This paper deals with an experimental and theoretical investigation into the resistance components in calm water of trimaran with symmetric outriggers. Experimental determination of total resistance and form factor for the ten different trimaran configurations of Wigley's series C98-349 and for Wigley monohull C87-267 (DINMA-Trieste) has been carried out over the Froude number range 0.082-0.495. Wave pattern measurements in six longitudinal cuts for the ten trimaran configurations have been conducted. Wave pattern resistance for the ten trimaran configurations have been experimentally determined by applying LFT, MEM and MES method, over the Froude number range 0.239-0.488. The tests have been conducted over a wide range of relative lateral and longitudinal positions, with model free to trim and sink conditions. Calculation of theoretical wave resistance based on the linear wave theory and the Michell approximation has been undertaken. The comparison between the theoretical predictions and the experimentally determined wave pattern resistance has shown poor agreement. The results of the investigation, over a wide range of Froude number, provide a better understanding of the hydrodynamical content of the components of trimaran resistance including the influence of mutual hull positions on the resistance components. Experimentally determined values of total resistance, form factor and wave pattern resistance for the ten trimaran configurations have made possible the determination of Froude number range, where positive total resistance interference occurs. Wave pattern and viscous interference have been quantified as well.