A catamaran (10) has two spaced demihulls (12) which are connected by an upper superstructure above the waterline. The catamaran (10) includes a main hydrofoil (26) extending between the demihulls at keel level slightly forward of the longitudinal center of gravity LCG (22) of the catamaran. Each demihull (12) defines an aft swept step formation (28) located slightly forward of the LCG (22) and extending transversely relative to a longitudinal center line CL defined between the demihulls. Each step formation (28) defines a step extending along a straight line and having a height dimension which tapers from an inner position at the keel (18) of the hull towards an outer position at the chine (20) of the hull. Each demihull defines a planing region immediately in front of the step formation (28), which has a concave hydrodynamic profile configured to generate lift on a wetted area of the hull.