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Large medium-speed catamarans are a new class of vessel currently under development as fuel-efficient ferries for sustainable fast sea transportation. Appropriate data to derive design guidelines for such vessels are not available and therefore a wide range of demihull slenderness ratios were studied to investigate the design space for fuel-efficient operation. Computational fluid dynamics for viscous free-surface flow simulations were utilised to investigate resistance properties of different catamaran configurations having a similar deadweight at light displacement, but with lengths ranging from 110 m to 190 m. The simulations were conducted at full-scale Reynolds numbers (log(Re) = 8.9 – 9.6) and Froude numbers ranged from Fr = 0.25 to 0.49. Hulls of 130 m and below had high transport efficiency below 26 knots and in light loading conditions while hulls of 150 m and 170 m showed benefits for heavier displacement cases and speeds up to 35 knots. Furthermore, the study concluded that the lowest drag was achieved with demihull slenderness ratios between 11 and 13.