Saturday, February 20, 2010

Propeller blade thickness ordinates

When a naval architect has finished the design calculation of a propeller, he will continue his work by drawing it. The method of drawing a marine screw propeller is usually based on Holst. Please, read page 204 of the Resistance, Propulsion and Steering of Ships by Prof. W.P.A. Van Lammeren or Marine Propellers and Propulsion for more explanation about it.
Those who want to be a propeller designer must know the basics of propeller geometry. There is a very good Youtube video which explains the science of propeller geometry. Watch the following video before you proceed to the design calculation and the drawing of a marine screw propeller.

Before the developed blade sections can be drawn, the propeller designer must obtain the values of  the  positions of Center Line (CL) of the propellers at each blade section radius (usually from r/R = 0.2 to 0.9 or 1.0) relative to the TE (Trailing Edge) and LC (Leading Edge). In theory, the Developed Area Ratio (AD/A0) is assumed to be similar to the Expanded Area Ratio of the marine screw propeller. By determining the positions of CL, we can place the positions of the maximum blade thickness ordinates for every radius (r/R) based on the standard percentages that are provided by the book.  For the thickness ordinates relative to trailing and leading edge of each blade section radius , the values are derived as percentages of thicknesses compared to the maximum thickness at each radius of the blade. The safe value for blade thickness diameter ratio is obtained from Propeller Strength Calculation usually using Taylor formulas.
Please keep in mind that the percentage of the blade thickness ordinates that I discuss in this post is for the standard  B-Series propeller which is provided by NSMB (the Netherlands Ship Model Basin). The table and the drawing of the newly designed propeller presented in this article is from by undergraduate final assignment The Propeller Design of Open Hatch Bulk Carrier (OHBC) DWT 45,000 metric tons. by Charles Roring