Abstract: A self-propelled waterborne vessel has a generally flat hull undersurface aft of the bow region from which extend side walls (10) and keels (11, 12) defining longitudinally extending channels (8, 9) of which the length approximates to one-third of the length of the vessel. Ducted portions of the channels contain propellers (21) aft of which the ducted portion cross-section is selectively variable by transversely pivoted vanes (18). The attitude of the vessel is controllable by the vanes (18) and by further vanes (36) at the stern. Steering is effected by a propulsive steering unit (25) comprising ducts containing propellers (32) and transversely pivoted vanes (34). Additional ducted propellers (46) can be provided aft and may be retractable into the hull.