must perform.
  1. The calculation of Bp or power coefficient
  2. The determination of delta value (usually read from Bp delta diagram of B- series propellers at the optimum propeller efficiency)
  3. The calculation of delta value which is obtained from the product of propeller's RPM (N) and diameter (D) divided by speed of advance Va 
  4. The reading of optimum efficiency from the Bp delta diagram based on delta value obtained at point 3 above.
  5. The determination of propeller pitch ratio
  6. The calculation of Ku or thrust loading coefficient
  7. The calculation of Y7 or conversion factor of speed resultant
  8. The determination of Kv or thrust loading coefficient whose value is obtained from Ku divided by Y7
  9. The calculation of static pressure (po-e) measured at propeller axis 
  10. The calculation of cavitation number sigma A
  11. The calculation of cavitation number sigma R at the fraction radius x = 0.7
  12. The reading of Developed Area Ratios from the diagram of blade areas. 
 Diagram of thrust loading coefficients and developed area ratios according to Burril and NSMB (TPO Brien - 1962)
The values of DAR are proposed by Burril (read Cavitation of Marine Propeller) and NSMB (the Netherlands Ship Model Basin). Usually propeller designer choose the value from Burril that is larger than that shown in the diagram by NSMB. Because the difference between Expanded Area Ratio (EAR) and DAR is small, propeller designers usually ignore the value of EAR. EAR can be obtained through calculation using simpson's rule after the ordinates of DAR have been developed into the expanded blade in the drawing of propeller using Holst method.
If you have never performed a propeller design calculation, you will be confused about this explanation. As a propeller designer, I can only suggest that you read page 194 of a book entitled the Design of Marine Screw Propellers written by T.P.O. Brien.
This is an old book but it contains a very good explanation about important steps a naval architect has to perform to design a ship or boat propeller with optimum performance. Another book, Marine Propellers and Propulsion by John Charlton is also good but lack of samples of manual calculation. So, Mr. Brien's book is better in explaining the the design philosophy behind a good ship or boat propeller. by Charles Roring
Also read: How to Design Ship Propeller