The structural analysis of multihulls is relatively complex due to the bridge connecting the hulls, this structure introduces additional stresses to those typical of a monohull. The aluminium trimaran presented in this paper was designed within the research project Conceptual Design of a High Performance Craft for Passenger Transportation in the Southern Region of Chile. The trimaran was structurally calculated according to the rules of Germanischer Lloyd, Det Norske Veritas and Lloyd’s Register classification societies. A Finite Element Model was created for each set of scantlings and the structural analysis was carried out introducing slamming, splitting and torsional moment loads. Results were analysed and high stress regions were determined and compared to admissible stresses to conclude the suitability of the structural arrangement to safely withstand the design loads. Finally, a structural optimisation was developed using a discreet approach, where, after an iterative process, geometry of structural elements under high stresses was improved, changing shapes and decreasing material thickness, where it was found to be possible. Through this process, the initial structural sizing was optimised, obtaining a reduction between 17% and 25% in terms of material required to build the boat, without exceeding allowable stresses prescribed by classification rules.