Abstract: The action of upstream and downstream foil sets rotating on a common shaft within a fluid is enhanced by fixing certain interrelations of the foils such as the axial distance between the foils, the radial index between foils and pitch of one foil relative to the other so that the foil stages are fixedly mounted with the foils of each stage being symmetrically and equidistantly spaced from one another and from the foils of the next adjacent foil stage so that the interacting uniform adjacent foil pressures and the upstream foil wake beneficially interacts optimally with at least one of the surfaces or boundary layers of the foils of the next adjacent downstream foil stage.

Abstract: Application of at least one slot notch or groove to or near the rear of a single or pair of towed planing boards or surfaces. Such slots or the like induce a water geyser at speed that improves the appearance and/or directional control of such boards. Such slots or the like are especially effective and practical for deep displacement type boards whose zero and low speed efficiency and stability is especially improved for paired boards or ski when such are fitted with a rigid or preferably semi-rigid connector as hereinafter described.