A device for linear propulsion comprising a support structure for carrying a payload, and one or more wave propulsion modules, wherein the or each wave propulsion module comprises a first individually-pivoted substantially planar blade that is capable of producing a reactive force by self-attenuating by spring or dynamics of buoyancy means in response to an incoming fluid flow in the horizontal plane, and a second individually-pivoted substantially planar blade that is capable of producing a reactive force by self-attenuating by spring or dynamics of buoyancy means in response to an incoming fluid flow in the vertical plane; characterised in that (a) the first and second blades are constructed to be positively buoyant; and the first blade is provided with pivot means that is inclined forward at the top of the pivot, such that the orbit of rotation of the blade is on a forwardly-inclined plane, thereby allowing the centre of positive buoyancy to provide a return force when the blade is deflected by lateral flows; and wherein the combination of the second blade and its pivot means is constructed so as to exhibit positive buoyancy, the centre of positive buoyancy being vertically above the pivot axis of the blade, and forward of the centre of lateral resistance in the vertical plane; or (b) the first and second blades are constructed to be negatively buoyant; and the first blade is provided with pivot means that is inclined rearward at the top of the pivot, such that the orbit of rotation of the blade is on a rearwardly-inclined plane, thereby allowing the centre of negative buoyancy to provide a return force when the blade is deflected by lateral flows; and the combination of the second blade and its pivot means is constructed so as to exhibit negative buoyancy, the centre of negative buoyancy being vertically below the pivot axis of the blade and forward of the centre of lateral resistance in the vertical plane. (a) the first and second blades are constructed to be positively buoyant; and the first blade is provided with pivot means that is inclined forward at the top of the pivot, such that the orbit of rotation of the blade is on a forwardly-inclined plane, thereby allowing the centre of positive buoyancy to provide a return force when the blade is deflected by lateral flows; and wherein the combination of the second blade and its pivot means is constructed so as to exhibit positive buoyancy, the centre of positive buoyancy being vertically above the pivot axis of the blade, and forward of the centre of lateral resistance in the vertical plane; or (b) the first and second blades are constructed to be negatively buoyant; and the first blade is provided with pivot means that is inclined rearward at the top of the pivot, such that the orbit of rotation of the blade is on a rearwardly-inclined plane, thereby allowing the centre of negative buoyancy to provide a return force when the blade is deflected by lateral flows; and the combination of the second blade and its pivot means is constructed so as to exhibit negative buoyancy, the centre of negative buoyancy being vertically below the pivot axis of the blade and forward of the centre of lateral resistance in the vertical plane.