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In my previous article, I said that the pitch of propeller is determined from power coefficient Bp whose parameters are the RPM of the propelling machinery of the ship, the delivered power of the main engine, and the speed of advance. In this case, the speed of advance is the speed of water flowing toward the propeller. Also I wrote that the pitch ratio of propeller is obtained from the Bp- diagram. We must understand that the value which we read from the diagram is the pitch ratio at the tip of the propeller.
If we use B-series propeller as our standard for the design of our propeller, then we must calculate the mean pitch ratio of the designed propeller. The pitch of B-series or Troost propeller varies according to its radius. It means the surface of the propeller is spiral. If the generatrix or generator line of a propeller has curved form then the pitch distribution of the propeller is not linear.
The following table is an example of mean pitch ratio calculation for an Open Hatch Bulk Carrier of 45,000 DWT. Based on the data of the B-series provided by T.P. O'Brien in his book, The Design of Marine Screw Propeller on page 132. The tip pitch ratio is 0.793 and the diameter of the propeller is 6.8 meters.
After tabulating the calculation of the pitch distribution based on P/D diagram, the product of pitch and radius x will have to be devided with the sum of the radius fraction of the propeller. Mean Pitch Ratio: xp/ x =4.2075/ 5.4 = 0.779.
So, the mean pitch ratio (P/D)mean of the designed propeller is 0.779