High-speed monohulls are extensively used as naval ships for near-shore patrol missions, as work boats, as private yachts and even, recently, as commercial car-ferries with lengths over 100 m and speeds in the 40 knot range. A new systematic series of planing hull forms, possessing improved characteristics with respect to both calm water resistance and seakeeping, is being generated at NTUA. In this paper the seakeeping characteristics of the double-chine parent hull form, with widetransom and bottom warp, of the NTUA series, extrapolated to a 108-metre car/passenger ferry and to a 65-metre passenger ship are investigated. Experimental results are compared with respective analytical results based on strip theory and a time domain method appropriate to planing hull forms.