In the proposed paper, a numerical method in the frequency domain based on the Rankine source formulation, has been extended for the calculation of motions in waves of multi-hulls. Also the added resistance is evaluated by means of the direct integration of the pressure over the wetted body surface. The added resistance is therefore computed as the time averaged value of the longitudinal component of the second order force. In particular the foregoing quantities are computed for catamaran hulls at different separations.. After a brief presentation of the methodology, the vertical motions and the added resistance are computed for a combination of Froude numbers and hull separations in regular head waves. Also the single demi-hull case is considered. The catamaran investigated in this study is mainly from the NPL series for which a comprehensive set of experimental data have been published by Molland et al. [1]. The numerical analysis is also driven in the field of trimarans and the computed motions are presented for a mathematically defined hull for which experimental data are available from Colagrossi et al. [2].