Abstract: A device, such as a boat hull and propulsion system, for effecting relative movement of a flowable substance, such as water. A channel having open upstream and downstream ends at least partially defines a flow path. The channel includes a fully laterally closed core section distal the upstream end with a fluid moving rotor rotatably mounted therein and a transition section extending forward from the upstream end of the core section. The channel configuration and rotor are adapted to permit a continual decrease in the transverse cross-sectional area of the flow path downstream along the transition section. If the device is a boat hull, the bow is configured to cause gas lubrication thereof by interaction with the water in motion. The transition sections of the channels may be open-bottomed, and ribs are provided to house drive shafts extending downwardly to the rotors.