Feature Extraction from B-Spline Marine Propeller Representations

Abstract

This paper presents accurate algorithms for the extraction of gross geometrical features of marine propeller blades represented in terms of B-spline surfaces. These features have important hydrodynamic function and include specifically the camber surface, the section thickness function, pitch, rake, skew, chord length, maximum thickness, maximum camber of the section and leading edge. The approximation of the camber surface, on which most of the remaining features are based, is an intricate problem relying on an extension of the concept of a Brooks ribbon and requiring the solution of a complex system of nonlinear differential equations, developed for the problem at hand, and a sophisticated error evaluation scheme. The above features provide an alternate abstract representation of propeller blades useful in geometric design through lofting and fairing processes, in idealization for hydrodynamic and structural dynamic analyses and in verification of the quality of a manufactured blade. Furthermore, the derivation of these design features provides an alternate shape interrogation method for propeller blades, which is more intuitive for propeller designers in comparison with existing surface interrogation techniques.