The added resistance in waves is, from a practical viewpoint, a key issue when selecting the propulsive plant of ships. Despite the fact that normal sea conditions correspond very rarely to calm water, most of the optimisation work performed at a design stage on the hull and on the propulsion system of ships relates to the calm sea condition. The actual motion resistance due to real sea conditions is often accounted for by using simplified empirical coefficients: a sea margin of about 15 to 20% is usually taken into account for selecting the maximum continuous rating of the propulsion engines. On the other hand, an evaluation of the increase in the motion resistance of hulls in a seaway is available from comparatively well assessed computational procedures. Some linear strip theory computer programs have this feature, despite the intrinsic non-linear nature of the force under consideration. In the present work a frequency domain strip theory program (PDStrip) has been adopted for a systematic investigation on a family of ships with different dimensions and speeds. The ship is a gas tanker and all the hull geometries are derived by transformation of a parent hull. The paper briefly recalls the literature available on the subject of the added resistance and the basic elements of the theory adopted. Results are provided of a systematic variation in dimensions and in design speed of the ship. The output in terms of added resistance is used in conjunction with the IACS scatter diagram of North Atlantic for a statistical prediction of the average annual speed and fuel consumption.