Abstract: 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.

Abstract: A propulsion system 10 for a watercraft, comprises a surface-piercing propeller 12, a propeller drive 14, a shroud unit 16 which surrounds the propeller and a mounting arrangement 18 for mounting the propeller and the shroud unit to a watercraft. The shroud unit comprises an inner shroud 31 and an outer shroud 33 which define leading and trailing ends of the shroud unit and which define an air gap between them in the form of a rearwardly-opening annular recess 34. In use, due to sub-atmospheric pressure conditions created behind the trailing edge of the shroud unit as the watercraft travels through water, air is aspirated into the recess below the waterline, forming a curved sheet of air in the water aft of the shroud unit which decouples water flowing within the shroud from water flow externally thereof, thereby decreasing drag induced by the shroud.