12 - Propeller design via computer and practical considerations

Summary

Here we present the essential steps in the problems posed by the design and analysis of propellers. In design we are required to develop the diameter, pitch, camber and blade section to deliver a required thrust at maximum efficiency (minimum torque). There are other criteria such as to design a propeller to drive a given hull (of known or predicted resistance over a range of speeds) with a specified available shaft horsepower and to determine the ship speed.

The analysis procedure requires prediction of the thrust, torque and efficiency of propellers of specified geometry and inflow.

We begin with the development of the criteria for the radial distribution of thrust-density to achieve maximum efficiency in uniform and non-uniform inflows. This is followed by methods for determining optimum diameter for given solutions and optimum solutions for a given diameter.

The derivation and reason for the induction factors in the lifting-line theory of discrete number of blades, as displayed in the previous chapter, is followed by formulas for the thrust and torque coefficients in terms of the circulation amplitude function G_n.

Applications are then made to the design and analysis of propellers. Means for selection of blade sections to avoid or mitigate cavitation are followed by extensive discussion of practical aspects of tip unloading via camber and pitch variation. Effects of blade form and skew on efficiency and pressure fluctuations at blade frequency (number of blades times revolution per second) are presented.