Seakeeping qualities, in terms of wellness and comfort onboard, are one of the most important factors for passenger ships, already at the preliminary project phase. Therefore it is important, both for designers and ship owners, to accurately predict and evaluate seasickness, in order to define the best hull forms and minimize relevant motions. From this point of view it is possible to achieve considerable seakeeping improvements, even if ship main dimensions and displacement have been already fixed. In the paper parametric hull modelling is used to generate, by Lackenby method, several ship variants, derived from a parent hull, but with different hull form parameters, without penalizing the bare hull resistance. Differently from classical seakeeping optimization procedures, based on motion RAOs peak values minimization in regular head waves, the paper deals with a new optimization technique, that accounts for all heading angles in a seaway, characterized by various wave heights and zero-crossing periods, each one with a certain probability of occurrence. A new objective function to be minimized is introduced, namely the Overall Motion Sickness Incidence (OMSI), defined as the mean MSI value on passenger decks, and two operating scenarios are considered. As test case, the procedure is applied to improve the seakeeping qualities of a twin-deck Ro-ro/Pax ship.
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