Apparatus and method for harnessing fluid energy from a fluid current and converting it into mechanical energy. The invention comprises a big wheel and a little wheel where the little wheel rotates about on axis parallel to but offset from an axis about which the big wheel rotates. The little wheel rotates within the big wheel. Flexible sails are fastened between masts of the big wheel and masts of the little wheel so that when the wheels rotate each sail is unfurled and extended during one phase of operation and said sail is folded upon itself during another phase of operation. When extended or stretched open, the sail derives power from the fluid current. When the sail is folded, it offers a reduced resistance to the fluid current into which said sail is moving.