Essentially it is a linear turbine blade.  Or a rotary one of infinite diameter,  if that makes more sense.,(reply>), it's sailing using water currents instead of air currents/wind. Nothing new to a sailor, sailors of sailboats basically have to take this into account on fast flowing water as they need to air to be the one giving more force anchored against the reactions on the keel/center board to go in the right direction.,(reply>), Or a very slow water-skier.,(reply>), Infinite diameter always makes more sense.