A constrained buoy experiencing vortex-induced, in-line and transverse angular motions and designed to absorb and attenuate the energies of streams, rivers and localized ocean currents is described. Referred to as a Finned-Spar-Buoy (FSB), the buoy design can be considered an exoskeleton, in that vertical fins are externally mounted on a vertical cylindrical float. The fins increase the drag coefficient by enhancing the wake losses. The FSB operates as a single unit or as a component of an array, depending on the application. The FSB can adjust to high-water events caused by tides, storm surges or spring-melting runoffs because the FSB can move axially along a center-staff which is attached to an anchor pole at a pivot point. The buoy-staff system is allowed to rotate in any angular direction from the vertical, still-water orientation of the center-staff. The FSB has a relatively small diameter-to-draft ratio, analytically qualifying the buoy as a slender-body.