A Rankine source method is applied for the calculation of hydrodynamic forces and resultant motions of a ship advancing in waves. The emphasis is placed on the calculation of added resistance due to waves with the future extension in mind for the calculation of motions and drift of a manoeuvring ship in waves. Although the calculation procedure differs little from that of the conventional Rankine source method, the sensitives of the results on such parameters as the size of a computation domain or the size of panels are examined in detail. The motions as well as the added resistance of a vertical cylinder and of a Wigley model are calculated. The results are compared with existing experimental results. The agreements are fairly good except at high-speed, shortwave range, where the calculated added resistance differs from the corresponding experimental results. Some discussions are made on these discrepancies.