Theultimatestrengthofashiphullgirderdependsongeometric,materialcharacteristics,boundaryandloadconditionsaswellasinitialimperfectionsofplateandstiffeners.Theultimatebendingmomentsofamidshipcrosssectionareobtainedfromnonlinearfiniteelementanalysis(NFEA).Acomparisonbetweentheseresultswithtestedboxgirdermodelsunderpurebendingloadingisalsoperformed.Asthesmallerrorsandtheyshowthattheadvantageofmodelsimulation,theNFEAcandeterminerapidlytheultimatelimitstatewhenlaboratorycannotsetuptheexperiments.ThispaperfocusontheassessmentoftheultimatebendingmomentofMST-3boxwithvariouslengthoftestedmodelsandtheeffectoflateralpressuresarealsoappliedtocatamaranhullstructures.Theseresultscontributestheinputdataforcatamaranstructuraloptimizationanalysis.