Fish-like Propulsion and Control Articulated by Elastomeric Bearing-Seals
Elastomeric thrust bearings comprise stacks of alternate rubber and metal layers, all bonded together and under compression from a normal load. Illustrated layers are greatly exaggerated in thickness, but are actually so thin (tenths of 1 mm) that they can't squeeze out from between equally thin metal layers. Each rubber layer allows a small parallel shifting movement due to its resilience, adding up to a total angle depending on the number of layers; e.g., +/-15 degrees. An opposing torque results from the rubber shear stress.
These devices are actually hermetic seals. The laminate stack has a central hole, and the annular solid rubber and metal between interior hole and exterior of the bearing acts as a barrier against water. If the top and bottom surfaces of the bearing are sealed against their respective loading members, then seawater can't move from outside to inside or vice-versa. A shaft can be supported and sealed as it penetrates the bearing and on through the hull of a submersible without any sand-averse rubbing surfaces as in a lip or face seal. An external appendage can be moved or oscillated from an internal actuator - a hydrofoil, or even a fishtail propulser.
These bearing-seals can support very high compression pressures, and have been tested at crushing hydrostatic pressures of up to 17,000 psi -full sea depth - under continuous oscillation for hundreds of hours. Bearing-seals do not become stiffer in torsion with added depth.
The flat laminates shown represent the simplest type of bearing-seal. Instead of being planar, the layers may instead have cylindrical, conical or hemispherical shapes. Those bearing-seals may then be capable of supporting radial, or combined radial and thrust forces, while permitting sealed angular motion about other axes.
Implementation of tailfin propulsion as seen in the Autonomous Underwater Vehicle (AUV) illustration would use spherical bearing-seals. They support and seal a (yellow) shaft that extends longitudinally from the aft body of the AUV, The extended shaft is tipped by compliant tailfins, and has a (black) rubber fairing for smooth hydrodynamic flow.
Details of that arrangement are shown in an expanded cross-section having back-to-back spherical bearing-seals. They surround, support and doubly-seal a ball-like enlargement that is located mid-length on the shaft. It can be seen that the shaft can teeter back and forth within the plane of the picture and out of it, depending upon the internal actuating mechanisms (not shown). A cruciform tailfin as shown could permit vertical as well as horizontal flapping motions for propulsion as well as pitch and direction control of the craft.
Pectoral fins could also be mounted upon and sealed by spherical bearing-seals, thereby making possible flapping or rowing motions, as well as pitch control.
These sealed effectors would make possible a high degree of controllability for inspection missions as well as efficient range in the burgeoning undersea realm.