Run of River or Tidal Current “Flutter Fin” Adaptation of Flo-pump

The Flo’Pump concept is suited to inversion to form an oscillating reciprocating water current mill below a small floating platform. The weight in water of the inverted fin does not have to be counterbalanced as it forms its own roll pendulum and also provides good pitch stability without wave excitation by a high pitch metacenter platform. The semicircular swept area better covers a typical small channel than the circular or rectangular swept areas of turbines especially for mimimum obstruction of navigation from a surface base. The high speed ratio part of its cycle is at the deepest depth so cavitation and surface ventilation will not occur as they might on insufficiently submerged rotors . The large blunt fins moving at modest speed ratio are most unlikely to cause any fish damage versus the sharp fast spinning and translating blades of boat propellers.

Unlike rotors oscillating fins do not wrap weeds or errant ropes to jam. Nor need there be any bearings at all in the silt-laden if not corrosive stream. Bow and stern anchors on bridles give fairly tight positioning for nearby shipping with no wrapping or excess movement of the output line. It is better to have well marked visible audible presence and warning rather than being hidden underwater at the mercy of boat anchors and nets.

To reduce the reaction requirements of the floating platform, two fins would counteroscillate on two slightly offset axles with engaged ring gears and two pinion shafts would wind spiral leaf chain coils to nonlinearly pull down a central unsealed rod pushing a piston into a central submerged cylinder with lower intake. A further pinion can be used to lower the fins after assembly at the surface and to crank them back up for cleaning. Since a single wing extracts at most ½ the Betz limit, there is an equal amount of power left for the second fin, especially if the solidity is reduced a bit over the windmill wing. The fin planform can be closer to the segment optimum which keeps the pitch damping power a constant fraction of the roll power and also is indicated by robust BEM theory. The Wing’d Mill had to have chord close to the root for extra pitch damping to ensure high wind cutout whereas the tidal stream at least has a definite and low max speed

Underwater (HA) rotors have bearings, gearboxes, and generators submerged in seawater. If the seals fail these expensive components will be ruined by corrosion and the oil will pollute the water. So the component, installation foundation and maintenance costs are much higher than the above. The only downside is the Flutter Fins have inherently uneven and so higher peak, and reversing useful torque . On the other hand the reversing gravity bending moments are much smaller, so that heavier construction such as solid ferrocement is possible. The pitch counterweight to achieve near pitch weight balances will be much smaller than in the windmill case as there the wing CG must be ahead of ¼ chord but here at mean specific gravity of 2 say, the CG just needs to be midway between the CB and ¼ chord for no net moment. Likely the CG shift would be by having an internal foam aft wedge. Any pitch counterweight for fine adjustment would be above the water and likewise a surface roll counterweight would be used to reduce the roll frequency for sites with mean currents less than the rated design current. (Since frequent extreme flow feathering is not needed, starting can be assured and adjusted by a simple pitch partial cam track above the water that is cleared once the flutter begins.)

The fin’s principal longitudal reinforcing rods would be welded around the circumference of a root pipe before being encased by the plaster and could be pretensionned easily in factory production. The galvanised rootpipe would be sealed at positive pressure to ensure no water leaks in. The cement would have high fibre content for crack resistance and be primed with a rubber antifouling paint for bridging any service cracks to prevent saltwater intrusion to the mesh which would be galvanised too.

The Flutterfin’s prime niche would be pumping river water ashore.either for use or to elevated hydroelectric storage , as Verdant already use with a propeller mill. Of the rotor designs for generating tidal electricity the best option would seem to be a Vawt below a floating platform, synchronously started from the grid. The tidal current power spectrum is usually sufficiently narrowbanded that the Vawt’s narrow operating peak won’t lose much and stall can be avoided just beyond the max tidal speed.