HIPER2004 - 4th International Conference on High-Performance Marine Vehicles INSEAN, Rome, Italy - 27 - 29 September 2004 Automatic Optimisation of a New Fast Catamaran with Bulbous Bow Stefano Brizzolara, Dario Bruzzone Università di Genova - DINAV, Italy, brizzolara@dinav.unige.it Abstract The paper presents a theoretical investigation into the wave resistance characteristics of a new typology of fast catamarans with an internal underwater body between the two demihulls, which can grant a considerable reduction in total resistance. The effect of the underwater body is investigated in a systematic way by the application of an automatic parametric optimisation method on the longitudinal and vertical position of the body. The wave pattern and wave making resistance of each investigated solution is evaluated by means of a free surface potential flow panel method. The inviscid resistance reductions predicted by the numerical method show comparable trends with the experimental results obtained by Zotti on the same bulbcat hull form. The optimum “catamaran with bulb” configuration, at different design speeds, is found by the automatic optimisation method, based on a differential evolution algorithm, which drives the entire numerical procedure (i.e. variation of free parameters, the panel mesh discretisation, the prediction of wave resistance) varying each iteration the free parameters in an “intelligent way”. The analysis of the results is made at two different levels: a preliminary comprehensive investigation of the wave patterns generated by each “bulbcat” hull component and their quantitative interference effects; and the discussion of the optimised configurations for different speeds through the correlation between the parameters considered in the optimisation problem and the analysis of the objective function variation over the free variable design space. Interesting conclusions are drawn about the effectiveness of the underwater bulb, of the optimisation method, and practical guidelines are given for the application of a “bulbcat” typology in the design of new fast catamarans. 1. Introduction A cooperative research project, funded by the Italian Ministry of Research and held by the three Italian Universities of Naval Architecture, by Genova, Napoli and Trieste, is dedicated to the study of modern and efficient multi-hull vessels for fast sea transportation of passengers and cars, among which trimaran, pentamarans and unconventional catamarans are taken under consideration. This particular study, just initiated by the authors, is part of the activity of this research project and deals with a novel typology of catamaran first presented by Zotti (2003). In fact, it was proved by him, through a systematic series of towing tank tests, that some typologies of fast catamaran hulls can improve their residual resistance by the use of an underwater body of revolution placed at the bow, as an isolated submerged bulb between the two demihulls. The effect of the underwater body is beneficial over a large speed range and not just around a single speed. This new catamaran hull typology is briefly referred to as “BulbCat” in the rest of the paper. An example of this kind of new composite multihull is presented in figure 2 and described in more detail in the next section. The intentions of this study is to further analyse the hydrodynamic behaviour and the interference effects of the underwater body on the whole ship wave-making resistance and to complement the inevitably limited experimental investigation with a higher number of configuration, especially for what regards the influence of the longitudinal and vertical position of the submerged internal body. Other motivations of the present study regard: enhancing the experience in the application of parametric optimisation algorithms to the design of fast ship hulls with improved powering characteristics, that brought already interesting results in another study (Brizzolara, 2004); and extend the application of the free surface potential flow method on a new multi-hull typology and verify the accuracy and stability of the results.