PROPELLERS GEOMETRY MODELING THROUGH B-SPLINES SURFACES

This work presents a new design methodology for modelling the blades of ship propellers using B-spline surfaces that are a standard tool in CAD and Naval Architecture software products. Propeller blades of a ship are good examples of free form surfaces, designed specifically considering several parameters that control their performance. Traditional tools for surface design in CAD such as control point manipulation, are not appropriated for blade design, and the designers prefer to work with a collection of propeller parameters that ultimately represent its surface and that possess a clear hydrodynamic meaning. This method uses common design parameters for the geometry of propellers and produces a final B-spline surface for the geometry of the blades that can be used for the visualization, calculations, and construction of the propeller. The method starts with the definition of a 3D grid of points that form the propeller blades based on the 2D definition of a series of cross-sectional profiles at several radial locations. These 3D points consider the inclination and twist of the blades are given by rake and pitch angles, quite common in the design procedures, and also different propellers parameters such as skew and blade thickness distribution. Propeller blades are very thin objects with great changes of curvature, and if standard B-spline techniques are used, they cannot be modelled well under a tolerance unless a large number of control points is used, producing very complex surfaces. The method stresses the fitting of the blade's leading edge which has great effect on the propeller behavior and geometrically has a small curvature radius in comparison with the rest of the blades. The leading edge is difficult to reproduce with standard techniques.