A parametric methodology for the preliminary design of SWATH hulls

Abstract

This paper presents a parametric computer-based technique to build a B-spline model of a SWATH (Small Waterplane Area Twin Hull) ship hull. The method that is presented uses naval architecture parameters such as Displacement, Waterplane Area (Awp), Centre of Buoyancy (LCB), and Centre of Flotation (LCF) to obtain a geometric definition of the hull that is not based on conic shapes (cylinders, cones or paraboloids) as it is normally used in the literature, but on Chebychev polynomials and B-spline curves and surfaces. With the use of Chebychev functions, a given area, centroid, and even inertia can be imposed on a curve that is later modelled with a B-spline curve. In this way, the SWATH struts can be designed with a given Waterplane area, LCF and inertia, and then combined with a SWATH Lower Hull definition to obtain the ship's displacement and LCB. The positions of LCB and LCF gain importance in the seakeeping behaviour of SWATH ships, whilst the waterplane plays a role on their stability characteristics. Applying the method presented in this work, all these variables can be controlled.