Fast monohulls are currently in operation as naval ships for near-shore and off-shore patrol missions, as commercial work boats, as private yachts and even, recently, as commercial car/passenger ferries. In the latter case their length and speed exceeds 100 m and 40 knot, respectively. Thus, it is worthwhile to develop hull forms of this kind with improved characteristics with respect to both calm water resistance and seakeeping. This was the prime scope of the new systematic series of highspeed double-chine monohulls, with wide transom and bottom warp, which is being generated at NTUA. In this paper the seakeeping operability performance of the parent hull form of the NTUA series, extrapolated to a 108-metre car / passenger ferry and to a 65-metre passenger vessel, are investigated. Experimental results are compared with respective analytical results based on strip theory and a time domain method appropriate to planing hull forms.