NUMERIC DESIGN AND EVALUATION OF SWATH FORM

Article type: Research Article

Authors: Subramanian, V. Anantha | Beena, V.I.

Affiliations: Department of Ocean Engineering, Indian Institute of Technology, Madras, India

Abstract: This paper describes a quick method of surface modelling coupled with interactive fairing of the SWATH hull based on 'optimal' geometry based inputs. This design approach is used for hydrodynamic evaluation to obtain motion and dynamic effects. A combination of typical hull form variants is assessed to obtain relative merits. In the absence of large database support from past experience, it is necessary to have a more fundamental approach to designing the SWATH form. This paper reports the use of hitherto accepted polynomial coefficients (Chebychev's polynomial) to link the basic owner's requirements with optimal geometry based characteristics such as displacement, centre of buoyancy, centre of flotation, distribution of strut waterplane area and underwater displacement hull sectional area. Once these are obtained, a computer based form generation scheme is deployed for generation of the hull form. The scheme uses bi-quintic B-splines (Basis splines) with their characteristic features of flexibility and controllability as an important tool in the representation of the body surface. Since the B-spline shape is influenced by the knot vector and the order of the curve, the obtained form is capable of being changed locally by removal or insertion of knots (KR – KI algorithm). This algorithm helps in controlling not only local section shapes, but also formation or elimination of knuckles or sharp corners such as the joint of strut and lower hull or upper hull. Application of this new method to typical SWATH forms with circular, elliptical or 'golf club' section shapes or any arbitrary variation of sectional area distribution is demonstrated. Coarse grid inputs for the polygon net enable development of the targetted form. The technique is illustrated through different stages of development of the hull form. The obtained form is subjected to hydrodynamic performance analysis using a package. Performance based on different basic section shapes and different sectional area distribution curves is evaluated for stationary and speed conditions. The results bring out relative merits of the hull forms and provide insights on their intrinsic advantages. The method serves as a rapid assessment tool in preliminary design.

Journal: International Shipbuilding Progress, vol. 49, no. 2, pp. 95-125, 2002