Sunday, February 28, 2010

How to design ship propeller

Last week someone sent me an email requesting help on how to design a ship propeller. He has been writing his final assignment for an undergraduate program for 5 years. He still cannot finish it just because he does not have literature or textbooks about propeller design. Well, propeller design is not a subject that all naval architects studied when they were taking an undergraduate program. Yes, they study resistance and propulsion of ships but going deeper into the subject or the design of marine screw propeller is not always the choice that every naval architect will do. The science of propeller design, actually lies between two fields of co-related engineering which are naval architecture and marine engineering. And  the guy who asked for help is not studying any of them. He studies mechanical engineering. I think that's the reason why he finds that propeller design is very complicated. It is difficult even for naval architects and marine engineers, so it is not surprising to see how difficult it will be for someone who does not study ship propulsion and tries to design a propeller.
A propeller might look like a fan that you use in your house or office. But the complexity of designing a propulsion device that moves or drives a ship in a sea water is huge. Well, I will not blame him for choosing such difficult subject for his final assignment which we in Indonesia like to call Skripsi. What I am going to say now is read the following steps to design a ship's propeller:
Kamome Propeller from Japan
  1. First of all, you need a number of data for the calculation of ship resistance. They are LBP or Length Between Perpendiculars; LWL: Length on Load Waterline; Breadth of Ship; Draft of Ship; Displacement of ship in cubic meters; Position of Center of Buoyancy; Waterplane area coefficient; midship section coefficient, wetted hull area (including the rudder and the appendages; and then the maximum diameter of a propeller. For the calculation of ship resistance, please find an article entitled: An Approximate Power Prediction Method by J. Holtrop and GGJ Mennen, and A Statistical Re-analysis of Resistance and Propulsion Data by J. Holtrop. After performing the calculation, you will get a curve depicting the relationship between effective power and ship speed.
  2. From the EHP or now better known as effective power (PE), find the delivered power, shaft power, and brake power. Perform two calculations which are forward calculation for determining the engine rate of the ship and then the backward calculation for redetermining the delivered power that will be used for calculating the coefficents of Bp and delta.
  3. Most propeller designer will use Troost series propeller which is now known as standard B series propeller from Netherlands Ship Model Basin (NSMB) which is now known as MARIN.
  4. After pitch diameter ratio, the rpm, and the pitch of the propeller the next step in propeller design is calculating the blade area ratios. These ratios can only be obtained after the designer has performed propeller cavitation calculation which is usually based on Burril Method. Then, we get Projected Area Ratio, Developed Blade Area Ratio or Expanded Area Ratio from Burril Cavitation Chart.
  5. Now the principal dimensions of the propellers have been obtained. But we cannot put these numbers on paper in the form of design drawing. We must perform propeller strength calculation to obtain blade thickness diameter ratio. The method that we, as naval architects or marine engineers, use here is Taylor Method.
  6. The last task is drawing. We draw the design or our new propeller based on the standard B-Series propeller using Holst Method. Here, I can say that the design stage is finished.
Still confused about my explanation? Read two reference books that I recommend to you: the first, a very old book, Resistance, Propulsion and Steering of Ships written by Professor Van Lammeren and the second, also an old book The Design of Marine Screw Propeller by T.P.O. Brien.
I hope the one who asked the procedure of designing and calculating the blade thickness of propeller read this post. by Charles Roring 
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1 comment:

Joseph Praful said...

hey..where can i get the bp-delta charts?